Remarkable blue shifts of C-H and N-H stretching frequencies in the interaction of monosubstituted formaldehyde and thioformaldehyde with nitrosyl hydride.

Weak interactions of monosubstituted formaldehydes and thioformaldehydes with nitrosyl hydride were investigated by using ab initio MO calculations at the MP2/aug-cc-pVTZ level. Thirty two equilibrium structures having different complex forms were located on the corresponding potential energy surfaces (all having C(s) symmetry). Obtained binding energies, which include both ZPE and BSSE corrections, range from 7 to 14 kJ x mol(-1) and 6 to 12 kJ x mol(-1) for complexes of substituted formaldehydes and thioformaldehydes, respectively. In each geometrical structure, the (XCHO,HNO) complex is consistently more stable than the (XCHS,HNO) complex. The H-bond strength significantly increases when one H atom is replaced by a methyl group in both formaldehyde and thioformaldehyde. When replacing H by a halogen atom, the binding energy tends to decrease. It is remarkable that all the C-H and N-H bonds are shortened upon complexation, resulting in an increase of their stretching frequencies. Furthermore, the blue shifts are consistently observed for the interacting N-H bonds in N-H...X, Z, with X = F, Cl, Br, and Z = O, S; such contraction of a covalent N-H bond is extremely rare. In addition, the N-H bond length contraction and its frequency blue shift in the N-H...S complex have been revealed for the first time.