Unusual intermolecular T-shaped X-H…π interactions between CH3CN/CH3NC and H2O, NH3 or C2H2: A B3LYP and MP2 theoretical study

The four types of unusual T-shaped X-H…π hydrogen bonds, which are O-H…CN, O-H…NC, N-H…CN and C-H…NC, are found using B3LYP and MP2 methods at 6-311++G(2df, 2p) and aug-cc-pVTZ levels in CH3CN…H2O, CH3NC…H2O, CH3CN…H3N and CH3NC…H2C2 complexes. In the CH3CN…H2O and CH3NC…H2O, the -CN or -NC triple bond offers the p1.00-hybridization π-electrons of the carbon and nitrogen atoms to the contacting σ(OH)∗ antibonding orbital of the H2O, apart from the interaction formed by which the oxygen offers its lone pair electrons to the σ(CH)∗ antibonding orbital of the methyl group. These two complexes are comparatively stable, in which the CH3CN…H2O is the most stable one because of the strongest intermolecular interaction coming from O-H…CN hydrogen bond. In the CH3CN…H3N and CH3NC…H2C2 complexes, the -CN or -NC triple bond offers the pure p electrons of π orbital to the σNH)∗ or σCH)∗ antibonding orbital of the H3N or H2C2, and the intermolecular hydrogen-bonded interactions in these two complexes are weak. For the T-shaped X-H…π (X = O, N, C) hydrogen bonds, the ZPE correction for binding energies is very necessary for B3LYP and MP2 methods at 6-311++G(2df, 2p) and aug-cc-pVTZ levels. The net charge transfer is from carbon to nitrogen atom after complexation for the -CN group while reversely for -NC, and the former becomes more polarized and the latter favors weaker.