Structure and stability of HSNO, the simplest S-nitrosothiol

High-level ab initio calculations employing the CCSD and CCSD(T) coupled cluster methods with a series of systematically convergent correlation-consistent basis sets have been performed to obtain accurate molecular geometry and energetic properties of the simplest S-nitrosothiol (RSNO), HSNO. The properties of the S-N bond, which are central to the physiological role of RSNOs in the storage and transport of nitric oxide, are highlighted. Following corrections for quadruple excitations, core-valence correlation and relativistic effects, the CCSD(T) method extrapolated to the complete basis set (CBS) limit yielded values of 1.85 angstrom and 29.2 kcal mol(-1) for the S-N bond length and the dissociation energy for homolysis of the S-N bond, respectively, in the energetically most stable trans-conformer of HSNO. The properties of the S-N bond strongly depend on the basis-set size and the inclusion of triple, and, to a lesser extent, quadruple excitations in the coupled cluster expansion. CCSD calculations systematically underestimate the S-N equilibrium distance and S-N bond dissociation energy by 0.05-0.07 angstrom and 6-7 kcal mol(-1), respectively. The significant differences between the CCSD(T) and CCSD descriptions of HSNO, the high values of the coupled cluster T-1 (0.027) and D-1 (0.076) diagnostics, as well as the instability of the reference restricted Hartree-Fock (RHF) wavefunction indicate that the electronic structure of the SNO group possesses multireference character. Previous quantum-chemical data on RSNOs are reexamined based on the new insight into the SNO electronic structure obtained from the present high-level calculations on HSNO.