Acidity of HOCN, HSCN, HNCO, HNCS: A treatment from the viewpoint of ab initio approach

The electronic structures of the molecules HOCN, HSCN, HNCO, HNCS and the anions OCN - , SCN - have been investigated ab initio at the RHF/6-31G(d), RHF/6-31G(d,p), MP2/6-31G(d)//RHF/6-31G(d) and MP2/6-31G(d,p)//RHF/6-31G(d,p) theory levels. The thermodynamic stability of the HNCO and HNCS molecules was shown to be higher than that of the HOCH and HSCN molecules, respectively.The fol- lowing series of the alteration of the protolytes strength HSCN > HOCH, HNCS > HNCO, HOCN > HNCO, HSCN > HNCS was substantiated. The computations taking into account the electronic correlation (MP2/6-31G(d)//RHF/6-31G(d) and MP2/6-31G(d,p)//RHF/6-31G(d,p)) reflect the general sequence of change in the pro- ton-donor properties: HSCN > HOCN > HNCS > HNCO, coinciding with the order of descending hydrophobicity of the compounds. The comparative proton-donor ability of the above acids in aqueous solutions is determined basically from the electronic struc- ture and size of their molecules and anions OCN - , SCN - , but not on medium effects.