On the multireference character of S-nitrosothiols: A theoretical study of HSNO

The significant multireference character of the HSNO wavefunction is revealed from a comparison of its S–N bond properties calculated with CASSCF and Hartree–Fock methods. This is further supported by analysis of the CASSCF natural orbital occupation numbers and the configuration interaction expansion coefficients. Associating these results for HSNO with our description of S-nitrosothiols (RSNOs) as a combination of conventional R–S–NO, zwitterionic R–S+N–O−, and RS−/NO+ ion-pair resonance structures, we demonstrate that the multireference character of the RSNO wavefunction arises from the ion-pair resonance structure.