Origin of blue and red shifts of C-H and C-N stretching vibrations in formamide-HF/H2O/H2S/NH3 complexes

The complexes of formamide with four small molecules (HF, H2O, H2S, NH3) are studied by MP2 and B3LYP methods with 6-311++G(d,p) basis set. The optimized geometric parameters and intermolecular interaction energies for all complexes are estimated. The origin of blue and red shifts of C-H and C-N bonds not participating in H-bonding are analyzed by using natural bond orbital (NBO) theory. Large electron density transfer from electron lone pair of the electron donor to electron acceptor leads to the formation of a new electronic structure and results in changes in geometry and vibrational frequency. These changes are attributed to two major factors: electron density redistribution and rehybridization. Our results suggest that Hobza's and Weinhold's theories can be also used to explain the stretching vibrational shifts of some bonds not directly participating in hydrogen bond formation.