Theoretical investigation of some O-nitrosyl carboxylate biologic molecules — A natural bond orbital study

Abstract

Theoretical study of several O-nitrosyl carboxylate compounds have been performed using quantum computational ab initio RHF and density functional B3LYP and B3PW91 methods with 6-31G** basis set. Geometries obtained from DFT calculations were used to perform the natural bond orbital (NBO) analysis. It is noted that weakness in the O3-N2 bond is due to $$ n_{O_1 } \to \sigma _{O_3 - N_2 }^* $$ delocalization and is responsible for the longer O3-N2 bond lengths in O-nitrosyl carboxylate compounds. It is also noted that decreased occupancy of the localized $$ \sigma _{O_3 - N_2 } $$ orbital in the idealized Lewis structure, or increased occupancy of $$ \sigma _{O_3 - N_2 }^* $$ of the non-Lewis orbital, and their subsequent impact on molecular stability and geometry (bond lengths) are related with the resulting p character of the corresponding sulfur natural hybrid orbital (NHO) of $$ \sigma _{O_3 - N_2 } $$ bond orbital. In addition, the charge transfer energy decreases with the increase of the Hammett constants of subsitutent groups.