Natural Bond Orbital Analysis of Some S-Nitrosothiols Biological Molecules

Theoretical study of several S-nitrosothiols biological molecules has been performed using quantum computational ab initio RHF and density functional B3LYP and B3PW91 methods with 6-31G(d,p) basis set. Geometries obtained from DFT calculations were used to perform natural bond orbital (NBO) analysis. It is noted that the weakness in the S-N sigma bond is due to n(O1) -> sigma(S3-N2)* delocalization and is responsible for the longer S-N bond length in S-nitrosothiols. It is also noted that decreased occupancy of the localized sigma(SN) orbital in the idealized Lewis structure or increased occupancy of sigma(SN)* 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 of sigma(SN) bond orbital. In addition, the charge transfer energy decreases with the increasing of the Hammett constants of substituent groups, and the partial charge distribution on the skeletal atoms shows that the electrostatic repulsion or attraction between atoms can give a significant contribution to the intramolecular and intermolecular interaction. (C) 2009 Wiley Periodicals, Inc. Int J Quantum Chem 110: 1565-1572, 2010