Structure, stability, and substituent effects in aromatic S-nitrosothiols: the crucial effect of a cascading negative...

Aromatic S-nitrosothiols (RSNOs) are of significant interest as potential donors of nitric oxide and related biologically active molecules. Here, we address a number of poorly understood properties of these species via a detailed density functional theory and the natural bond orbital (NBO) investigation of the parent PhSNO molecule. We find that the characteristic perpendicular orientation of the -SNO group relative to the phenyl ring is determined by a combination of the steric factors and the donor-acceptor interactions including, in particular, a cascading orbital interaction involving electron delocalization from the oxygen lone pair to the σ-antibonding S-N orbital and then to the π*-aromatic orbitals, an unusual negative hyperconjugation/conjugation long-range delocalization pattern. These interactions, which are also responsible for the relative weakness of the S-N bond in PhSNO and the modulation of -SNO group properties in substituted aromatic RSNOs, can be interpreted as a resonance stabilization of the ionic resonance component RS(-)/NO(+) of the RSNO electronic structure by the aromatic ring, similar to the resonance stabilization of PhS(-) anion. These insights into the chemistry and structure-property relationships in aromatic RSNOs can provide an important theoretical foundation for rational design of new RSNOs for biomedical applications.