| Abstract
| - There are conflicting reports on the origin of the effect of Y substituents on the S−H bond dissociationenthalpies (BDEs) in 4-Y-substituted thiophenols, 4-YC6H4S−H. The differences in S−H BDEs, [4-YC6H4S−H] − [C6H5S−H], are known as the total (de)stabilization enthalpies, TSEs, where TSE = RSE − MSE, i.e.,the radical (de)stabilization enthalpy minus the molecule (de)stabilization enthalpy. The effects of 4-Ysubstituents on the S−H BDEs in thiophenols and on the S−C BDEs in phenyl thioethers are expected to bealmost identical. Some S−C TSEs were therefore derived from the rates of homolyses of a few 4-Y-substitutedphenyl benzyl sulfides, 4-YC6H4S−CH2C6H5, in the hydrogen donor solvent 9,10-dihydroanthracene. TheseTSEs were found to be −3.6 ± 0.5 (Y = NH2), −1.8 ± 0.5 (CH3O), 0 (H), and 0.7 ± 0.5 (CN) kcal mol-1.The MSEs of 4-YC6H4SCH2C6H5 have also been derived from the results of combustion calorimetry, Calvet-drop calorimetry, and computational chemistry (B3LYP/6-311+G(d,p)). The MSEs of these thioethers were−0.6 ± 1.1 (NH2), −0.4 ± 1.1 (CH3O), 0 (H), −0.3 ± 1.3 (CN), and −0.8 ± 1.5 (COCH3) kcal mol-1.Although all the enthalpic data are rather small, it is concluded that the TSEs in 4-YC6H4SH are largelygoverned by the RSEs, a somewhat surprising conclusion in view of the experimental fact that the unpairedelectron in C6H5S• is mainly localized on the S. The TSEs, RSEs, and MSEs have also been computed for amuch larger series of 4-YC6H4SH and 4-YC6H4SCH3 compounds by using a B3P86 methology and havefurther confirmed that the S−H/S−CH3 TSEs are dominated by the RSEs. Good linear correlations wereobtained for TSE = ρ+σp+(Y), with ρ+ (kcal mol-1) = 3.5 (S−H) and 3.9 (S−CH3). It is also concluded thatthe SH substituent is a rather strong electron donor with a σp+(SH) of −0.60, and that the literature value of−0.03 is in error. In addition, the SH rotational barriers in 4-YC6H4SH have been computed and it has beenfound that for strong electron donating (ED) Ys, such as NH2, the lowest energy conformer has the S−Hbond oriented perpendicular to the aromatic ring plane. In this orientation the SH becomes an electronwithdrawing (EW) group. Thus, although the OH group in phenols is always in-plane and ED irrespective ofthe nature of the 4-Y substituent, in thiophenols the SH switches from being an ED group with EW and weakED 4-Ys, to being an EW group for strong ED 4-Ys.
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