S−H Bond Dissociation Enthalpies in Thiophenols:  A Time-Resolved Photoacoustic Calorimetry and Quantum Chemistry Study

Time-resolved photoacoustic calorimetry (TR-PAC) and quantum chemistry calculations were used to investigate the energetics of sulfur−hydrogen bonds in thiophenol and four para-substituted thiophenols, 4-XC6H4SH (X = CH3, OCH3, Cl, and CF3). The result obtained for the PhS−H gas-phase bond dissociation enthalpy, derived from the PAC experimental results in solution, is 349.4 ± 4.5 kJ mol-1. This value is significantly higher than recent literature values but agrees with a value suggested some 20 years ago in a widely used review. The PAC result also concurs with the value computed at a high theory level, G3(MP2), 346.8 kJ mol-1. The data obtained for the substituted thiophenols support the idea that substituent effects are less pronounced on the S−H bond dissociation enthalpy than on the O−H bond dissociation enthalpy of the corresponding phenols.