摘要
实验技术和检测手段的改进,使气相负离子化学在过去的几十年中得到了飞速发展。气相负离子反应凭借其反应速度快,选择性强等优点,在探讨新的有机合成机理和检测一些重要热力学数据等方面发挥了重要作用。1984年,Charles H.Depuy的实验组利用流动余辉和离子选择流动管连用的技术(Flowing afterglow-selected ion flow tube instrument)(FA-SIFT),对NH2-和一系列小分子(N20,CO2,CS2,SO2,OCS)在298 K的温度下的反应[3]进行研究。本文选取了其中的NH2和氧族系列含碳小分子反应NH2-+SCX(X=O,S),进行探讨。
本文以量子化学中的分子轨道理论、过渡态理论等为基础,利用密度泛函理论(B3LYP).二级微扰理论(MP2)、单点能计算(CCSD)和自然价键轨道(NBO)分析方法,对所选取的研究体系选择恰当的基组进行计算,从而确定各反应途径中所涉及到的中间体和过渡态的优化构型。
全文共分四章。第一章对理论研究的依据量子化学进行了简要的介绍,并且简述了对本文的研究体系-有机气相负离子-分子反应的理解。第二章介绍了本工作所依据的量子化学的基本理论和计算方法。这两章主要概括了本文工作的理论背景和理论依据,为我们的研究提供了可靠的理论和实践的基础。
第三章,以Charles等人的实验和机理推测为基础,结合气相离子反应理论研究的经验,采用MP2和B3LYP两种计算方法,在6-311++G(d,p)的基组下,对气相中NH2-与OCS反应的微观机理进行了较为系统的计算研究,并在相同基组下进一步用CCSD方法在MP2优化构型的基础上进行了单点能校正。结果表明,由关键中间体开始通过不同路径的多个步骤得到有竞争力的产物,(1)NH2-+OCS→H2NS-+CO;(2)NH2-+ OCS→HS-+HNCO;(3)NH2-+OCS→NCS-+H20;(4)NH2-+OCS→NCO-+H2S。同时,利用自然键轨道(NBO)和分子轨道理论对反应过程中的一些化学行为进行了分析研究。计算的结果与实验观察基本一致。
第四章,在6-311++G(d,p)的基组水平上,采用密度泛函(B3LYP)和二阶微扰理论(MP2)两种计算方法,优化得到标题反应路径上的反应物,过渡态,中间体和产物的几何构型,并通过振动频率分析对过渡态和中间体进行了确认。能量则采用单点能校正后的CCSD/6-311++G(d,p)∥MP2/6-311++G(d,p)+ZPE水平下得到的相对数值。结果表明,该反应经过缔合、H-转移和离解等过程,最终可以得到三种产物,分别为NCS-+H2S, HS-+HNCS和HS-+NCSH.由于形成产物NCS-+H2S的活化势垒较低,因而是主要反应通道;生成次产物HS-+HNCS的反应通道与主反应通道发生竞争;而生成产物HS-+NCSH的通道从动力学上看是最不利的。计算结果与实验观察完全一致。在第三、四章的研究基础上,对NH2-和氧族系列含碳小分子的两个反应的结果进行对比,发现差异。通过对反应机理的理解,阐述产生差异的原因。
Gas-phase anion molecule reaction was developed greatly during the past several decades for the improvement of experimental technique and test method. Gas-phase anion molecule reaction plays an important role in discovering the reaction mechanisms of the new synthetic and measuring some important thermochemical values because they are in general rapid, intense and selective. Experimentally, Charles H. Depuy et al. have studied, by means of the flowing afterglow-selected ion flow tube instrument (FA-SIFT), the reactions of NH2-with a series of oxygen species small molecules (N2O, CO2, CS2, SO2, OCS) at 298 K [3]. Two typical reactions NH2- + SCX (X=O, S), which were carried out by Charles were selected as the object of study.
In the paper, on the basis of the molecular orbital theory, the tradition transition state theory as well as quantum chemistry theory, the selected systems have been investigated using Moller-Plesset perturbation theory (MP2) and comparison DFT-B3LYP,the single point energy calculations (CCSD) calculations and the Natural Bond Orbital analysis(NBO). The structures of the intermediates and the transition states along the reaction paths were fully optimized. The thermodynamic datas were all used to obtain the potential surface. The information of orbitals was also used to explain the reaction mechanism.
The whole paper consists of four chapters. In Chapter 1, quantum chemistry as foundation of theory investigation was introducted in brief, and then discribed the understanding of organic vapor anion-molecular reaction which is the system of this study. In Chapter 2, we will introduce the theory and computation methods that we based on.
In Chapter 3, ab initio calculations of the title reactions have been made to study reactivity of OCS toward a nucleophile, NH2-. MP2/6-311++G(d, p) geometry optimizations on the singlet potential energy surface have demonstrated that respective channels start from key intermediates and have revealed that multistep paths give the most favorable products:(1) NH2-+OCS→H2NS-+CO; (2) NH2-+OCS→HS-+HNCO; (3) NH2-+OCS→NCS-+H2O; (4) NH2-+OCS→NCO-+H2S. Furthermore, to get more reliable energetic data, single-point calculations are carried out at CCSD/6-311++G(d, p)//MP2/6-311++G(d, p)+ZPE level. The calculated result is consistent with the measured large rate constant in experiment.
In Chapter 4, the mechanism for the ion-molecule reaction of NH2- anion with CS2 has been characterized in detail. Based on 6-311++G(d,p) basis set, using the second-order M(?)ller-Plesset perturbation theory (MP2) and comparison DFT-B3LYP, the single-point energies have also been refined at the CCSD/6-311++G(d, p)//MP2/6-311++G(d, p)+ZPE level to get more accurate energies using the MP2/6-311++G(d,p) optimized geometries. The computational results indicate that the reaction through the association, H-transfer and dissociation processes, the final product can be three, namely NCS-+ H2S, HS-+ HNCS and HS-+ NCSH. Since the formation channel of product NCS- + H2S has lower activation barrier, so it is the main reaction channel. The formation channel of minor product HS-+ HNCS compete with the main reaction channel. The fomation channel of product HS-+ NCSH is most unfavorable from the dynamic point of view. Results are consistent with experimental observations. Addition, based on the study of third and fourth chapters, compare reactivities of OCS and CS2 toward a nucleophile, NH2-, differents existed.By understanding the reaction mechanism to explain the reasons for the discrepancy.
引文
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