三接触弯曲构型的蓝移氢键CH_3…Y的理论研究  

Theoretical Study of the CH_3…Y Trifurcated Blue-Shifted Hydrogen Bonds

作　　者：黎安勇[1] 闫秀花[1] 王素纹[1] 

机构地区：[1]西南大学化学化工学院,重庆400715

出　　处：《化学学报》2007年第24期2809-2816,共8页Acta Chimica Sinica

基　　金：重庆市自然科学基金(No.230413515)资助项目.

摘　　要：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(AtomsinMole-cules,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.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.

关 键 词：CH3…Y弯曲氢键 蓝移氢键 电子密度拓扑分析 自然键轨道分析 

分 类 号：O641.1[理学—物理化学]