Estimation of S–H bond dissociation enthalpies and free energies of thiophenols : an ab initio Hartree-Fock study

Ab initio Hartree–Fock method (UHF/6-31G(d)) was used to estimate the gas phase S–H bond dissociation enthalpies (BDEs) and free energies (BDFEs) of thiophenols with a considerable success. This was carried out by identifying the unimolecular dissociative states (DSs) of thiophenols, in a similar way recently described for hydroxylic antioxidants. The DSs of thiophenols forerunning S–H bond homolysis were vibrationally excited singlet or triplet states. In the DS, < C–S–H bond angle was fixed at 180º. The BDE of a given thiophenol was calculated as the relative change in enthalpy accompanying the formation of its DS with respect to a ground state of same multiplicity. BDFE was the corresponding change in Gibbs free energy. The calculated BDEs of thiophenol, its 12 monosubstituted derivatives (2-Me, 3-Me, 4-Me, 2-Cl, 3-Cl, 4-Cl, 4-Br, 3-CF3, 4-NH2, 4-MeO, 4-NO2 and 2-CO2Me) and 2-naphthalenethiol were remarkably similar to literature experimental values. The calculated BDE and BDFE of thiophenol were 79.1 and 80.1 kcal/mol, respectively. The average BDE and BDFE of the 13 thiophenols studied were 79.5±0.8 and 80.5±0.8 kcal/mol, respectively, the lowest and highest BDE/BDFE values being those of 2-methylthiophenol (77.7/78.5 kcal/ mol) and 2-chlorothiophenol (81.3/81.9 kcal/mol). The BDE and BDFE of 2-naphthalenethiol were virtually identical with those of thiophenol’s. The lower literature BDE values of 4-aminothiophenol (-9.3 kcal/mol) and 4-methoxythiophenol (-2.2 kcal/mol) in DMSO were satisfactorily explained by the stabilization of their respective dissociative states with hydrogen bonding of the para substituents with 2 and 1 DMSO molecules