One Hydrogen Bond鈥擳wo Ways To Build a Structure. The Role of N鈥揌路路路O Hydrogen Bonds in Crystal Structures of N,N-Dimethylglycine

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• 作者：Eugene A. Kapustin ; Vasily S. Minkov ; Jernej Stare ; Elena V. Boldyreva
• 刊名：Crystal Growth & Design
• 出版年：2014
• 出版时间：April 2, 2014
• 年：2014
• 卷：14
• 期：4
• 页码：1851-1864
• 全文大小：804K
• 年卷期：v.14,no.4(April 2, 2014)
• ISSN：1528-7505

文摘

Crystal structures of amino acids are considered to mimic important interactions in peptides; therefore the studies of the structure-forming factors in these systems attract much attention. N,N-Dimethylglycine is an interesting model compound that was used to test the role of the N鈥揌路路路O hydrogen bonds in forming the head-to-tail chains, the main structural unit in the crystals of amino acids. It was hypothesized previously [Kolesov, B. A.; Boldyreva, E. V. J. Raman Spectrosc. 2010, 41, 670鈭?77] that additional side N鈥揌路路路O hydrogen bonds play an important role in forming the head-to-tail chains of amino acid zwitterions linked via N鈥揌路路路O hydrogen bonds between the charged 鈭扤H₃⁺ and 鈭扖OO^{鈥?/sup> terminal groups. The twice methylated amino group of N,N-dimethylglycine is able to form only one N鈥揌路路路O hydrogen bond in the crystal structure, so this hypothesis could be tested. In the present article, we describe the crystal structures of two polymorphs of N,N-dimethylglycine, in which the zwitterions are packed in two different ways. In one polymorph (orthorhombic, Pbca), they form finite four-membered ring motifs not linked to each other via any hydrogen bonds but only by weak van der Waals interactions. However, in the second polymorph (monoclinic, P2₁/n, which was never described before), the zwitterions do form infinite head-to-tail chains though the N鈥揌路路路O bond is the only interaction and is not assisted via any additional hydrogen bonds. The effect of cooling on the two crystal structures was followed by single-crystal X-ray diffraction combined with polarized Raman spectroscopy of oriented single crystals, in order to compare the response of the N鈥揌路路路O bonds to temperature variations. The crystal structure of the monoclinic polymorph with infinite chain motifs compresses anisotropically on cooling, whereas that of the orthorhombic polymorph with finite ring motifs undergoes a reversible single-crystal to single-crystal phase transition at 200 K accompanied by nonmerohedral twinning, reducing the space symmetry to monoclinic (P2₁/b) and doubling the asymmetric unit from two to four molecules. This phase transition could not be detected by Raman spectroscopy and DSC. The temperature dependent structure and relative stability of both polymorphs were studied by periodic DFT calculations. The monoclinic polymorph appears to be more stable (by 0.8鈥?.2 kcal/mol, depending on the density), but with the increasing density and decreasing temperature, the difference decreases. The phase transition of the orthorhombic polymorph has no detectable impact on its relative stability.}