Netzel Jeanette, van Smaalen Sander
Laboratory of Crystallography, University of Bayreuth, D-95440 Bayreuth, Germany.
Acta Crystallogr B. 2009 Oct;65(Pt 5):624-38. doi: 10.1107/S0108768109026767. Epub 2009 Aug 28.
Charge densities have been determined by the Maximum Entropy Method (MEM) from the high-resolution, low-temperature (T approximately 20 K) X-ray diffraction data of six different crystals of amino acids and peptides. A comparison of dynamic deformation densities of the MEM with static and dynamic deformation densities of multipole models shows that the MEM may lead to a better description of the electron density in hydrogen bonds in cases where the multipole model has been restricted to isotropic displacement parameters and low-order multipoles (l(max) = 1) for the H atoms. Topological properties at bond critical points (BCPs) are found to depend systematically on the bond length, but with different functions for covalent C-C, C-N and C-O bonds, and for hydrogen bonds together with covalent C-H and N-H bonds. Similar dependencies are known for AIM properties derived from static multipole densities. The ratio of potential and kinetic energy densities |V(BCP)|/G(BCP) is successfully used for a classification of hydrogen bonds according to their distance d(H...O) between the H atom and the acceptor atom. The classification based on MEM densities coincides with the usual classification of hydrogen bonds as strong, intermediate and weak [Jeffrey (1997). An Introduction to Hydrogen Bonding. Oxford University Press]. MEM and procrystal densities lead to similar values of the densities at the BCPs of hydrogen bonds, but differences are shown to prevail, such that it is found that only the true charge density, represented by MEM densities, the multipole model or some other method can lead to the correct characterization of chemical bonding. Our results do not confirm suggestions in the literature that the promolecule density might be sufficient for a characterization of hydrogen bonds.
通过最大熵方法（MEM），从六种不同氨基酸和肽晶体的高分辨率、低温（T约20K）X射线衍射数据中确定了电荷密度。将MEM的动态变形密度与多极模型的静态和动态变形密度进行比较表明，在多极模型已被限制为H原子的各向同性位移参数和低阶多极（l(max)=1）的情况下，MEM可能会更好地描述氢键中的电子密度。发现键临界点（BCP）处的拓扑性质系统地依赖于键长，但对于共价C-C、C-N和C-O键，以及对于氢键与共价C-H和N-H键，其函数不同。从静态多极密度导出的AIM性质也有类似的依赖性。势能密度与动能密度之比|V(BCP)|/G(BCP)已成功用于根据H原子与受体原子之间的距离d(H...O)对氢键进行分类。基于MEM密度的分类与通常将氢键分为强、中、弱的分类一致[杰弗里（1997年）。《氢键导论》。牛津大学出版社]。MEM密度和前晶体密度在氢键BCP处的密度值相似，但差异普遍存在，因此发现只有由MEM密度、多极模型或其他方法表示的真实电荷密度才能导致化学键的正确表征。我们的结果不支持文献中关于前分子密度可能足以表征氢键的建议。
