A theoretical study on SCN− + XH reactions (X = O, S): Hemi bonded vs. H-bonded intermediate

Ab initio molecular electronic structure calculations are employed to elucidate the structure, bonding and spectra (IR and UV-VIS) of various intermediates formed on the reaction of OH, SH radicals and atomic O with thiocyanate anion (SCN−) and thiocyanate radical (SCN) in gas phase. Geometry optimizations are carried out following second order Moller-Plesset perturbation theory (MP2) adopting 6-311++G(d,p) basis set. Hessian calculations are performed at the same level of theory to check the nature of the equilibrium geometry as well as to generate IR spectrum. Visualization of the highest doubly occupied molecular orbital (HDOMO) in a few SCNX− adducts (X = OH, SH) is able to depict clearly the presence of a two-center three-electron (2c-3e) σ bond between O…S, S…S and S…N pairs. Excited state calculations are performed at configuration interaction with single-electron excitation (CIS) level for all the species. Strong optical absorption predicted for 2c-3e bonded species is due to σ → σ∗ electronic transition. Structures having 2c-3e bonding in SCNX− systems (X = OH and SH) show strong CN stretching IR band. In case of neutral SCNX and NCSSH adducts (X = OH and SH), a strong CN stretching band and weak OH or SH stretching bands are obtained. Strong ON or OS stretching bands are observed in case of anionic ONCS− or OSCN− adducts. But strong CN stretching band is predicted for radical adducts, ONCS or OSCN.