Hydrogen bond nature in formamide (CYHNH_2···XH; Y=O, S, Se, Te; X=F, HO, NH_2) complexes at their ground and low-lying excited states

摘要

A theoretical study on the nature of hydrogen bond for formamide and its heavy complexes (CYHNH_2···XH; Y=O, S, Se, Te; X=F, HO, NH_2) was performed on the basis of density functional theory and the quantum chemistry analysis. Except for the CYHNH_2···NH_3 complexes, the substitution of O atom at formamide with less electronegative atoms (S, Se, and Te) is found to weaken the hydrogen bond (H-bond). This substitution results in cyclic structure of hydrated and ammoniated formamide complexes by the formation of bifunctional H-bonds (Y···H_4X; X···H_3C). Natural bond orbital analysis indicates that the H-bond is weakened because of less charge transfer from a lone pair orbital of H-bond acceptor to antibonding orbital of H-bond donor. The quantum theory of atoms in molecules analysis reveals that the acyclic structure with single H-bond stabilizes the complexes more than the cyclic structure formed by bifunctional H-bonds. Natural energy decomposition analysis (NEDA) and block-localized wavefunction energy decomposition (BLW-ED) analyses show that the H-bond stabilization energies of NEDA and BLW-ED have good correlation with the dissociation energy of formamide complexes and charge transfer from donor to acceptor atom play an important role in H-bonding. We have also studied the low-lying electronic excited states (T_1, T_2, and S_1) for CYHNH_2···H_2O complexes to explore the nature of H-bond on the basis of electronegativity and found that NEDA also establishes a good correlation with relative electronic energy (with respect to their ground state) and H-bond strength at their excited states.