乙醛二聚体内蓝移型氢键的理论研究

Theoretical Study on Blue-shifted Hydrogen Bonds in CH_3CHO Dimers

利用理论方法研究了乙醛二聚体内的氢键.在MP2/6-31+G(d),B3LYP/6-31+G(d),B3LYP/6-311++G(d,p)和B3LYP/6-311++G(3df,2p)水平上,利用常规方法和均衡校正方法对3种稳定的乙醛二聚体进行了几何优化和振动频率计算.计算结果表明:在二聚体A和C中乙醛中C—H键强烈收缩,存在显著的C—H…O蓝移型氢键.自然键轨道(NBO)分析表明,电子供体轨道和电子受体轨道之间相互作用的稳定化能、分子内电子密度重排、轨道再杂化和结构重组是决定氢键红移和蓝移的主要因素.其中,轨道间稳定化能属于键伸长效应,分子内电子密度重排、轨道再杂化和电子受体内部结构重组属于键收缩效应.在二聚体A和C中,由于键收缩效应处于优势地位导致C—H…O蓝移氢键存在.

Theoretical calculations were performed to study the nature of the hydrogen bonds in the CH3CHO dimers. The geometric structures and vibrational frequencies of its three stable dimers at the MP2/6-31 ＋ G（d）, B3LYP/6-31 ＋ G（d）, B3LYP/6-311 ＋ ＋ G（d,p） and B3LYP/6-311 ＋ ＋ G（3df,2p） levels were calculated by standard and counterpoise-corrected methods, respectively. The results indicate that in the dimers A and C the C-H bond is strongly contracted and C-H……O blue-shifted hydrogen bonds were observed. From the natural bond orbital analysis it can be seen that the X-H bond length in the X-H…Y hydrogen bond is controlled by a balance of four main factors in the opposite directions： hyperconjugation, electron density redistribution, rehybridization and structural reorganization. Among them the hyperconjugation has the effect of elongating the X-H bond, but the other three factors belong to the bond shortening effects. In the dimers A and C, the shortening effects dominate which lead to the blue shift of the C-H stretching frequencies.