The S-H bond dissociation enthalpies and acidities of para and meta substituted thiophenols: A quantum chemical study

The accurate S-H bond dissociation enthalpies [BDE(S-H)] for a series of meta and para substituted thiophenols (X-C6H4-SH, X = H, F, Cl, CH3, OCH3, NH2, CF3, CN, and NO2) have been estimated by using the (RO)B3LYP procedure with 6-311++G(2df,2p) basis set. The proton affinities [PA(S-)] at the sulfur site of the corresponding thiophenoxide ions (X-C6H4-S-) and adiabatic electron affinities (EA(a)) of thiophenoxy radicals have also been evaluated at the same level of calculations. The BDE(S-H) of the parent thiophenol has been evaluated to be 332.6 +/- 4.0 kJ/mol, which is in agreement rather with the earlier experimental value of 331.0 kJ/mol (Bordwell et al., J. Am. Chem. Soc. 1994, 116, 6605) than the more recent value of 349.4 kJ/mol (Santos et al., J. Phys. Chem. A 2002, 106, 9883). The effects of para and meta substituents on the BDE(S-H) and PA(S-) have been critically analyzed considering the change in stabilities of the neutral molecule, radical and anions with the change in substituent, and its position. The correlations of Hammett's substituent constants with the BDE(S-H) and PA(S-) values have also been explored.