Toward reliable modeling of S-nitrosothiol chemistry: Structure and properties of methyl thionitrite (CH3SNO), an S-nitrosocysteine model

Methyl thionitrite CH3SNO is an important model of S-nitrosated cysteine aminoacid residue (CysNO), a ubiquitous biological S-nitrosothiol (RSNO) involved in numerous physiological processes. As such, CH3SNO can provide insights into the intrinsic properties of the -SNO group in CysNO, in particular, its weak and labile S-N bond. Here, we report an ab initio computational investigation of the structure and properties of CH3SNO using a composite Feller-Peterson-Dixon scheme based on the explicitly correlated coupled cluster with single, double, and perturbative triple excitations calculations extrapolated to the complete basis set limit, CCSD(T)-F12/CBS, with a number of additive corrections for the effects of quadruple excitations, core-valence correlation, scalar-relativistic and spin-orbit effects, as well as harmonic zero-point vibrational energy with an anharmonicity correction. These calculations suggest that the S-N bond in CH3SNO is significantly elongated (1.814 angstrom) and has low stretching frequency and dissociation energy values, nu(S-N) = 387 cm(-1) and D-0 = 32.4 kcal/mol. At the same time, the S-N bond has a sizable rotation barrier, Delta E-0(not equal) = 12.7 kcal/mol, so CH3SNO exists as a cis-or trans-conformer, the latter slightly higher in energy, Delta E-0 = 1.2 kcal/mol. The S-N bond properties are consistent with the antagonistic nature ofCH(3)SNO, whose resonance representation requires two chemically opposite (antagonistic) resonance structures, CH3-S+= N-O- fi and CH3-S-/NO+, which can be probed using external electric fields and quantified using the natural resonance theory approach (NRT). The calculated S-N bond properties slowly converge with the level of correlation treatment, with the recently developed distinguished cluster with single and double excitations approximation (DCSD-F12) performing significantly better than the coupled cluster with single and double excitations (CCSD-F12), although still inferior to the CCSD(T)-F12 method that includes perturbative triple excitations. Double-hybrid density functional theory (DFT) calculations with mPW2PLYPD/def2-TZVPPD reproduce well the geometry, vibrational frequencies, and the S-N bond rotational barrier in CH3SNO, while hybrid DFT calculations with PBE0/def2-TZVPPD give a better S-N bond dissociation energy.