A theoretical analysis of the weakly bound complexes HM center dot center dot center dot HXY (M=O and S; XY=CN and NC): comparison with H2M center dot center dot center dot HXY complexes

An ab initio study of the complexes formed between MH (M=O and S) and HXY (XY=CN and NC) was carried out at the UQCISD/6-311++G(2df,2p) level. For comparison, the corresponding H2M-HXY complexes were also studied. Two minima were found for each molecular pair. The results show the necessity of electron correlation and larger basis sets in the study of open-shell hydrogen-bonded complexes. As the proton donor and acceptor, the OH radical is favourable for the formation of a hydrogen bond with HXY than is the SH radical. The MH radical is more likely to donate a proton than H2M, whereas H2M is more likely to accept a proton than the MH radical. Natural bond orbital and atoms in molecules analyses were performed for these systems. It is shown that the complexes are held together mainly by electrostatic interactions.