In-plane and out-of-plane conformational preferences of the sulfhydryl group in some halothiophenol derivatives

Long-range spin-spin coupling constants between sulfhydryl protons and ring protons in some halothiophenol derivatives in CCl 4 solutions are reported. In contrast to the corresponding phenol derivatives, substantial amounts of out-of-plane conformers are present at 305 K for all but 2,6-dichlorothiophenol. The cis and trans conformers differ by only about 0.2 kcal/mol in free energy for 2,4-dibromothiophenol and 2,4-dichlorothiophenol, in good agreement with a recent analysis of the dipole moment observed for the latter compound. The barrier to internal rotation of the sulfhydryl group is considerably smaller than for a hydroxyl group and rough estimates are given for the barrier in a few compounds. For example, the barrier in 2,3,5,6-tetrafluorothiophenol is lower than in 2,6-dichlorothiophenol. STO-3G MO calculations overestimate the internal barrier to rotation of the sulfhydryl group, but yield charge densities for this group which indicate that a major cause of the relative weakness of its intramolecular hydrogen bonds resides in its lack of polarity.