Department of Chemistry, School of Education, Shizuoka University, 836 Ohya, Shizuoka 422-8529, Japan.
J Chem Phys. 2010 Jul 21;133(3):034504. doi: 10.1063/1.3456183.
The changes in the vibrational properties of the peptide group upon formation of O...X and N...X halogen bonds are studied theoretically. Calculations are carried out for complexes of N-methylacetamide (NMA), a well known model molecule of the peptide group, with halogen-containing molecules. For comparison, calculations are also carried out for some NMA-water hydrogen-bonding complexes. It is shown that the infrared (IR) intensity of the amide I mode of the peptide group is enhanced significantly (up to about 520 km mol(-1) or 2.6 times) upon C=O...X halogen-bond formation, in spite of rather modest magnitudes of the intermolecular electric field and of the changes in the C=O bond length and in the amide I vibrational frequency as compared with the cases of the C=O...H(D) hydrogen bonding. From the analysis of the changes in the dipole derivative and in the electronic structure, it is shown that this IR intensity enhancement arises from the intermolecular charge flux. For the N...X halogen bonding complexes, some characteristic changes in the vibrational properties are seen, among which the IR intensity enhancement of the ND out-of-plane wagging mode is most notable. The reason why such large IR intensity enhancements are seen for these particular vibrational modes is examined.
理论研究了形成 O...X 和 N...X 卤键时肽基团振动性质的变化。对含卤分子与 N-甲基乙酰胺（NMA）的复合物进行了计算，NMA 是肽基团的一个众所周知的模型分子。为了进行比较，还对一些 NMA-水氢键复合物进行了计算。结果表明，尽管与 C=O...H(D) 氢键形成相比，分子间电场的幅度以及 C=O 键长和酰胺 I 振动频率的变化相当温和，但在 C=O...X 卤键形成时，肽基团的酰胺 I 模式的红外（IR）强度显著增强（高达约 520 km mol(-1) 或 2.6 倍）。通过对偶极导数和电子结构变化的分析，表明这种 IR 强度增强源于分子间电荷转移。对于 N...X 卤键复合物，观察到一些振动性质的特征变化，其中 ND 面外摇摆模式的 IR 强度增强最为显著。检查了为什么这些特殊的振动模式会出现如此大的 IR 强度增强。
