The prominent enhancing effect and mechanism of the methyl group in the XY (X=O, S, H3CO, H3CS, (H3C)2O, (H3C)2S; Y=HCN, HNC) hydrogen-bonded complex

An ab initio computational study of the enhancing role of the methyl group in the M center dot center dot center dot H (M = S and O) hydrogen bond has been carried out at the QCISD/6-311++G(2df, 2p) level. The bond lengths, frequency shifts, and interaction energies were analysed. The methyl group of the electron donor plays a positive role in the formation of the hydrogen bond. Its enhancing role is stronger in the O center dot center dot center dot H hydrogen bond than in the S center dot center dot center dot H hydrogen bond. The results show that the methyl group has a prominent effect on the strength of the hydrogen bond. The interaction energy is increased by 347% for the Me2O-HCN complex relative to that for the O-HCN complex. The enhancing mechanism of the methyl group has been analysed by means of natural bond orbital (NBO) theory. The electrostatic interaction is of more importance to the O center dot center dot center dot H hydrogen bond, whereas dispersion and charge-transfer interactions play a more significant role in the S center dot center dot center dot H hydrogen bond.