Toward Accurate Characterization of the Puzzling NSO and SNO Moieties

The structural and spectroscopic properties in the gas phase of prototypical compounds containing the NSO and SNO moieties have been analyzed by a general computational strategy based on recent Pisa composite schemes (PCS). In a first step, an accurate semiexperimental (SE) equilibrium structure has been derived for cis-HNSO and employed, together with the already available SE equilibrium structures of cis- and trans-HSNO, to validate the geometrical parameters delivered by different quantum chemical methods. The results confirm the accuracy of the proposed composite schemes, provided that the complementary auxiliary basis set correction is included for the Hartree–Fock component. However, perturbative inclusion of quadruple excitations is mandatory for obtaining a correct S–N bond length in the case of HSNO. In this way, it is possible to obtain accurate geometrical parameters and ground state rotational constants, employing in the latter case vibrational corrections obtained by methods rooted in density functional theory (DFT) in the framework of second-order vibrational perturbation theory. The results delivered by a much cheaper model based on DFT geometry optimizations and one-parameter bond corrections, while slightly less accurate, represent a remarkable improvement with respect to current methods of comparable cost. The nearly identical correction induced by quadruple excitations on the S–N bond length of HSNO and CH3SNO paves the way toward the study of larger compounds of biochemical interest containing the NSO or SNO moieties.