高压下硫化氢和氨硼烷结构和动力学性质的从头算分子动力学研究
|
摘要
含氢小分子是一种在自然界中广泛存在的简单物质,对它们的研究可以提高人们对物质科学的基本认识。同时,由于含氢小分子在地球内部和其他星体内部大量存在,其在压力下的结构和性质的研究也具有很重要的实际意义。通过压力,含氢小分子在常压下的结构、性质可以被大大的改变,从而带来很多新奇的现象和性质。含氢小分子在压力的影响下会发生复杂的结构转变,呈现出一系列性质迥异的新相。
含氢小分子在压力下存在着的一些相似的变化规律,比如压致分子解离,更是引起了人们的广泛关注。其中最著名的就是水的压致解离。大量的高压实验和理论研究证明,水的压致解离和水中的氢原子在其压致解离曲线周围的三个高压固体相中的动力学行为息息相关。在水解离前的两个高压相VII相和VIII相中,由于氢原子处在特殊的双势阱中运动,从而表现为在低温相VIII时,氢位于其中一个势阱中,表现为有序相;在常温和高温时,由于热运动,氢原子在两个势阱间等几率分布,而其运动就表现为氢原子在两个势阱点来回运动。随着压力的升高,由于氧氧间距被压缩,这种特殊的双势阱势垒渐渐变成单势阱,从而约束了氢原子的运动,使得氢原子处在两个氧原子中间点的位置,形成了对称氢键,从而完整了分子解离。在水之后,人们发现其姐妹分子硫化氢——在分子水平上和水分子相似——在高压下却变现出迥异的性质。首先水在常压下具有很强的氢键,而硫化氢却没有;其次,硫化氢的高压相图远没有水的高压相图复杂;最重要的是,硫化氢在高压下将发生分解,从而形成硫、氢和硫化氢的混合物,而不会生成具有对称氢键的分子解离相。前人的实验虽然定出了硫化氢的解离曲线,但是由于混相的缘故,无法得到硫化氢在解离后的晶体相结构,也没有像水那样给出解离的微观机制。这给予了我们研究动机及研究思路,即:通过从头算分子动力学方法,从硫化氢未解离前的结构出发,先得到稳定的相结构,再通过改变温度压力条件,模拟其解离过程。我们首先从一个实验建议的结构出发(其具有I41/acd空间对称群),在15GPa,100K下进行动力学模拟。计算发现实验建议的结构并不稳定,在200fs的动力学模拟时间内,其氢键键连网络发生了变化。新得到的结构具有Ibca的对称性。我们得到的结构的XRD谱和实验符合的非常好。实验给出了错误的IV相结构的原因是无法测定氢原子的位置,而是在解出了硫原子的位置后手动加入了氢原子,从而得到了不稳定的氢键键连网络结构。我们接着从Ibca结构出发,通过升温的方式穿过实验建议的解离边界。之所以这样做是因为:1.硫化氢的解离压力随温度变化非常大。前人实验发现在室温时(300K),硫化氢在27GPa解离;而在150K时,解离压高达42GPa。这充分说明温度对硫化氢解离的重要性;2.实验建议,在压力超过25GPa时,由于硫硫原子间距被压缩到硫原子轨道相接触的程度,会促使硫化氢分解而不是解离,正像实验中所观察的那样。因此我们选择在15GPa时,通过升温动力学穿过扩展的解离区线,从而期望能得到硫化氢的V相。我们的Parrinello-Raham变胞动力学表明,在15GPa,350K时,动力学超胞和原子均方根位移同时发生了突变,得到了一个新的稳定相。同时,通过对分布函数(RDF)和角关联(ACF)函数的计算,我们发现,在15GPa,350K时,S-S RDF的第一主峰和第二主峰合并为一个峰,显示了硫晶格的一个突变;S-H RDF的第一主峰和第二主峰在300K以下时有明显的零值分界,因为它们分别代表S-H共价键和S…H氢键,但在350K时却有明显的交叠,这说明硫化氢的共价键和氢键有相互转化的迹象,也就是说,氢原子在两个S原子之间跳跃。而ACF函数显示,在350K时,硫化氢分子表现出明显的转动无序。由于存在明显的氢无序,于是我们先求出平均硫原子晶格,得到其晶格对称群为P63/mmc。接着,我们将9个ps的平衡动力学轨迹中超胞内的氢原子通过P63/mmc的周期性边界条件移动到其单胞内,从而得到一个时间和空间平均的氢原子空间分布。我们发现,氢原子空间密度分布在每个硫原子周围都有12个密度极大值。而且这些极大值都位于P63/mmc对称群的Wyckoff位置上。综上结果我们认为,硫化氢在15GPa350K时由IV相转变到V相。我们得到的新结构是一个纯硫化氢晶体相,具有P63/mmc对称群。氢原子在V相中是无序的,并且以分数占据的形式分布在硫原子周围。计算得到的V相的XRD谱也和实验符合。在V相中,氢无序主要表现在两方面,一是H原子在两个S原子之间的跳跃,而这和水的VII相没有不同;二是硫化氢分子的转动无序,而这在水的VII相中不曾发现。我们认为新发现的这个具有氢无序的相结构是分析和理解硫化氢的压致分解的关键。随着压力的升高,和水不一样,氢原子不会被固定在两个硫原子中间,从而不会随着硫原子间距的减小而被固定在两个硫原子之间而形成对称氢键。相反的,由于转动无序的存在,氢原子会被带离两个硫原子之间,甚至是形成两个硫原子直接接触的情况。再加上由于压力带来的硫硫轨道交叠,硫硫共价键终将形成。因此,我们发现的硫化氢的V相具有水的VII、VIII相不具有的动力学性质,从而导致硫化氢在高压下分解而不是形成对称氢键的分子解离相。
含氢小分子引起广泛关注的另一个原因涉及到能源问题。现在的能源结构是建立在化石能源(包括煤、石油、天然气等)基础上的,其缺陷就是污染环境,并且储量有限。而氢能源由于其燃烧生成的是水,无任何污染,因此是未来的能源材料。应用氢能源的途径主要是用其做载体,因为地球上的氢主要是以稳定化合物的形式存在,不能直接利用。用氢做能源载体需要满足无污染和方便简单。特别是像汽车等小型机械设备,对以氢为载体的能源应用来说是再合适不过。因此,寻找合适的材料去储存氢是迫切之需。现在储存氢的方式主要集中在将氢固化方面,因为无论是气态还是液态形式的氢,都不利于实际应用。现在对氢的固化储存主要集中在三个方面,一个是利用金属和氢化合生成金属氢化物。金属氢化物的优点就是,通过加热可以很快的放出氢气,并且可以反复充氢,达到循环利用。缺点就是储氢比例不高,且金属成本较高,质量较大不便运输。第二个是通过各种纳米材料做成的孔筛式载体,使得氢吸附在纳米材料表面。但此方法同样面临着材料合成方面的难度。第三个方法是化学储氢,就是通过轻元素和氢的化合物,例如含氢小分子,达到储氢的目的。化学储氢的优点就是材料来源广泛,并且由于轻质元素(例如硼、碳、氮等)较低的质量数,能使氢的储量百分比达到一个很高的值。化学储氢的缺点就是放氢速率不高,放氢温度较高,还有就是重复利用较困难。综合来看,现在还没有一种方式可以在所有的方面达到最合理的要求。对于化学储氢来说,其将来的研究重点就是如何提高储放氢效率,从而达到重复利用的目的。化学储氢里最具前景的材料之一就是氨硼烷(NH3BH3),它的含氢体积百分比和质量百分比都非常高。已有的实验研究主要集中在改进催化剂和应用纳米材料改性等方面。但直到目前为止,还未能达到实际应用的要求。同时,对于氨硼烷本身的基础研究也大量开展起来。包括从实验上测定其结构,观察结构随温度和压力的变化,拉曼光谱的温度和压力效应等。人们发现在氨硼烷中,最重要的相互作用是双氢键,它是由带正电的、和氮相连的+δH与带负电的、和硼相连的Hδ-之间相互吸引造成的。而固态氨硼烷和气态氨硼烷最重要的区别就是在气态时,氨硼烷分子的旋转是整个分子作为一个整体而来的;在固态时,其氨基的旋转和硼基的旋转却是分离的。氨硼烷在常压下有两个固态相。其中低温相具有Pmn21对称群,常温相具有I4mm对称群。在更高的温度下,氨硼烷将分解,释放氢气。氨硼烷的高压研究也很重要,因为压力带来的普遍的结构和性质变化可能对其储氢性能有所提高。更重要的是,通过研究此类储氢材料的压力效应,可以提高人们对储氢材料的理解,进而对储氢材料的设计提供重要参考。人们发现在常温下加压时,氨硼烷会从I4mm相转变到一个具有Cmc21对称群的高压相,转变压大概在1.1GPa~1.4GPa,误差主要来自于不同实验小组用的样品的纯度、性状的不同。在更高的压力下,拉曼实验认为氨硼烷将在5GPa,12GPa时分别发生相变,而XRD实验认为直到12.1GPa,氨硼烷只会发生在~1.1GPa时的一个相变。在更高的压力下,没有相应的研究。为了解释实验上的差异和寻求氨硼烷在更高压力下的结构和动力学性质,我们应用从头算分子动力学,首先从氨硼烷已知的低温常压相出发,通过缓慢加压,发现在3GPa、100K时,体系发生了结构性转变。得到的新的稳定结构具有Cmc21对称性,和常温XRD实验发现的高压相一致。接着,我们在常温条件下对Cmc21结构进行加压。动力学模拟显示,Cmc21相在15GPa之前都保持稳定。而在15GPa时,动力学计算显示,体系的超胞在8个皮秒的模拟时间内没有收敛,而是表现为a、b两个晶轴交替振荡。我们发现这种交替振荡正好对应着Cmc21相单位晶格的一个剪切形变。于是我们又进行了弹性常数的计算,发现体系在~15.3GPa时发生C66的软化,而且C66正好对应着动力学模拟中发现的超胞的形变。当压力继续升高到20GPa,动力学显示超胞迅速收敛到稳定值,得到了一个具有P21对称群的新结构,由于该结构单包中有两个分子,我们将其记为P21(Z=2)。随着压力继续升高,在50GPa、300K时体系又发生了一次结构性转变,体系的超晶胞和均方根位移都发生了突变,得到了一个新相,同样具有P21对称群。由于其单包中有4个分子,记为P21(Z=4)结构。我们的分子动力学一直进行到60GPa,没有发现任何分子解离或分解的迹象。我们还进行了零温焓差曲线的计算。计算结果和动力学结果是大致符合的。通过零温结构分析,我们发现,在Cmc21相中,体系的双氢键数目并不是如最近一篇XRD实验研究文章所宣称的那样一直保持在12个每分子上,而是随着压力的升高而增多。我们利用一篇CSD结构库搜索研究文献提出的双氢键的作用范围和作用角度为判据,得出在6.4GPa时,体系中每个分子参与的双氢键数目从12增大到14。我们认为因为这种双氢键数目的变化而带来的结构变化,对应着Lin等人在拉曼光谱实验中发现的光谱突变,并且这种变化并没有造成体系相结构的变化。当体系转变到P21(Z=4)结构时,我们发现氨硼烷分子的交叉排列构象被破坏。虽然这种分子稳定构象的破坏会带来能量的升高,但相变同时双氢键网络的变化却大大降低了体系的能量。通过对Pmn21、Cmc21、P21(Z=2)和P21(Z=4)结构的动力学模拟轨迹的分析我们发现,在所有相结构中,双氢键键角的NHH部分随压力变化不大,而BHH部分随着压力先增大,在P21(Z=4)相中,由于分子构象的改变,BHH角又大大减小。我们还发现在所有相结构中,双氢键键角的分布都符合前人提出的双氢键键和规律,即:NHH角较直(接近160度),BHH角较弯(接近120度)。最后,我们的从头算分子动力学研究表明,氨硼烷的三个高压相中都存在分子的120度跳跃旋转。由于氨硼烷分子的C3v对称性,这种旋转并不会破坏氨硼烷的有序结构。动力学显示,在常压相下存在的NH3和BH3基团旋转的分离在高压下被保留。并且保持着NH3旋转优先于BH3旋转和分子整体旋转的格局。通过分子旋转势能面的计算,我们发现这种格局可以被它们相差很大的旋转势垒所解释。
Hydrogen-containing molecules are of fundamental importance for their widely existence in nature. The structural and dynamical properties of hydrogen-containing molecules under high pressure can provide basic understanding to interior earth and planetary physics, because high-pressure can greatly modify the structures and chemical bonds including hydrogen bonding which is one of the most important interactions in nature.
One of the most interesting phenomena in the hydrogen-containing molecules is the molecular dissociation which turns the system from molecular to atomic at high pressure. For ice, dissociation coincides with hydrogen bond symmetrization, forming ice X, in which the proton occupies the midpoint between two neighboring O atoms. Path integral and ab initio molecular dynamics showed that the proton behavior in ice VIII, VII, and X plays a crucial role in understanding the nature of dissociation. For hydrogen sulfide, which is an analog of water at molecular level, the dissociation turns out to be an intermediate process of the decomposition. Experimental works based on analyzing the decomposed products cannot give a microscopy description to this process and thus failed to provide dissociation mechanism. In this study, by using ab initio molecular dynamics, the stable structure of phase IV was found along with a new-found structure of phase V. The proton behavior in phase V is the key to understanding the decomposition process of hydrogen sulfide.
We first chose an experimental proposal for the structure of phase IV (with symmetry of I41/acd) by XRD experiments as our starting structure. We found that the I41/acd structure has very high potential energy. The hydrogen bonding network was reformed in less than 300 fs in a simulation at 15 GPa and 100 K. A structure with a space-symmetry of Ibca and a stable hydrogen bonding network was reached. We found that the hydrogen bonding geometry in the Ibca structure is common in many ice phases and is more energy favorable during geometry optimization. The calculated enthalpy of the Ibca structure is 262 meV/molecule lower than that of I41/acd structure at 15 GPa. Moreover, the Ibca structure was also proposed for an intermediate phase, named phase IV’by Fujihisa et al. In the latter work, the authors found that in the range of 4-10 GPa and 30-250 K, the strongest peak of the X-ray diffraction patterns is doublet, while singlet in phase IV. In our Ibca model, we find that whether the strongest peak is singlet or doublet depends on the b/a ratio. When this ratio is very close to one, the 202 and 022 planes have very similar d-spacing, leading the strongest X-ray peak to appear as a singlet above 12 GPa. However, our calculations suggest that the space group is still the orthorhombic Ibca due to positions of hydrogen atoms also when the Bravais lattice turns tetragonal (a=b). Therefore we propose that phase VI’and phase IV are actually the same phase. The Ibca structure was then heated up to 400 K with a temperature interval of 50 K at 15 GPa by a series of simulations. The radial distribution functions (RDF) and two self-defined angle correlation functions (ACF) were calculated simultaneously to monitor the structural change during the heating. We found that at 350 K, the cell shape changed significantly and the lattice parameters stabilized to new values in about 2 ps. In the S-S RDF, the shoulder of the first main peak disappears above 350 K, indicating a change in the S-S distance distribution. In the S-H RDF, the sharp separation between the first and second peaks which represent the S-H covalent bond and the S…H hydrogen bond, respectively, disappears at 350 K. This points to the occurrence of occasional hopping of protons between neighboring molecules. The calculated ACF indicate that molecular orientations change rapidly above 350 K and the structures become orientationally disordered. To obtain this disordered structure, we first calculated the averaged sulfur lattice which turns out to have a space-symmetry of P63/mmc. Next, the density distribution of H atoms from our MD trajectory was extracted by translating H atoms to the P63/mmc unit cell with periodic conditions. It turns out that there are 12 density maxima of hydrogen around each sulfur atom which correspond to the Wyckoff positions of space group P63/mmc. This suggests that our model is a proton-disordered structure with fractional occupation of hydrogen positions. The calculated X-ray diffraction curves consist with the experimental XRD patterns very well. We believe that our obtained P63/mmc structure is the experimental suggested unreacted crystalline structure of phase V. The behavior of the protons in equilibrated phase IV and V is expatiated by the calculated distribution of proton positions along the S-H…S bond. In phase IV, the protons are well localized, meaning that the hydrogen sulfide molecules are well defined as stable units and the structure of phase IV is ordered. While in phase V, both the rotational disorder and thermal jumps of protons exist. The protons in phase V are delocalized and can move back and forth along the S-S separation. The proton behavior in phase V provides a possible way to the decomposition of hydrogen sulfide, i.e., when the S-S covalent bonds are formed, protons can be expelled from the sulfur lattice by a combination of molecular rotation and proton jumping.
The importance of hydrogen-containing molecules also originates from their application in hydrogen storage materials. One of the most promising candidates is ammonia borane (NH3BH3, AB) which contains remarkable high gravimetric and volumetric hydrogen density, and has a moderate dehydrogenation temperature. Experimental attempts were focused on the use of acid, transition metal catalysts, ionic liquids, nano-scaffolds, and etc. to improve the efficiency of discharging H2 from AB. The structural and dynamical factors that govern the stability and the intermolecular interactions are thus essential in understanding and improving the rate of hydrogen releasing. The structures, phase transitions, and hydrogen dynamics of AB at ambient pressure were studied extensively by former researchers. It turns out that AB has two ambient-pressure solid phases with Pmn21 symmetry and I4mm symmetry, respectively. The Pmn21-Cmc21 phase transition is proved to be to be a first-order phase transition leading by progressive displacement of the borane group under the amine group. Using different techniques based on nuclear magnetic resonance (NMR), it is suggested that the rotational dynamics of the NH3 and BH3 groups in the ambient-pressure phases plays a crucial role in the phase transition and shows distinct rotational barriers compared with gas state. The separation of the NH3 and BH3 rotation in the ambient-pressure phases is also supported by both theoretic study and neutron scattering experiments and is attributed to the different inter- and intramolecular torsional forces. The dihydrogen bond, which is formed between the protonic and hydridic hydrogen atoms at adjacent NH3 and BH3 ends respectively plays a significant role in determining the properties of solid AB. The existence of dihydrogen bonds in AB is also supported by high-pressure Raman studies, in which a red shift in the N-H stretch frequency and a blue shift in the B-H stretch frequency with increasing pressure are observed. According to these Raman spectroscopic studies, AB shows a complex vibrational behavior at high pressure and room temperature, and has at least three phase transitions up to 20 GPa at room temperature. However, a recent X-ray diffraction study proposed that only one phase transition occurred at ~1.1 GPa up to 12.1 GPa at room temperature. To the best of our knowledge, no theoretical studies were focused on the phase transitions at high pressure. To filling in this gap, ab initio molecular dynamics were performed extensively in this study. Another interest in this study is the hydrogen dynamics at high pressure, including proton distribution and rotation of the NH3 and BH3 groups. The dihydrogen bond which plays a significant role in determining the behavior of AB is discussed.
Gradually applying pressures at 100 K, the Pmn21 structure transformed to the Cmc21 structure at 3 GPa. This indicates that the Cmc21 phase observed by a recent room-temperature X-ray diffraction study can also be obtained at low temperature and at high pressure. The phase transition is found to be preceded by displacement of the NH3 and BH3 groups in opposite directions, resulting new dihydrogen bonding arrangement. Next, a series of simulations at raising pressure and at room temperature were performed up to 60 GPa in order to search new high-pressure phases. We found that the Cmc21 structure is stable until the pressure reaches 15 GPa, at which the supercell as and bs exhibit alternately up-and-down undulating. As the pressure reaches 20 GPa, the supercell lattice as and bs converged to different values within 2 ps. A new structure with monoclinic P21 symmetry emerged, denoted as P21(Z=2) for there are 2 AB molecules in the unit cell. We also found that the deformation of the supercell at 15 GPa and 300 K is corresponding to a shear strain of the Cmc21 unit cell. The calculated elastic constants indicate that there is a softening of C66 at 15.3 GPa and zero-temperature which is in consistent with the MD simulations. In considering the uncertainties due to the finite simulation time and the temperature effect, we propose that AB undergoes a mechanical instability induced phase transition at 15 GPa and 300 K, and this transition corresponds to the transition at 12 GPa suggested by recent Raman spectrum study. A further phase transition took place at 50 GPa and 300 K, a new structure with P21 symmetry and 4 AB molecules in its unit cell emerged (denoted as P21(Z=4)). In the P21(Z=4) phase, the staggered conformation of AB molecules was broken, resulting in different dihydrogen bonding geometry. We also calculated the enthalpy difference curves which support our MD results. A structural analyse suggests that the dihydrogen bonds per molecule in the Cmc21 structure increase with rising pressure. We use an empirical rule for the dihydrogen bonding geometry as a criterion. It turns out that the number of dihydrogen bonds increases from 12 to 14 at 6.4 GPa, which is in contrast to the proposal by Y. Filinchuk et al. The calculated N-H-H-B dihedral angle distribution derived from MD trajectories indicates that the AB molecules no longer stay in their stable molecular conformation in the P21(Z=4) phase. Further, the calculated dihydrogen bond angles suggest that the NHH angle is more linear and the BHH angle is more bent in all high-pressure phases, which is in agreement with the empirical rule established by W. Klooster et al. The NHH angle change little under compression, while the BHH angles first increase with rising pressure, and become almost as linear as the NHH angle in the P21(Z=2) phase. However, in the P21(Z=4) phase, by breaking the staggered molecular conformation, the BHH angle decreases significantly which is due to the reorientation of dihydrogen bonding network. Our MD simulations also indicate that the rotational angles of the NH3 and BH3 groups represent layered distribution in all high-pressure phases at room temperature. The angle interval is approximate 120°which does not break the symmetry of the high pressure phases due to the C3v point symmetry of AB molecules. As a result, all the three-pressure phases remain ordered structures. We also found that the separation of the NH3 and BH3 rotation observed in the ambient-pressure phases is remained in the high-pressure phases. Moreover, we calculated the potential energy surfaces for rotation of the NH3 and BH3 groups by rotating the NH3 and BH3 groups recurrently over the range of 0≤.θNH3≤120°and 0≤.θBH3≤120°in 10°increments with respect to the equilibrious configurations. Thus, the energy barriers for rotation of the NH3 and BH3 groups can be estimated from theθBH3=0 path and theθNH3=0 path, respectively. As for the correlated rotation of the whole molecule, it is theθBH3=θNH3 path. It is found that the rotational barriers of NH3 is much small than those of BH3 in all high-pressure phases, which is in agreement with the observation in the MD that the NH3 groups rotate more frequently than BH3.
含氢小分子在压力下存在着的一些相似的变化规律,比如压致分子解离,更是引起了人们的广泛关注。其中最著名的就是水的压致解离。大量的高压实验和理论研究证明,水的压致解离和水中的氢原子在其压致解离曲线周围的三个高压固体相中的动力学行为息息相关。在水解离前的两个高压相VII相和VIII相中,由于氢原子处在特殊的双势阱中运动,从而表现为在低温相VIII时,氢位于其中一个势阱中,表现为有序相;在常温和高温时,由于热运动,氢原子在两个势阱间等几率分布,而其运动就表现为氢原子在两个势阱点来回运动。随着压力的升高,由于氧氧间距被压缩,这种特殊的双势阱势垒渐渐变成单势阱,从而约束了氢原子的运动,使得氢原子处在两个氧原子中间点的位置,形成了对称氢键,从而完整了分子解离。在水之后,人们发现其姐妹分子硫化氢——在分子水平上和水分子相似——在高压下却变现出迥异的性质。首先水在常压下具有很强的氢键,而硫化氢却没有;其次,硫化氢的高压相图远没有水的高压相图复杂;最重要的是,硫化氢在高压下将发生分解,从而形成硫、氢和硫化氢的混合物,而不会生成具有对称氢键的分子解离相。前人的实验虽然定出了硫化氢的解离曲线,但是由于混相的缘故,无法得到硫化氢在解离后的晶体相结构,也没有像水那样给出解离的微观机制。这给予了我们研究动机及研究思路,即:通过从头算分子动力学方法,从硫化氢未解离前的结构出发,先得到稳定的相结构,再通过改变温度压力条件,模拟其解离过程。我们首先从一个实验建议的结构出发(其具有I41/acd空间对称群),在15GPa,100K下进行动力学模拟。计算发现实验建议的结构并不稳定,在200fs的动力学模拟时间内,其氢键键连网络发生了变化。新得到的结构具有Ibca的对称性。我们得到的结构的XRD谱和实验符合的非常好。实验给出了错误的IV相结构的原因是无法测定氢原子的位置,而是在解出了硫原子的位置后手动加入了氢原子,从而得到了不稳定的氢键键连网络结构。我们接着从Ibca结构出发,通过升温的方式穿过实验建议的解离边界。之所以这样做是因为:1.硫化氢的解离压力随温度变化非常大。前人实验发现在室温时(300K),硫化氢在27GPa解离;而在150K时,解离压高达42GPa。这充分说明温度对硫化氢解离的重要性;2.实验建议,在压力超过25GPa时,由于硫硫原子间距被压缩到硫原子轨道相接触的程度,会促使硫化氢分解而不是解离,正像实验中所观察的那样。因此我们选择在15GPa时,通过升温动力学穿过扩展的解离区线,从而期望能得到硫化氢的V相。我们的Parrinello-Raham变胞动力学表明,在15GPa,350K时,动力学超胞和原子均方根位移同时发生了突变,得到了一个新的稳定相。同时,通过对分布函数(RDF)和角关联(ACF)函数的计算,我们发现,在15GPa,350K时,S-S RDF的第一主峰和第二主峰合并为一个峰,显示了硫晶格的一个突变;S-H RDF的第一主峰和第二主峰在300K以下时有明显的零值分界,因为它们分别代表S-H共价键和S…H氢键,但在350K时却有明显的交叠,这说明硫化氢的共价键和氢键有相互转化的迹象,也就是说,氢原子在两个S原子之间跳跃。而ACF函数显示,在350K时,硫化氢分子表现出明显的转动无序。由于存在明显的氢无序,于是我们先求出平均硫原子晶格,得到其晶格对称群为P63/mmc。接着,我们将9个ps的平衡动力学轨迹中超胞内的氢原子通过P63/mmc的周期性边界条件移动到其单胞内,从而得到一个时间和空间平均的氢原子空间分布。我们发现,氢原子空间密度分布在每个硫原子周围都有12个密度极大值。而且这些极大值都位于P63/mmc对称群的Wyckoff位置上。综上结果我们认为,硫化氢在15GPa350K时由IV相转变到V相。我们得到的新结构是一个纯硫化氢晶体相,具有P63/mmc对称群。氢原子在V相中是无序的,并且以分数占据的形式分布在硫原子周围。计算得到的V相的XRD谱也和实验符合。在V相中,氢无序主要表现在两方面,一是H原子在两个S原子之间的跳跃,而这和水的VII相没有不同;二是硫化氢分子的转动无序,而这在水的VII相中不曾发现。我们认为新发现的这个具有氢无序的相结构是分析和理解硫化氢的压致分解的关键。随着压力的升高,和水不一样,氢原子不会被固定在两个硫原子中间,从而不会随着硫原子间距的减小而被固定在两个硫原子之间而形成对称氢键。相反的,由于转动无序的存在,氢原子会被带离两个硫原子之间,甚至是形成两个硫原子直接接触的情况。再加上由于压力带来的硫硫轨道交叠,硫硫共价键终将形成。因此,我们发现的硫化氢的V相具有水的VII、VIII相不具有的动力学性质,从而导致硫化氢在高压下分解而不是形成对称氢键的分子解离相。
含氢小分子引起广泛关注的另一个原因涉及到能源问题。现在的能源结构是建立在化石能源(包括煤、石油、天然气等)基础上的,其缺陷就是污染环境,并且储量有限。而氢能源由于其燃烧生成的是水,无任何污染,因此是未来的能源材料。应用氢能源的途径主要是用其做载体,因为地球上的氢主要是以稳定化合物的形式存在,不能直接利用。用氢做能源载体需要满足无污染和方便简单。特别是像汽车等小型机械设备,对以氢为载体的能源应用来说是再合适不过。因此,寻找合适的材料去储存氢是迫切之需。现在储存氢的方式主要集中在将氢固化方面,因为无论是气态还是液态形式的氢,都不利于实际应用。现在对氢的固化储存主要集中在三个方面,一个是利用金属和氢化合生成金属氢化物。金属氢化物的优点就是,通过加热可以很快的放出氢气,并且可以反复充氢,达到循环利用。缺点就是储氢比例不高,且金属成本较高,质量较大不便运输。第二个是通过各种纳米材料做成的孔筛式载体,使得氢吸附在纳米材料表面。但此方法同样面临着材料合成方面的难度。第三个方法是化学储氢,就是通过轻元素和氢的化合物,例如含氢小分子,达到储氢的目的。化学储氢的优点就是材料来源广泛,并且由于轻质元素(例如硼、碳、氮等)较低的质量数,能使氢的储量百分比达到一个很高的值。化学储氢的缺点就是放氢速率不高,放氢温度较高,还有就是重复利用较困难。综合来看,现在还没有一种方式可以在所有的方面达到最合理的要求。对于化学储氢来说,其将来的研究重点就是如何提高储放氢效率,从而达到重复利用的目的。化学储氢里最具前景的材料之一就是氨硼烷(NH3BH3),它的含氢体积百分比和质量百分比都非常高。已有的实验研究主要集中在改进催化剂和应用纳米材料改性等方面。但直到目前为止,还未能达到实际应用的要求。同时,对于氨硼烷本身的基础研究也大量开展起来。包括从实验上测定其结构,观察结构随温度和压力的变化,拉曼光谱的温度和压力效应等。人们发现在氨硼烷中,最重要的相互作用是双氢键,它是由带正电的、和氮相连的+δH与带负电的、和硼相连的Hδ-之间相互吸引造成的。而固态氨硼烷和气态氨硼烷最重要的区别就是在气态时,氨硼烷分子的旋转是整个分子作为一个整体而来的;在固态时,其氨基的旋转和硼基的旋转却是分离的。氨硼烷在常压下有两个固态相。其中低温相具有Pmn21对称群,常温相具有I4mm对称群。在更高的温度下,氨硼烷将分解,释放氢气。氨硼烷的高压研究也很重要,因为压力带来的普遍的结构和性质变化可能对其储氢性能有所提高。更重要的是,通过研究此类储氢材料的压力效应,可以提高人们对储氢材料的理解,进而对储氢材料的设计提供重要参考。人们发现在常温下加压时,氨硼烷会从I4mm相转变到一个具有Cmc21对称群的高压相,转变压大概在1.1GPa~1.4GPa,误差主要来自于不同实验小组用的样品的纯度、性状的不同。在更高的压力下,拉曼实验认为氨硼烷将在5GPa,12GPa时分别发生相变,而XRD实验认为直到12.1GPa,氨硼烷只会发生在~1.1GPa时的一个相变。在更高的压力下,没有相应的研究。为了解释实验上的差异和寻求氨硼烷在更高压力下的结构和动力学性质,我们应用从头算分子动力学,首先从氨硼烷已知的低温常压相出发,通过缓慢加压,发现在3GPa、100K时,体系发生了结构性转变。得到的新的稳定结构具有Cmc21对称性,和常温XRD实验发现的高压相一致。接着,我们在常温条件下对Cmc21结构进行加压。动力学模拟显示,Cmc21相在15GPa之前都保持稳定。而在15GPa时,动力学计算显示,体系的超胞在8个皮秒的模拟时间内没有收敛,而是表现为a、b两个晶轴交替振荡。我们发现这种交替振荡正好对应着Cmc21相单位晶格的一个剪切形变。于是我们又进行了弹性常数的计算,发现体系在~15.3GPa时发生C66的软化,而且C66正好对应着动力学模拟中发现的超胞的形变。当压力继续升高到20GPa,动力学显示超胞迅速收敛到稳定值,得到了一个具有P21对称群的新结构,由于该结构单包中有两个分子,我们将其记为P21(Z=2)。随着压力继续升高,在50GPa、300K时体系又发生了一次结构性转变,体系的超晶胞和均方根位移都发生了突变,得到了一个新相,同样具有P21对称群。由于其单包中有4个分子,记为P21(Z=4)结构。我们的分子动力学一直进行到60GPa,没有发现任何分子解离或分解的迹象。我们还进行了零温焓差曲线的计算。计算结果和动力学结果是大致符合的。通过零温结构分析,我们发现,在Cmc21相中,体系的双氢键数目并不是如最近一篇XRD实验研究文章所宣称的那样一直保持在12个每分子上,而是随着压力的升高而增多。我们利用一篇CSD结构库搜索研究文献提出的双氢键的作用范围和作用角度为判据,得出在6.4GPa时,体系中每个分子参与的双氢键数目从12增大到14。我们认为因为这种双氢键数目的变化而带来的结构变化,对应着Lin等人在拉曼光谱实验中发现的光谱突变,并且这种变化并没有造成体系相结构的变化。当体系转变到P21(Z=4)结构时,我们发现氨硼烷分子的交叉排列构象被破坏。虽然这种分子稳定构象的破坏会带来能量的升高,但相变同时双氢键网络的变化却大大降低了体系的能量。通过对Pmn21、Cmc21、P21(Z=2)和P21(Z=4)结构的动力学模拟轨迹的分析我们发现,在所有相结构中,双氢键键角的NHH部分随压力变化不大,而BHH部分随着压力先增大,在P21(Z=4)相中,由于分子构象的改变,BHH角又大大减小。我们还发现在所有相结构中,双氢键键角的分布都符合前人提出的双氢键键和规律,即:NHH角较直(接近160度),BHH角较弯(接近120度)。最后,我们的从头算分子动力学研究表明,氨硼烷的三个高压相中都存在分子的120度跳跃旋转。由于氨硼烷分子的C3v对称性,这种旋转并不会破坏氨硼烷的有序结构。动力学显示,在常压相下存在的NH3和BH3基团旋转的分离在高压下被保留。并且保持着NH3旋转优先于BH3旋转和分子整体旋转的格局。通过分子旋转势能面的计算,我们发现这种格局可以被它们相差很大的旋转势垒所解释。
Hydrogen-containing molecules are of fundamental importance for their widely existence in nature. The structural and dynamical properties of hydrogen-containing molecules under high pressure can provide basic understanding to interior earth and planetary physics, because high-pressure can greatly modify the structures and chemical bonds including hydrogen bonding which is one of the most important interactions in nature.
One of the most interesting phenomena in the hydrogen-containing molecules is the molecular dissociation which turns the system from molecular to atomic at high pressure. For ice, dissociation coincides with hydrogen bond symmetrization, forming ice X, in which the proton occupies the midpoint between two neighboring O atoms. Path integral and ab initio molecular dynamics showed that the proton behavior in ice VIII, VII, and X plays a crucial role in understanding the nature of dissociation. For hydrogen sulfide, which is an analog of water at molecular level, the dissociation turns out to be an intermediate process of the decomposition. Experimental works based on analyzing the decomposed products cannot give a microscopy description to this process and thus failed to provide dissociation mechanism. In this study, by using ab initio molecular dynamics, the stable structure of phase IV was found along with a new-found structure of phase V. The proton behavior in phase V is the key to understanding the decomposition process of hydrogen sulfide.
We first chose an experimental proposal for the structure of phase IV (with symmetry of I41/acd) by XRD experiments as our starting structure. We found that the I41/acd structure has very high potential energy. The hydrogen bonding network was reformed in less than 300 fs in a simulation at 15 GPa and 100 K. A structure with a space-symmetry of Ibca and a stable hydrogen bonding network was reached. We found that the hydrogen bonding geometry in the Ibca structure is common in many ice phases and is more energy favorable during geometry optimization. The calculated enthalpy of the Ibca structure is 262 meV/molecule lower than that of I41/acd structure at 15 GPa. Moreover, the Ibca structure was also proposed for an intermediate phase, named phase IV’by Fujihisa et al. In the latter work, the authors found that in the range of 4-10 GPa and 30-250 K, the strongest peak of the X-ray diffraction patterns is doublet, while singlet in phase IV. In our Ibca model, we find that whether the strongest peak is singlet or doublet depends on the b/a ratio. When this ratio is very close to one, the 202 and 022 planes have very similar d-spacing, leading the strongest X-ray peak to appear as a singlet above 12 GPa. However, our calculations suggest that the space group is still the orthorhombic Ibca due to positions of hydrogen atoms also when the Bravais lattice turns tetragonal (a=b). Therefore we propose that phase VI’and phase IV are actually the same phase. The Ibca structure was then heated up to 400 K with a temperature interval of 50 K at 15 GPa by a series of simulations. The radial distribution functions (RDF) and two self-defined angle correlation functions (ACF) were calculated simultaneously to monitor the structural change during the heating. We found that at 350 K, the cell shape changed significantly and the lattice parameters stabilized to new values in about 2 ps. In the S-S RDF, the shoulder of the first main peak disappears above 350 K, indicating a change in the S-S distance distribution. In the S-H RDF, the sharp separation between the first and second peaks which represent the S-H covalent bond and the S…H hydrogen bond, respectively, disappears at 350 K. This points to the occurrence of occasional hopping of protons between neighboring molecules. The calculated ACF indicate that molecular orientations change rapidly above 350 K and the structures become orientationally disordered. To obtain this disordered structure, we first calculated the averaged sulfur lattice which turns out to have a space-symmetry of P63/mmc. Next, the density distribution of H atoms from our MD trajectory was extracted by translating H atoms to the P63/mmc unit cell with periodic conditions. It turns out that there are 12 density maxima of hydrogen around each sulfur atom which correspond to the Wyckoff positions of space group P63/mmc. This suggests that our model is a proton-disordered structure with fractional occupation of hydrogen positions. The calculated X-ray diffraction curves consist with the experimental XRD patterns very well. We believe that our obtained P63/mmc structure is the experimental suggested unreacted crystalline structure of phase V. The behavior of the protons in equilibrated phase IV and V is expatiated by the calculated distribution of proton positions along the S-H…S bond. In phase IV, the protons are well localized, meaning that the hydrogen sulfide molecules are well defined as stable units and the structure of phase IV is ordered. While in phase V, both the rotational disorder and thermal jumps of protons exist. The protons in phase V are delocalized and can move back and forth along the S-S separation. The proton behavior in phase V provides a possible way to the decomposition of hydrogen sulfide, i.e., when the S-S covalent bonds are formed, protons can be expelled from the sulfur lattice by a combination of molecular rotation and proton jumping.
The importance of hydrogen-containing molecules also originates from their application in hydrogen storage materials. One of the most promising candidates is ammonia borane (NH3BH3, AB) which contains remarkable high gravimetric and volumetric hydrogen density, and has a moderate dehydrogenation temperature. Experimental attempts were focused on the use of acid, transition metal catalysts, ionic liquids, nano-scaffolds, and etc. to improve the efficiency of discharging H2 from AB. The structural and dynamical factors that govern the stability and the intermolecular interactions are thus essential in understanding and improving the rate of hydrogen releasing. The structures, phase transitions, and hydrogen dynamics of AB at ambient pressure were studied extensively by former researchers. It turns out that AB has two ambient-pressure solid phases with Pmn21 symmetry and I4mm symmetry, respectively. The Pmn21-Cmc21 phase transition is proved to be to be a first-order phase transition leading by progressive displacement of the borane group under the amine group. Using different techniques based on nuclear magnetic resonance (NMR), it is suggested that the rotational dynamics of the NH3 and BH3 groups in the ambient-pressure phases plays a crucial role in the phase transition and shows distinct rotational barriers compared with gas state. The separation of the NH3 and BH3 rotation in the ambient-pressure phases is also supported by both theoretic study and neutron scattering experiments and is attributed to the different inter- and intramolecular torsional forces. The dihydrogen bond, which is formed between the protonic and hydridic hydrogen atoms at adjacent NH3 and BH3 ends respectively plays a significant role in determining the properties of solid AB. The existence of dihydrogen bonds in AB is also supported by high-pressure Raman studies, in which a red shift in the N-H stretch frequency and a blue shift in the B-H stretch frequency with increasing pressure are observed. According to these Raman spectroscopic studies, AB shows a complex vibrational behavior at high pressure and room temperature, and has at least three phase transitions up to 20 GPa at room temperature. However, a recent X-ray diffraction study proposed that only one phase transition occurred at ~1.1 GPa up to 12.1 GPa at room temperature. To the best of our knowledge, no theoretical studies were focused on the phase transitions at high pressure. To filling in this gap, ab initio molecular dynamics were performed extensively in this study. Another interest in this study is the hydrogen dynamics at high pressure, including proton distribution and rotation of the NH3 and BH3 groups. The dihydrogen bond which plays a significant role in determining the behavior of AB is discussed.
Gradually applying pressures at 100 K, the Pmn21 structure transformed to the Cmc21 structure at 3 GPa. This indicates that the Cmc21 phase observed by a recent room-temperature X-ray diffraction study can also be obtained at low temperature and at high pressure. The phase transition is found to be preceded by displacement of the NH3 and BH3 groups in opposite directions, resulting new dihydrogen bonding arrangement. Next, a series of simulations at raising pressure and at room temperature were performed up to 60 GPa in order to search new high-pressure phases. We found that the Cmc21 structure is stable until the pressure reaches 15 GPa, at which the supercell as and bs exhibit alternately up-and-down undulating. As the pressure reaches 20 GPa, the supercell lattice as and bs converged to different values within 2 ps. A new structure with monoclinic P21 symmetry emerged, denoted as P21(Z=2) for there are 2 AB molecules in the unit cell. We also found that the deformation of the supercell at 15 GPa and 300 K is corresponding to a shear strain of the Cmc21 unit cell. The calculated elastic constants indicate that there is a softening of C66 at 15.3 GPa and zero-temperature which is in consistent with the MD simulations. In considering the uncertainties due to the finite simulation time and the temperature effect, we propose that AB undergoes a mechanical instability induced phase transition at 15 GPa and 300 K, and this transition corresponds to the transition at 12 GPa suggested by recent Raman spectrum study. A further phase transition took place at 50 GPa and 300 K, a new structure with P21 symmetry and 4 AB molecules in its unit cell emerged (denoted as P21(Z=4)). In the P21(Z=4) phase, the staggered conformation of AB molecules was broken, resulting in different dihydrogen bonding geometry. We also calculated the enthalpy difference curves which support our MD results. A structural analyse suggests that the dihydrogen bonds per molecule in the Cmc21 structure increase with rising pressure. We use an empirical rule for the dihydrogen bonding geometry as a criterion. It turns out that the number of dihydrogen bonds increases from 12 to 14 at 6.4 GPa, which is in contrast to the proposal by Y. Filinchuk et al. The calculated N-H-H-B dihedral angle distribution derived from MD trajectories indicates that the AB molecules no longer stay in their stable molecular conformation in the P21(Z=4) phase. Further, the calculated dihydrogen bond angles suggest that the NHH angle is more linear and the BHH angle is more bent in all high-pressure phases, which is in agreement with the empirical rule established by W. Klooster et al. The NHH angle change little under compression, while the BHH angles first increase with rising pressure, and become almost as linear as the NHH angle in the P21(Z=2) phase. However, in the P21(Z=4) phase, by breaking the staggered molecular conformation, the BHH angle decreases significantly which is due to the reorientation of dihydrogen bonding network. Our MD simulations also indicate that the rotational angles of the NH3 and BH3 groups represent layered distribution in all high-pressure phases at room temperature. The angle interval is approximate 120°which does not break the symmetry of the high pressure phases due to the C3v point symmetry of AB molecules. As a result, all the three-pressure phases remain ordered structures. We also found that the separation of the NH3 and BH3 rotation observed in the ambient-pressure phases is remained in the high-pressure phases. Moreover, we calculated the potential energy surfaces for rotation of the NH3 and BH3 groups by rotating the NH3 and BH3 groups recurrently over the range of 0≤.θNH3≤120°and 0≤.θBH3≤120°in 10°increments with respect to the equilibrious configurations. Thus, the energy barriers for rotation of the NH3 and BH3 groups can be estimated from theθBH3=0 path and theθNH3=0 path, respectively. As for the correlated rotation of the whole molecule, it is theθBH3=θNH3 path. It is found that the rotational barriers of NH3 is much small than those of BH3 in all high-pressure phases, which is in agreement with the observation in the MD that the NH3 groups rotate more frequently than BH3.
引文
[1] X. Jin, X. Meng, Z. He, Y. Ma, B. Liu, T. Cui, G. Zou, and Ho-kwangMao (2010) Superconducting high-pressure phases of disilane. PNAS 107, (22) 9969-9973.
[2] R. J. Hemley, A. P. Jephcoat, H. K. Mao, C. S. Zha, L. W. Finger, and D. E. Cox (1987) Static compression of H2O-ice to 128 GPa. Nature 330, 737– 740
[3] M. Benoit, D. Marx, and M. Parrinello (1998) Tunnelling and zero-point motion in high-pressure ice. Nature 392, 258-261
[4] M. Bernasconi, P. L. Silvestrelli, and M. Parrinello (1998) Ab Initio Infrared Absorption Study of the Hydrogen-Bond Symmetrization in Ice. Phys. Rev. Lett. 81, 1235-1238.
[5] J. K. Cockkroftand and A. N. Fitch (1990) Z. Kristallogr. 193, 1
[6] S. Endo, N. Ichimiya, K. Koto, S. Sasaki and H. Shimizu, (1994) X-ray-diffraction study of solid hydrogen sulfide under high pressure, Phys. Rev. B 50, 5865.
[7] H. Fujihisa, H. Yamawaki, M. Sakashita, K. Aoki, S. Sasaki and H. Shimizu, (1998) Structures of H2S: Phases I’and IV under high pressure, Phys. Rev. B 57, 2651.
[8] J. S. Loveday, R. J. Nelmes, S. Klotz, J. M. Besson and G. Hamel, (2000) Pressure-Induced Hydrogen Bonding: Structure of D2S Phase I’, Phys .Rev. Lett. 85, 1024.
[9] H. Shimizu, Y. Nakamichi, S. Sasaki, (1991) Pressure‐induced phase transition in solid hydrogen sulfide at 11 GPa, J. Chem. Phys. 95, 2036;(1991) High‐pressure Raman study of solid deuterium sulfide up to 17 GPa, J. Chem. Phys. 97, 7137.
[10] S. Endo, A. Honda, K. Koto, O. Shimomura, T. Kikegawa and N. Hamaya, (1998) Crystal structure of high-pressure phase-IV solid hydrogen sulfide, Phys. Rev. B 57, 5699.
[11] S. Endo, A. Honda, S. Sasaki, H. Shimizu, O. Shimomura and T. Kikegawa, (1996) High-pressure phase of solid hydrogen sulfide, Phys. Rev. B 54, R717.
[12] H. Shimizu, T. Ushida, S. Sasaki, M. Sakashita, H. Yamawaki and K. Aoki, (1997) High-pressure phase transitions of solid H2S probed by Fourier-transform infrared spectroscopy, Phys. Rev. B 55, 5538.
[13] M. Sakashita, H. Yamawaki, H. Fujihisa, K. Aoki, S. Sasaki and H. Shimizu, (1997) Pressure-Induced Molecular Dissociation and Metallization in Hydrogen-Bonded H2S Solid, Phys. Rev. Lett. 79, 1082.
[14] H. Fujihisa, H. Yamawaki, M. Sakashita, A. Nakayama, T. Yamada and K. Aoki, (2004) Molecular dissociation and two low-temperature high-pressure phases of H2S, Phys. Rev. B 69, 214102.
[15] R. Rousseau, M. Boero, M. Bernasconi, M. Parrinello and K. Terakura, (1999) Static Structure and Dynamical Correlations in High Pressure H2S, Phys. Rev. Lett. 83, 2218.
[16] T. Ikeda, (2001) Pressure-induced phase transition of hydrogen sulfide at low temperature: Role of the hydrogen bond and short S-Scontacts, Phys. Rev. B 64, 104103.
[17] M. Sakashita, H. Fujihisa, H. Yamawaki and K. Aoki, (2000) Molecular Dissociation in Deuterium Sulfide under High Pressure: Infrared and Raman Study, J. Phys. Chem. A 104, 8838.
[18] R. Rousseau, M. Boero, M. Bernasconi, M. Parrinello and K. Terakura, (2000) Ab initio Simulation of Phase Transitions and Dissociation of H2S at High Pressure Phys. Rev. Lett. 85, 1254
[19] G. Wolf, J. Baumann, F. Baitalow, and F. P. Hoffmann, (2000) Calorimetric process monitoring of thermal decomposition of B–N–H compounds, Thermochim. Acta 343, 19.
[20] F. Baitalow, J. Baumann, G. Wolf, K. Jaeniche-Rossler, and G. Leitner (2002) Thermal decomposition of B–N–H compounds investigated by using combined thermoanalytical methods. Thermochim. Acta 391, 159.
[21] P. A. Storozhenko, R. A. Svitsyn, V. A. Ketsko, A. K. Buryak, and A. V. Ulyanov (2005) Russ. J. Inorg. Chem. 50, 980.
[22] J. Baumann, F. Baitalow, and G. Wolf (2005) Thermal decomposition of polymeric aminoborane (H2BNH2)x under hydrogen release. Thermochim. Acta 430, 9.
[23] M. G. Hu, R. A. Geanangel, and W. W. Wendlandt (1978) The thermal decomposition of ammonia borane. Thermochim. Acta 23, 249.
[24] H. C. Kelly and V. B. Marriott, (1979) Reexamination of themechanism of acid-catalyzed amine-borane hydrolysis. The hydrolysis of ammonia-borane, Inorg. Chem. 18, 2875.
[25] F. H. Stephens, R. T. Baker, M. H. Matus, D. J. Grant, D. A. Dixon, (2007) Acid Initiation of Ammonia–Borane Dehydrogenation for Hydrogen Storage. Angew. Chem. Int. Ed. 46, 746.
[26] R. J. Keaton, J. M. Blacquiere, and R. T. Baker, (2007) Base Metal Catalyzed Dehydrogenation of Ammonia?Borane for Chemical Hydrogen Storage, J. Am. Chem. Soc. 129, 1844.
[27] F. Y. Cheng, H. Ma, Y. M. Li, and J. Chen, (2007) Ni1-xPtx (x = 0?0.12) Hollow Spheres as Catalysts for Hydrogen Generation from Ammonia Borane, Inorg. Chem. 46, 788.
[28] M. E. Bluhm, M. G. Bradley, R. Butterick, U. Kusari, and L. G. Sneddon, (2006) Amineborane-Based Chemical Hydrogen Storage: Enhanced Ammonia Borane Dehydrogenation in Ionic Liquids, J. Am. Chem. Soc. 128, 7748.
[29] A. Gutowska, L. Li, Y. Shin, C. M. Wang, X. S. Li, J. C. Linehan, R. S. Smith, B. D. Kay, B. Schmid, W. Shaw, M. Gutowski, and T. Autrey, (2005) Nanoscaffold Mediates Hydrogen Release and the Reactivity of Ammonia Borane, Angew. Chem. Int. Ed. 44, 3578.
[30] C. F. Hoon and E. C. Reynhardt, (1983) Molecular dynamics and structures of amine borane of the type R3N.BH3: I. X-ray investigation of HN3.BH3 at 295 K and 110 K, J. Phys. C 16, 6129.
[31] M. E. Bowden, G. J. Gainsford, and W. T. Robinson, (2007) Room-Temperature Structure of Ammonia Borane, Aust. J. Chem. 60, 149.
[32] W. T. Klooster, T. F. Koetzle, Per E. M. Siegbahn, T. B. Richardson, and R. H. Crabtree, (1999) Study of the N?H...H?B Dihydrogen Bond Including the Crystal Structure of BH3NH3 by Neutron Diffraction, J. Am. Chem. Soc. 121, 6337.
[33] D. G. Allis, M. E. Kosmowski, and B. S. Hudson, (2004) The Inelastic Neutron Scattering Spectrum of H3BNH3 and the Reproduction of Its Solid-State Features by Periodic DFT, J. Am. Chem. Soc. 126, 7756
[34] J. B. Yang, J. Lamsal, Q. Cai, W. J. James, and W. B. Yelon, (2008) Structural evolution of ammonia borane for hydrogen storage, Appl. Phys. Lett. 92, 091916.
[35] N. J. Hess, G. K. Schenter, M. R. Hartman, L. L. Daemen, T. Proffen, S. M. Kathmann, C. J. Mundy, M. Hartl, D. J. Heldebrant, A. C. Stowe, and T. Autrey, (2009) Neutron Powder Diffraction and Molecular Simulation Study of the Structural Evolution of Ammonia Borane from 15 to 340 K, J. Phys. Chem. A 113, 5723.
[36] N. J. Hess, M. R. Hartman, C. M. Brown, E. Mamontov, A. Karkamkar, D. J. Heldebrant, L. L. Daemen, T. Autrey, (2008) Quasielastic neutron scattering of–NH3 and–BH3 rotational dynamics in orthorhombic ammonia borane, Chem. Phys. Lett. 459, 85.
[37] S. M. Kathmann, V. Parvanov, G. K. Schenter, A. C. Stowe, L. L. Daemen, M. Hartl, J. Linehan, N. J. Hess, A. Karkamkar, and T. Autrey, (2009) Experimental and computational studies on collective hydrogen dynamics in ammonia borane--Incoherent inelastic neutron scattering, J. Chem. Phys. 130, 024507.
[38] E. C. Reynhardt and C. F. Hoon, (1983) Molecular dynamics and structures of amine boranes of the type R3N.BH3: II. NMR investigation of H3N.BH3, J. Phys. C 16, 6137.
[39] G. H. Penner, Y. C. Phillis Chang, and J. Hutzal, (1999) A Deuterium NMR Spectroscopic Study of Solid BH3NH3, Inorg. Chem. 38, 2868.
[40] O. Gunaydin-Sen, R. Achey, N. S. Dalal, A. Stowe, and T. Autrey, (2007) High Resolution 15N NMR of the 225 K Phase Transition of Ammonia Borane (NH3BH3)--Mixed Order-Disorder and Displacive Behavior, J. Phys. Chem. B 111, 677.
[41] H. Cho, W. J. Shaw, V. Parvanov, G. K. Schenter, A. Karkamkar, N. J. Hess, C. Mundy, S. Kathmann, J. Sears, A. S. Lipton, P. D. Ellis, and S. T. Autrey, (2008) Molecular Structure and Dynamics in the Low Temperature (Orthorhombic) Phase of NH3BH3, J. Phys. Chem. A 112, 4277.
[42] N. J. Hess, M. E. Bowden, V. M. Parvanov, C. Mundy, S. M. Kathmann, G. K. Schenter, and T. Autrey, (2008) Spectroscopic studies of the phase transition in ammonia borane--Raman spectroscopy of singlecrystal NH3BH3 as a function of temperature from 88 to 330 K, J. Chem. Phys. 128, 034508.
[43] A. Paolone, O. Palumbo, P. Rispoli, R. Cantelli, and T. Autrey, (2009) Hydrogen Dynamics and Characterization of the Tetragonal-to-Orthorhombic Phase Transformation in Ammonia Borane, J. Phys. Chem. C 113, 5872.
[44] V. M. Parvanov, G. K. Schenter, N. J. Hess, L. L. Daemen, M. Hartl, A. C. Stowe, D. M. Camaioni, and T. Autrey, Materials for hydrogen storage--structure and dynamics of borane ammonia complex, Dalton Trans. 2008, 4514.
[45] J. Dillen and P. Verhoeven, (2003) The end of a 30-year-old controversy? A computational study of the B-N stretching frequency of BH3-NH3 in the solid state, J. Phys. Chem. A 107, 2570.
[46] S. Trudel and Denis F. R. Gilson, (2003) High-Pressure Raman Spectroscopic Study of the Ammonia?Borane Complex. Evidence for the Dihydrogen Bond, Inorg. Chem. 42, 2814.
[47] R. Custelcean and Z. A. Dreger, (2003) Dihydrogen Bonding under High Pressure--A Raman Study of BH3NH3 Molecular Crystal, J. Phys. Chem. B 107, 9231.
[48] Y. Lin, W. L. Mao, V. Drozd, J. Chen, and L. L. Daemen, (2008) Raman spectroscopy study of ammonia borane at high pressure, J. Chem. Phys. 129, 234509.
[49] S. Xie, Y. Song, and Z. Liu, (2009) In situ high-pressure study of ammonia borane by Raman and IR spectroscopy, Can. J. Chem. 87, 1235.
[50] A. Jayaraman, (1983) Diamond anvil cell and high-pressure physical investigations, Rev. Mod. Phys. 55, 65.
[51] Y. M. Ma, M. Eremets, A. R. Oganov, Y. Xie, I. Trojan, S. Medvedev, A. O. Lyakhov, M. Valle, and V. Prakapenka (2009) Transparent dense sodium, Nature 458, 182.
[1] W. Kohn (1999) Nobel Lecture: Electronic structure of matter—wave functions and density functionals. Rev. Mod. Phys. 71:1253–1266
[2] C. David Sherrill (2005) The Born-Oppenheimer Approximation
[3] D. R. Hartree (1928) Mathematical Proceedings of the Cambridge Philosophical Society. Cambridge University Press Proc. 24:89-110
[4] Parr R G , Yong W. (1989) Density-Functional Theory of Atoms and Molecules. Clarendon Press Oxford
[5] Thomas H. Proc. Camb. Phil. Soc. (1927) 23:542
[6] Fermi E. Accad. Naz. Lincei (1927) 6:602
[7] P. Hohenberg, W. Kohn (1964) Inhomogeneous Electron Gas. Phys. Rev. 136:B864–B871
[8] W. Kohn, L. J. Sham (1965) Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 140:A1133–A113
[9] J. P. Perdew, Alex Zunger (1981) Self-interaction correction to density-functional approximations for many-electron systems. Phys. Rev. B 23:5048–5079
[10] John P. Perdew, Wang Yue (1986) Accurate and simple density functional for the electronic exchange energy: Generalized gradient approximation. Phys. Rev. B 33:8800–8802
[11] John P. Perdew, J. A. Chevary, S. H. Vosko, Koblar A. Jackson, MarkR. Pederson, D. J. Singh (1992) Atoms, molecules, solids, and surfaces: Applications of the generalized gradient approximation for exchange and correlation. Phys. Rev. B 46:6671–6687
[12] J. P. Perdew, K. Burke, and M. Ernzerhof (1996) Generalized Gradient Approximation Made Simple, Phys. Rev. Lett. 77, 3865–3868, Phys. Rev. Lett. 78, 1396–1396
[13] N. Troullier and J. L. Martins (1991) Efficient pseudopotentials for plane-wave calculations, Phys. Rev. B 43, 1993–2006
[14] D. Vanderbilt (1990) Soft self consistent pseudopotentials in a generalized eigenvalue formalism, Phys. Rev. B 41, 7892
[14] W. C. Swope, H. C. Andersen, P. H. Berens, and K. R. Wilson, (1982) J. Chem. Phys. 76, 637.
[15] H. C. Andersen, (1983) J. Comput. Phys. 52, 24
[16] H. J. C. Berendsen, J. P. M. Postma, W. F. van Gunsteren, A. DiNola, and J. R. Haak (1984) Molecular dynamics with coupling to an external bath, J. Chem. Phys., 81, 3684-3690
[17] Nosé, S. (1984) A molecular dynamics method for simulations in the canonical ensemble, Mol. Phys., 52, 255-268.
[18] Nosé, S. (1984) A unified formulation of the constant temperature molecular dynamics methods, J. Chem. Phys., 81, 511-519.
[19] Nosé, S. (1991) Constant temperature molecular dynamics methods, Prog. Theor. Phys., Suppl., 103, 1-46.
[20] Hoover, W. (1985) Canonical dynamics: Equilibrium phase-space distributions, Phys. Rev. A, 31, 1695-1697
[21] R. Car and M. Parrinello (1985) Unified Approach for Molecular Dynamics and Density-Functional Theory. Phys. Rev. Lett. 55, 2471–2474
[22] http://www.cpmd.org/
[1] J. K. Cockkroftand and A. N. Fitch (1990) Z. Kristallogr. 193, 1
[2] H. Fujihisa, H. Yamawaki, M. Sakashita, A. Nakayama, T. Yamada and K. Aoki, (2004) Molecular dissociation and two low-temperature high-pressure phases of H2S, Phys. Rev. B 69, 214102.
[3] H. Fujihisa, H. Yamawaki, M. Sakashita, K. Aoki, S. Sasaki and H. Shimizu, (1998) Structures of H2S: Phases I’and IV under high pressure, Phys. Rev. B 57, 2651.
[4] T. Ikeda, (2001) Pressure-induced phase transition of hydrogen sulfide at low temperature: Role of the hydrogen bond and short S-S contacts, Phys. Rev. B 64, 104103.
[5] R. Rousseau, M. Boero, M. Bernasconi, M. Parrinello and K. Terakura, (1999) Static Structure and Dynamical Correlations in High Pressure H2S, Phys. Rev. Lett. 83, 2218.
[6] M. Sakashita, H. Fujihisa, H. Yamawaki and K. Aoki, (2000) Molecular Dissociation in Deuterium Sulfide under High Pressure: Infrared and Raman Study, J. Phys. Chem. A 104, 8838.
[7] H. Fujihisa, H. Yamawaki, M. Sakashita, A. Nakayama, T. Yamada and K. Aoki, (2004) Molecular dissociation and two low-temperature high-pressure phases of H2S, Phys. Rev. B 69, 214102.
[8] M. L. Huggins, (1953) Atomic Radii. IV. Dependence of InteratomicDistance on Bond Energy, J. Am. Chem. Soc. 75, 4126.
[9] R. Rousseau, M. Boero, M. Bernasconi, M. Parrinello and K. Terakura, (2000) Ab initio Simulation of Phase Transitions and Dissociation of H2S at High Pressure, Phys. Rev. Lett. 85, 1254.
[10] T. Cui, E. Cheng, B. J. Alder, and K. B. Whaley, Rotational ordering in solid deuterium and hydrogen: A path integral Monte Carlo study, (1997) Phys. Rev. B 55, 12253.
[11] S. Endo, A. Honda, S. Sasaki, H. Shimizu, O. Shimomura and T. Kikegawa, (1996) High-pressure phase of solid hydrogen sulfide. Phys. Rev. B 54, R717.
[12] T. Kume, Y. Fukaya, S. Sasaki, and H. Shimizu, (2002) High pressure ultraviolet-visible-near study of colored solid hydrogen sulfide, Rev. Sci. Instrum. 73, 2355.
[1] C. F. Hoon and E. C. Reynhardt, (1983) Molecular dynamics and structures of amine borane of the type R3N.BH3: I. X-ray investigation of HN3.BH3 at 295 K and 110 K, J. Phys. C 16, 6129.
[2] M. E. Bowden, G. J. Gainsford, and W. T. Robinson, (2007) Room-Temperature Structure of Ammonia Borane, Aust. J. Chem. 60, 149.
[3] W. T. Klooster, T. F. Koetzle, Per E. M. Siegbahn, T. B. Richardson, and R. H. Crabtree, (1999) Study of the N?H...H?B Dihydrogen Bond Including the Crystal Structure of BH3NH3 by Neutron Diffraction, J. Am. Chem. Soc. 121, 6337.
[4] D. G. Allis, M. E. Kosmowski, and B. S. Hudson, (2004) The Inelastic Neutron Scattering Spectrum of H3BNH3 and the Reproduction of Its Solid-State Features by Periodic DFT, J. Am. Chem. Soc. 126, 7756.
[5] J. B. Yang, J. Lamsal, Q. Cai, W. J. James, and W. B. Yelon, (2008) Structural evolution of ammonia borane for hydrogen storage, Appl. Phys. Lett. 92, 091916.
[6] N. J. Hess, G. K. Schenter, M. R. Hartman, L. L. Daemen, T. Proffen, S. M. Kathmann, C. J. Mundy, M. Hartl, D. J. Heldebrant, A. C. Stowe, and T. Autrey, (2009) Neutron Powder Diffraction and Molecular Simulation Study of the Structural Evolution of Ammonia Borane from15 to 340 K, J. Phys. Chem. A 113, 5723.
[7] N. J. Hess, M. R. Hartman, C. M. Brown, E. Mamontov, A. Karkamkar, D. J. Heldebrant, L. L. Daemen, T. Autrey, (2008) Quasielastic neutron scattering of–NH3 and–BH3 rotational dynamics in orthorhombic ammonia borane, Chem. Phys. Lett. 459, 85.
[8] S. M. Kathmann, V. Parvanov, G. K. Schenter, A. C. Stowe, L. L. Daemen, M. Hartl, J. Linehan, N. J. Hess, A. Karkamkar, and T. Autrey, (2009) Experimental and computational studies on collective hydrogen dynamics in ammonia borane--Incoherent inelastic neutron scattering, J. Chem. Phys. 130, 024507.
[9] E. C. Reynhardt and C. F. Hoon, (1983) Molecular dynamics and structures of amine boranes of the type R3N.BH3: II. NMR investigation of H3N.BH3, J. Phys. C 16, 6137.
[10] G. H. Penner, Y. C. Phillis Chang, and J. Hutzal, (1999) A Deuterium NMR Spectroscopic Study of Solid BH3NH3, Inorg. Chem. 38, 2868.
[11] O. Gunaydin-Sen, R. Achey, N. S. Dalal, A. Stowe, and T. Autrey, (2007) High Resolution 15N NMR of the 225 K Phase Transition of Ammonia Borane (NH3BH3)--Mixed Order-Disorder and Displacive Behavior, J. Phys. Chem. B 111, 677.
[12] H. Cho, W. J. Shaw, V. Parvanov, G. K. Schenter, A. Karkamkar, N. J. Hess, C. Mundy, S. Kathmann, J. Sears, A. S. Lipton, P. D. Ellis, and S. T. Autrey, (2008) Molecular Structure and Dynamics in the LowTemperature (Orthorhombic) Phase of NH3BH3, J. Phys. Chem. A 112, 4277.
[13] N. J. Hess, M. E. Bowden, V. M. Parvanov, C. Mundy, S. M. Kathmann, G. K. Schenter, and T. Autrey, (2008) Spectroscopic studies of the phase transition in ammonia borane--Raman spectroscopy of single crystal NH3BH3 as a function of temperature from 88 to 330 K, J. Chem. Phys. 128, 034508.
[14] A. Paolone, O. Palumbo, P. Rispoli, R. Cantelli, and T. Autrey, (2009) Hydrogen Dynamics and Characterization of the Tetragonal-to-Orthorhombic Phase Transformation in Ammonia Borane, J. Phys. Chem. C 113, 5872.
[15] L. R. Thorne, R. D. Suenram, and F. J. Lovas, (1983) Microwave spectrum, torsional barrier, and structure of BH3NH3. J. Chem. Phys. 78, 167.
[16] J. Dillen and P. Verhoeven, (2003) The end of a 30-year-old controversy? A computational study of the B-N stretching frequency of BH3-NH3 in the solid state, J. Phys. Chem. A 107, 2570.
[17] R. Car and M. Parrinello, (1985) Unified Approach for Molecular Dynamics and Density-Functional Theory, Phys. Rev. Lett. 55, 2471.
[18] CPMD, http://www.cpmd.org/, Copyright IBM Corp 1990-2008, Copyright MPI für Festk?rperforschung Stuttgart 1997-2001.
[19] A. D. Becke, (1988) Density-functional exchange energyapproximation with correct asymptotic behavior, Phys. Rev. A 38, 3098.
[20] C. Lee, W. Yang, and R. G. Parr, (1988) Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density, Phys. Rev. B 37,785.
[21] N. Troullier and J. L. Martins, (1991) Efficient pseudopotentials for plane-wave calculations, Phys. Rev. B 43, 1993.
[22] M. Parrinello and A. Rahman, (1980) Crystal Structure and Pair Potentials: A Molecular-Dynamics Study, Phys. Rev. Lett. 45, 1196.
[23] P. Focher, G. L. Chiarotti, M. Bernasconi, E. Tosatti, and M. Parrinello, (1994) Structural Phase Transformations via First-Principles Simulation, Europhys. Lett. 26, 345.
[24] S. Nosé, (1984) A unified formulation of the constant temperature molecular dynamics methods, J. Chem. Phys. 81, 511; W. G. Hoover, (1985) Canonical dynamics: Equilibrium phase-space distributions, Phys. Rev. A 31, 1695.
[25] M. D. Segall, P. L. D. Lindan, M. J. Probert, C. J. Pickard, P. J. Hasnip, S. J. Clark, and M. C. Payne, (2002) First-principles simulation: ideas, illustrations and the CASTEP code, J. Phys.: Condens. Matter 14, 2717.
[26] J. P. Perdew, K. Burke, and M. Ernzerhof (1996) Generalized Gradient Approximation Made Simple, Phys. Rev. Lett. 77, 3865.
[27] J. P. Perdew, K. Burke, and M. Ernzerhof (1997) Phys. Rev. Lett. 78,1396.
[28] Y. Lin, W. L. Mao, V. Drozd, J. Chen, and L. L. Daemen, (2008) Raman spectroscopy study of ammonia borane at high pressure, J. Chem. Phys. 129, 234509.
[29] Y. Filinchuk, A. H. Nevidomskyy, D. Chernyshov, and V. Dmitriev (2009) High-pressure phase and transition phenomena in ammonia borane NH3BH3 from X-ray diffraction, Landau theory, and ab initio calculations, Phys. Rev. B, 79, 214111.
[30] T. Cui, E. Cheng, B. J. Alder, and K. B. Whaley, (1997) Rotational ordering in solid deuterium and hydrogen: A path integral Monte Carlo study, Phys. Rev. B 55, 12253.
[31] V. M. Parvanov, G. K. Schenter, N. J. Hess, L. L. Daemen, M. Hartl, A. C. Stowe, (2008) Materials for hydrogen storage: structure and dynamics of borane ammonia cpmplex. D. M. Camaioni, and T. Autrey, Dalton Trans. 2008, 4514.
[32] M. Benoit, D. Marx, and M. Parrinello, (1998) Tunnelling and zero-point notion in high-preesure ice, Nature 392, 258.
[33] M. Bernasconi, P. L. Silvestrelli, and M. Parrinello, (1998) Ab initio infrared absorption study of the hydrogen-bond symmetrization in ice, Phys. Rev. Lett. 81, 1235.
[2] R. J. Hemley, A. P. Jephcoat, H. K. Mao, C. S. Zha, L. W. Finger, and D. E. Cox (1987) Static compression of H2O-ice to 128 GPa. Nature 330, 737– 740
[3] M. Benoit, D. Marx, and M. Parrinello (1998) Tunnelling and zero-point motion in high-pressure ice. Nature 392, 258-261
[4] M. Bernasconi, P. L. Silvestrelli, and M. Parrinello (1998) Ab Initio Infrared Absorption Study of the Hydrogen-Bond Symmetrization in Ice. Phys. Rev. Lett. 81, 1235-1238.
[5] J. K. Cockkroftand and A. N. Fitch (1990) Z. Kristallogr. 193, 1
[6] S. Endo, N. Ichimiya, K. Koto, S. Sasaki and H. Shimizu, (1994) X-ray-diffraction study of solid hydrogen sulfide under high pressure, Phys. Rev. B 50, 5865.
[7] H. Fujihisa, H. Yamawaki, M. Sakashita, K. Aoki, S. Sasaki and H. Shimizu, (1998) Structures of H2S: Phases I’and IV under high pressure, Phys. Rev. B 57, 2651.
[8] J. S. Loveday, R. J. Nelmes, S. Klotz, J. M. Besson and G. Hamel, (2000) Pressure-Induced Hydrogen Bonding: Structure of D2S Phase I’, Phys .Rev. Lett. 85, 1024.
[9] H. Shimizu, Y. Nakamichi, S. Sasaki, (1991) Pressure‐induced phase transition in solid hydrogen sulfide at 11 GPa, J. Chem. Phys. 95, 2036;(1991) High‐pressure Raman study of solid deuterium sulfide up to 17 GPa, J. Chem. Phys. 97, 7137.
[10] S. Endo, A. Honda, K. Koto, O. Shimomura, T. Kikegawa and N. Hamaya, (1998) Crystal structure of high-pressure phase-IV solid hydrogen sulfide, Phys. Rev. B 57, 5699.
[11] S. Endo, A. Honda, S. Sasaki, H. Shimizu, O. Shimomura and T. Kikegawa, (1996) High-pressure phase of solid hydrogen sulfide, Phys. Rev. B 54, R717.
[12] H. Shimizu, T. Ushida, S. Sasaki, M. Sakashita, H. Yamawaki and K. Aoki, (1997) High-pressure phase transitions of solid H2S probed by Fourier-transform infrared spectroscopy, Phys. Rev. B 55, 5538.
[13] M. Sakashita, H. Yamawaki, H. Fujihisa, K. Aoki, S. Sasaki and H. Shimizu, (1997) Pressure-Induced Molecular Dissociation and Metallization in Hydrogen-Bonded H2S Solid, Phys. Rev. Lett. 79, 1082.
[14] H. Fujihisa, H. Yamawaki, M. Sakashita, A. Nakayama, T. Yamada and K. Aoki, (2004) Molecular dissociation and two low-temperature high-pressure phases of H2S, Phys. Rev. B 69, 214102.
[15] R. Rousseau, M. Boero, M. Bernasconi, M. Parrinello and K. Terakura, (1999) Static Structure and Dynamical Correlations in High Pressure H2S, Phys. Rev. Lett. 83, 2218.
[16] T. Ikeda, (2001) Pressure-induced phase transition of hydrogen sulfide at low temperature: Role of the hydrogen bond and short S-Scontacts, Phys. Rev. B 64, 104103.
[17] M. Sakashita, H. Fujihisa, H. Yamawaki and K. Aoki, (2000) Molecular Dissociation in Deuterium Sulfide under High Pressure: Infrared and Raman Study, J. Phys. Chem. A 104, 8838.
[18] R. Rousseau, M. Boero, M. Bernasconi, M. Parrinello and K. Terakura, (2000) Ab initio Simulation of Phase Transitions and Dissociation of H2S at High Pressure Phys. Rev. Lett. 85, 1254
[19] G. Wolf, J. Baumann, F. Baitalow, and F. P. Hoffmann, (2000) Calorimetric process monitoring of thermal decomposition of B–N–H compounds, Thermochim. Acta 343, 19.
[20] F. Baitalow, J. Baumann, G. Wolf, K. Jaeniche-Rossler, and G. Leitner (2002) Thermal decomposition of B–N–H compounds investigated by using combined thermoanalytical methods. Thermochim. Acta 391, 159.
[21] P. A. Storozhenko, R. A. Svitsyn, V. A. Ketsko, A. K. Buryak, and A. V. Ulyanov (2005) Russ. J. Inorg. Chem. 50, 980.
[22] J. Baumann, F. Baitalow, and G. Wolf (2005) Thermal decomposition of polymeric aminoborane (H2BNH2)x under hydrogen release. Thermochim. Acta 430, 9.
[23] M. G. Hu, R. A. Geanangel, and W. W. Wendlandt (1978) The thermal decomposition of ammonia borane. Thermochim. Acta 23, 249.
[24] H. C. Kelly and V. B. Marriott, (1979) Reexamination of themechanism of acid-catalyzed amine-borane hydrolysis. The hydrolysis of ammonia-borane, Inorg. Chem. 18, 2875.
[25] F. H. Stephens, R. T. Baker, M. H. Matus, D. J. Grant, D. A. Dixon, (2007) Acid Initiation of Ammonia–Borane Dehydrogenation for Hydrogen Storage. Angew. Chem. Int. Ed. 46, 746.
[26] R. J. Keaton, J. M. Blacquiere, and R. T. Baker, (2007) Base Metal Catalyzed Dehydrogenation of Ammonia?Borane for Chemical Hydrogen Storage, J. Am. Chem. Soc. 129, 1844.
[27] F. Y. Cheng, H. Ma, Y. M. Li, and J. Chen, (2007) Ni1-xPtx (x = 0?0.12) Hollow Spheres as Catalysts for Hydrogen Generation from Ammonia Borane, Inorg. Chem. 46, 788.
[28] M. E. Bluhm, M. G. Bradley, R. Butterick, U. Kusari, and L. G. Sneddon, (2006) Amineborane-Based Chemical Hydrogen Storage: Enhanced Ammonia Borane Dehydrogenation in Ionic Liquids, J. Am. Chem. Soc. 128, 7748.
[29] A. Gutowska, L. Li, Y. Shin, C. M. Wang, X. S. Li, J. C. Linehan, R. S. Smith, B. D. Kay, B. Schmid, W. Shaw, M. Gutowski, and T. Autrey, (2005) Nanoscaffold Mediates Hydrogen Release and the Reactivity of Ammonia Borane, Angew. Chem. Int. Ed. 44, 3578.
[30] C. F. Hoon and E. C. Reynhardt, (1983) Molecular dynamics and structures of amine borane of the type R3N.BH3: I. X-ray investigation of HN3.BH3 at 295 K and 110 K, J. Phys. C 16, 6129.
[31] M. E. Bowden, G. J. Gainsford, and W. T. Robinson, (2007) Room-Temperature Structure of Ammonia Borane, Aust. J. Chem. 60, 149.
[32] W. T. Klooster, T. F. Koetzle, Per E. M. Siegbahn, T. B. Richardson, and R. H. Crabtree, (1999) Study of the N?H...H?B Dihydrogen Bond Including the Crystal Structure of BH3NH3 by Neutron Diffraction, J. Am. Chem. Soc. 121, 6337.
[33] D. G. Allis, M. E. Kosmowski, and B. S. Hudson, (2004) The Inelastic Neutron Scattering Spectrum of H3BNH3 and the Reproduction of Its Solid-State Features by Periodic DFT, J. Am. Chem. Soc. 126, 7756
[34] J. B. Yang, J. Lamsal, Q. Cai, W. J. James, and W. B. Yelon, (2008) Structural evolution of ammonia borane for hydrogen storage, Appl. Phys. Lett. 92, 091916.
[35] N. J. Hess, G. K. Schenter, M. R. Hartman, L. L. Daemen, T. Proffen, S. M. Kathmann, C. J. Mundy, M. Hartl, D. J. Heldebrant, A. C. Stowe, and T. Autrey, (2009) Neutron Powder Diffraction and Molecular Simulation Study of the Structural Evolution of Ammonia Borane from 15 to 340 K, J. Phys. Chem. A 113, 5723.
[36] N. J. Hess, M. R. Hartman, C. M. Brown, E. Mamontov, A. Karkamkar, D. J. Heldebrant, L. L. Daemen, T. Autrey, (2008) Quasielastic neutron scattering of–NH3 and–BH3 rotational dynamics in orthorhombic ammonia borane, Chem. Phys. Lett. 459, 85.
[37] S. M. Kathmann, V. Parvanov, G. K. Schenter, A. C. Stowe, L. L. Daemen, M. Hartl, J. Linehan, N. J. Hess, A. Karkamkar, and T. Autrey, (2009) Experimental and computational studies on collective hydrogen dynamics in ammonia borane--Incoherent inelastic neutron scattering, J. Chem. Phys. 130, 024507.
[38] E. C. Reynhardt and C. F. Hoon, (1983) Molecular dynamics and structures of amine boranes of the type R3N.BH3: II. NMR investigation of H3N.BH3, J. Phys. C 16, 6137.
[39] G. H. Penner, Y. C. Phillis Chang, and J. Hutzal, (1999) A Deuterium NMR Spectroscopic Study of Solid BH3NH3, Inorg. Chem. 38, 2868.
[40] O. Gunaydin-Sen, R. Achey, N. S. Dalal, A. Stowe, and T. Autrey, (2007) High Resolution 15N NMR of the 225 K Phase Transition of Ammonia Borane (NH3BH3)--Mixed Order-Disorder and Displacive Behavior, J. Phys. Chem. B 111, 677.
[41] H. Cho, W. J. Shaw, V. Parvanov, G. K. Schenter, A. Karkamkar, N. J. Hess, C. Mundy, S. Kathmann, J. Sears, A. S. Lipton, P. D. Ellis, and S. T. Autrey, (2008) Molecular Structure and Dynamics in the Low Temperature (Orthorhombic) Phase of NH3BH3, J. Phys. Chem. A 112, 4277.
[42] N. J. Hess, M. E. Bowden, V. M. Parvanov, C. Mundy, S. M. Kathmann, G. K. Schenter, and T. Autrey, (2008) Spectroscopic studies of the phase transition in ammonia borane--Raman spectroscopy of singlecrystal NH3BH3 as a function of temperature from 88 to 330 K, J. Chem. Phys. 128, 034508.
[43] A. Paolone, O. Palumbo, P. Rispoli, R. Cantelli, and T. Autrey, (2009) Hydrogen Dynamics and Characterization of the Tetragonal-to-Orthorhombic Phase Transformation in Ammonia Borane, J. Phys. Chem. C 113, 5872.
[44] V. M. Parvanov, G. K. Schenter, N. J. Hess, L. L. Daemen, M. Hartl, A. C. Stowe, D. M. Camaioni, and T. Autrey, Materials for hydrogen storage--structure and dynamics of borane ammonia complex, Dalton Trans. 2008, 4514.
[45] J. Dillen and P. Verhoeven, (2003) The end of a 30-year-old controversy? A computational study of the B-N stretching frequency of BH3-NH3 in the solid state, J. Phys. Chem. A 107, 2570.
[46] S. Trudel and Denis F. R. Gilson, (2003) High-Pressure Raman Spectroscopic Study of the Ammonia?Borane Complex. Evidence for the Dihydrogen Bond, Inorg. Chem. 42, 2814.
[47] R. Custelcean and Z. A. Dreger, (2003) Dihydrogen Bonding under High Pressure--A Raman Study of BH3NH3 Molecular Crystal, J. Phys. Chem. B 107, 9231.
[48] Y. Lin, W. L. Mao, V. Drozd, J. Chen, and L. L. Daemen, (2008) Raman spectroscopy study of ammonia borane at high pressure, J. Chem. Phys. 129, 234509.
[49] S. Xie, Y. Song, and Z. Liu, (2009) In situ high-pressure study of ammonia borane by Raman and IR spectroscopy, Can. J. Chem. 87, 1235.
[50] A. Jayaraman, (1983) Diamond anvil cell and high-pressure physical investigations, Rev. Mod. Phys. 55, 65.
[51] Y. M. Ma, M. Eremets, A. R. Oganov, Y. Xie, I. Trojan, S. Medvedev, A. O. Lyakhov, M. Valle, and V. Prakapenka (2009) Transparent dense sodium, Nature 458, 182.
[1] W. Kohn (1999) Nobel Lecture: Electronic structure of matter—wave functions and density functionals. Rev. Mod. Phys. 71:1253–1266
[2] C. David Sherrill (2005) The Born-Oppenheimer Approximation
[3] D. R. Hartree (1928) Mathematical Proceedings of the Cambridge Philosophical Society. Cambridge University Press Proc. 24:89-110
[4] Parr R G , Yong W. (1989) Density-Functional Theory of Atoms and Molecules. Clarendon Press Oxford
[5] Thomas H. Proc. Camb. Phil. Soc. (1927) 23:542
[6] Fermi E. Accad. Naz. Lincei (1927) 6:602
[7] P. Hohenberg, W. Kohn (1964) Inhomogeneous Electron Gas. Phys. Rev. 136:B864–B871
[8] W. Kohn, L. J. Sham (1965) Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 140:A1133–A113
[9] J. P. Perdew, Alex Zunger (1981) Self-interaction correction to density-functional approximations for many-electron systems. Phys. Rev. B 23:5048–5079
[10] John P. Perdew, Wang Yue (1986) Accurate and simple density functional for the electronic exchange energy: Generalized gradient approximation. Phys. Rev. B 33:8800–8802
[11] John P. Perdew, J. A. Chevary, S. H. Vosko, Koblar A. Jackson, MarkR. Pederson, D. J. Singh (1992) Atoms, molecules, solids, and surfaces: Applications of the generalized gradient approximation for exchange and correlation. Phys. Rev. B 46:6671–6687
[12] J. P. Perdew, K. Burke, and M. Ernzerhof (1996) Generalized Gradient Approximation Made Simple, Phys. Rev. Lett. 77, 3865–3868, Phys. Rev. Lett. 78, 1396–1396
[13] N. Troullier and J. L. Martins (1991) Efficient pseudopotentials for plane-wave calculations, Phys. Rev. B 43, 1993–2006
[14] D. Vanderbilt (1990) Soft self consistent pseudopotentials in a generalized eigenvalue formalism, Phys. Rev. B 41, 7892
[14] W. C. Swope, H. C. Andersen, P. H. Berens, and K. R. Wilson, (1982) J. Chem. Phys. 76, 637.
[15] H. C. Andersen, (1983) J. Comput. Phys. 52, 24
[16] H. J. C. Berendsen, J. P. M. Postma, W. F. van Gunsteren, A. DiNola, and J. R. Haak (1984) Molecular dynamics with coupling to an external bath, J. Chem. Phys., 81, 3684-3690
[17] Nosé, S. (1984) A molecular dynamics method for simulations in the canonical ensemble, Mol. Phys., 52, 255-268.
[18] Nosé, S. (1984) A unified formulation of the constant temperature molecular dynamics methods, J. Chem. Phys., 81, 511-519.
[19] Nosé, S. (1991) Constant temperature molecular dynamics methods, Prog. Theor. Phys., Suppl., 103, 1-46.
[20] Hoover, W. (1985) Canonical dynamics: Equilibrium phase-space distributions, Phys. Rev. A, 31, 1695-1697
[21] R. Car and M. Parrinello (1985) Unified Approach for Molecular Dynamics and Density-Functional Theory. Phys. Rev. Lett. 55, 2471–2474
[22] http://www.cpmd.org/
[1] J. K. Cockkroftand and A. N. Fitch (1990) Z. Kristallogr. 193, 1
[2] H. Fujihisa, H. Yamawaki, M. Sakashita, A. Nakayama, T. Yamada and K. Aoki, (2004) Molecular dissociation and two low-temperature high-pressure phases of H2S, Phys. Rev. B 69, 214102.
[3] H. Fujihisa, H. Yamawaki, M. Sakashita, K. Aoki, S. Sasaki and H. Shimizu, (1998) Structures of H2S: Phases I’and IV under high pressure, Phys. Rev. B 57, 2651.
[4] T. Ikeda, (2001) Pressure-induced phase transition of hydrogen sulfide at low temperature: Role of the hydrogen bond and short S-S contacts, Phys. Rev. B 64, 104103.
[5] R. Rousseau, M. Boero, M. Bernasconi, M. Parrinello and K. Terakura, (1999) Static Structure and Dynamical Correlations in High Pressure H2S, Phys. Rev. Lett. 83, 2218.
[6] M. Sakashita, H. Fujihisa, H. Yamawaki and K. Aoki, (2000) Molecular Dissociation in Deuterium Sulfide under High Pressure: Infrared and Raman Study, J. Phys. Chem. A 104, 8838.
[7] H. Fujihisa, H. Yamawaki, M. Sakashita, A. Nakayama, T. Yamada and K. Aoki, (2004) Molecular dissociation and two low-temperature high-pressure phases of H2S, Phys. Rev. B 69, 214102.
[8] M. L. Huggins, (1953) Atomic Radii. IV. Dependence of InteratomicDistance on Bond Energy, J. Am. Chem. Soc. 75, 4126.
[9] R. Rousseau, M. Boero, M. Bernasconi, M. Parrinello and K. Terakura, (2000) Ab initio Simulation of Phase Transitions and Dissociation of H2S at High Pressure, Phys. Rev. Lett. 85, 1254.
[10] T. Cui, E. Cheng, B. J. Alder, and K. B. Whaley, Rotational ordering in solid deuterium and hydrogen: A path integral Monte Carlo study, (1997) Phys. Rev. B 55, 12253.
[11] S. Endo, A. Honda, S. Sasaki, H. Shimizu, O. Shimomura and T. Kikegawa, (1996) High-pressure phase of solid hydrogen sulfide. Phys. Rev. B 54, R717.
[12] T. Kume, Y. Fukaya, S. Sasaki, and H. Shimizu, (2002) High pressure ultraviolet-visible-near study of colored solid hydrogen sulfide, Rev. Sci. Instrum. 73, 2355.
[1] C. F. Hoon and E. C. Reynhardt, (1983) Molecular dynamics and structures of amine borane of the type R3N.BH3: I. X-ray investigation of HN3.BH3 at 295 K and 110 K, J. Phys. C 16, 6129.
[2] M. E. Bowden, G. J. Gainsford, and W. T. Robinson, (2007) Room-Temperature Structure of Ammonia Borane, Aust. J. Chem. 60, 149.
[3] W. T. Klooster, T. F. Koetzle, Per E. M. Siegbahn, T. B. Richardson, and R. H. Crabtree, (1999) Study of the N?H...H?B Dihydrogen Bond Including the Crystal Structure of BH3NH3 by Neutron Diffraction, J. Am. Chem. Soc. 121, 6337.
[4] D. G. Allis, M. E. Kosmowski, and B. S. Hudson, (2004) The Inelastic Neutron Scattering Spectrum of H3BNH3 and the Reproduction of Its Solid-State Features by Periodic DFT, J. Am. Chem. Soc. 126, 7756.
[5] J. B. Yang, J. Lamsal, Q. Cai, W. J. James, and W. B. Yelon, (2008) Structural evolution of ammonia borane for hydrogen storage, Appl. Phys. Lett. 92, 091916.
[6] N. J. Hess, G. K. Schenter, M. R. Hartman, L. L. Daemen, T. Proffen, S. M. Kathmann, C. J. Mundy, M. Hartl, D. J. Heldebrant, A. C. Stowe, and T. Autrey, (2009) Neutron Powder Diffraction and Molecular Simulation Study of the Structural Evolution of Ammonia Borane from15 to 340 K, J. Phys. Chem. A 113, 5723.
[7] N. J. Hess, M. R. Hartman, C. M. Brown, E. Mamontov, A. Karkamkar, D. J. Heldebrant, L. L. Daemen, T. Autrey, (2008) Quasielastic neutron scattering of–NH3 and–BH3 rotational dynamics in orthorhombic ammonia borane, Chem. Phys. Lett. 459, 85.
[8] S. M. Kathmann, V. Parvanov, G. K. Schenter, A. C. Stowe, L. L. Daemen, M. Hartl, J. Linehan, N. J. Hess, A. Karkamkar, and T. Autrey, (2009) Experimental and computational studies on collective hydrogen dynamics in ammonia borane--Incoherent inelastic neutron scattering, J. Chem. Phys. 130, 024507.
[9] E. C. Reynhardt and C. F. Hoon, (1983) Molecular dynamics and structures of amine boranes of the type R3N.BH3: II. NMR investigation of H3N.BH3, J. Phys. C 16, 6137.
[10] G. H. Penner, Y. C. Phillis Chang, and J. Hutzal, (1999) A Deuterium NMR Spectroscopic Study of Solid BH3NH3, Inorg. Chem. 38, 2868.
[11] O. Gunaydin-Sen, R. Achey, N. S. Dalal, A. Stowe, and T. Autrey, (2007) High Resolution 15N NMR of the 225 K Phase Transition of Ammonia Borane (NH3BH3)--Mixed Order-Disorder and Displacive Behavior, J. Phys. Chem. B 111, 677.
[12] H. Cho, W. J. Shaw, V. Parvanov, G. K. Schenter, A. Karkamkar, N. J. Hess, C. Mundy, S. Kathmann, J. Sears, A. S. Lipton, P. D. Ellis, and S. T. Autrey, (2008) Molecular Structure and Dynamics in the LowTemperature (Orthorhombic) Phase of NH3BH3, J. Phys. Chem. A 112, 4277.
[13] N. J. Hess, M. E. Bowden, V. M. Parvanov, C. Mundy, S. M. Kathmann, G. K. Schenter, and T. Autrey, (2008) Spectroscopic studies of the phase transition in ammonia borane--Raman spectroscopy of single crystal NH3BH3 as a function of temperature from 88 to 330 K, J. Chem. Phys. 128, 034508.
[14] A. Paolone, O. Palumbo, P. Rispoli, R. Cantelli, and T. Autrey, (2009) Hydrogen Dynamics and Characterization of the Tetragonal-to-Orthorhombic Phase Transformation in Ammonia Borane, J. Phys. Chem. C 113, 5872.
[15] L. R. Thorne, R. D. Suenram, and F. J. Lovas, (1983) Microwave spectrum, torsional barrier, and structure of BH3NH3. J. Chem. Phys. 78, 167.
[16] J. Dillen and P. Verhoeven, (2003) The end of a 30-year-old controversy? A computational study of the B-N stretching frequency of BH3-NH3 in the solid state, J. Phys. Chem. A 107, 2570.
[17] R. Car and M. Parrinello, (1985) Unified Approach for Molecular Dynamics and Density-Functional Theory, Phys. Rev. Lett. 55, 2471.
[18] CPMD, http://www.cpmd.org/, Copyright IBM Corp 1990-2008, Copyright MPI für Festk?rperforschung Stuttgart 1997-2001.
[19] A. D. Becke, (1988) Density-functional exchange energyapproximation with correct asymptotic behavior, Phys. Rev. A 38, 3098.
[20] C. Lee, W. Yang, and R. G. Parr, (1988) Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density, Phys. Rev. B 37,785.
[21] N. Troullier and J. L. Martins, (1991) Efficient pseudopotentials for plane-wave calculations, Phys. Rev. B 43, 1993.
[22] M. Parrinello and A. Rahman, (1980) Crystal Structure and Pair Potentials: A Molecular-Dynamics Study, Phys. Rev. Lett. 45, 1196.
[23] P. Focher, G. L. Chiarotti, M. Bernasconi, E. Tosatti, and M. Parrinello, (1994) Structural Phase Transformations via First-Principles Simulation, Europhys. Lett. 26, 345.
[24] S. Nosé, (1984) A unified formulation of the constant temperature molecular dynamics methods, J. Chem. Phys. 81, 511; W. G. Hoover, (1985) Canonical dynamics: Equilibrium phase-space distributions, Phys. Rev. A 31, 1695.
[25] M. D. Segall, P. L. D. Lindan, M. J. Probert, C. J. Pickard, P. J. Hasnip, S. J. Clark, and M. C. Payne, (2002) First-principles simulation: ideas, illustrations and the CASTEP code, J. Phys.: Condens. Matter 14, 2717.
[26] J. P. Perdew, K. Burke, and M. Ernzerhof (1996) Generalized Gradient Approximation Made Simple, Phys. Rev. Lett. 77, 3865.
[27] J. P. Perdew, K. Burke, and M. Ernzerhof (1997) Phys. Rev. Lett. 78,1396.
[28] Y. Lin, W. L. Mao, V. Drozd, J. Chen, and L. L. Daemen, (2008) Raman spectroscopy study of ammonia borane at high pressure, J. Chem. Phys. 129, 234509.
[29] Y. Filinchuk, A. H. Nevidomskyy, D. Chernyshov, and V. Dmitriev (2009) High-pressure phase and transition phenomena in ammonia borane NH3BH3 from X-ray diffraction, Landau theory, and ab initio calculations, Phys. Rev. B, 79, 214111.
[30] T. Cui, E. Cheng, B. J. Alder, and K. B. Whaley, (1997) Rotational ordering in solid deuterium and hydrogen: A path integral Monte Carlo study, Phys. Rev. B 55, 12253.
[31] V. M. Parvanov, G. K. Schenter, N. J. Hess, L. L. Daemen, M. Hartl, A. C. Stowe, (2008) Materials for hydrogen storage: structure and dynamics of borane ammonia cpmplex. D. M. Camaioni, and T. Autrey, Dalton Trans. 2008, 4514.
[32] M. Benoit, D. Marx, and M. Parrinello, (1998) Tunnelling and zero-point notion in high-preesure ice, Nature 392, 258.
[33] M. Bernasconi, P. L. Silvestrelli, and M. Parrinello, (1998) Ab initio infrared absorption study of the hydrogen-bond symmetrization in ice, Phys. Rev. Lett. 81, 1235.