三接触弯曲构型的蓝移氢键CH3…Y的理论研究

运用量子化学从头算方法, 在MP2/6-311＋＋G(d,p), MP2/6-311＋＋G(2df,2p), MP2/6-311＋＋G(3df,3pd)和QCISD/6-311＋＋G(d,p)水平上, 研究了CH₃F, CH₃Cl和CH₃Br作为质子给体与Cl^－, Br^－作为质子接受体形成的氢键CH₃…Y. 计算结果表明: 6种复合物中C—H键收缩, 伸缩振动频率增大, 形成蓝移氢键. 分子中原子(Atoms in Molecules, AIM)分析表明, 这些复合物的电子密度拓扑性质与普通氢键有着本质的不同, 在Y…H之间不存在键临界点, 而在Y与C之间存在键临界点, 因此这些相互作用严格地不能称为氢键. 自然键轨道(Natural bond orbital, NBO)分析表明, 在这些复合物中弯曲的CH…Y的特殊结构使得分子间超共轭n(Y)®σ*(C—H)减小到可以忽略; 质子接受体的电子密度没有转移到σ*(C—H)上, 而是转移到了σ*(C—X) (X＝F, Cl, Br)上; 存在一定程度的重杂化; 分子内超共轭相互作用减小使得σ*(C—H)的电子密度减少. 这些因素共同导致C—H伸缩振动频率的蓝移.

Theoretical Study of the CH3 …Y Trifurcated Blue-Shifted Hydrogen Bonds

Ab initio quantum mechanics methods were applied to investigate the CH₃…Y bend hydrogen bonds in the complexes Cl^－…CH₃F, Cl^－…CH₃Cl, Cl^－…CH₃Br, Br^－…CH₃F, Br^－…CH₃Cl and Br^－…CH₃Br. The MP2 level of theory with the four basis sets 6-311＋＋G(d,p), 6-311＋＋G(2df,2p), 6-311＋＋G(3df,3pd) and QCISD/6-311＋＋G(d,p) and the standard gradient technique were used to optimize the geometries and analyze vibrational frequencies. The results show that there are three C—H…Y (Y＝Cl^－, Br^－) hydrogen bonds in each of the six complexes, and the C—H bond contracts with a concomitant blue shift of the corresponding stretching frequency. The natural bond orbital (NBO) analysis shows that four factors lead to the formation of these blue-shifted H-bonds: (a) intermolecular hyperconjugation in a bend H-bond configuration is very small and can be neglected; (b) there exists rehybridization; (c) the electron density in the Cl^－ or Br^－ lone pair is transferred not to σ*(C—H) but to the σ*(C—X) (X＝F, Cl, Br) antibonding orbital; (d) decrease of intramolecular hyperconjugation leads to decrease of occupancy in σ*(C—H). The analysis using the theory of atoms in molecules (AIM) shows that the topological properties of electron density for these trifurcated bent H-bonds have essential differences from those of usual H-bonds, where the bond critical point is not between H and Y, but between C and Y.