The para substituent dependence of the internal rotational barrier in benzenethiol

Abstract

. J. Chem. 55, 552 (1977).On the basis of the observed spin-spin coupling constants between the sulfhydryl and ring protons and a hindered rotor treatment of the twofold barrier to internal rotation in a series of para substituted benzenethiol derivatives, it is argued that V2 is essentially zero in p-aminobenzenethiol and is 2.5 + 0.2 kcal/mol in p-nitrobenzenethiol; having intermediate values for the methoxy, fluoro, methyl, and bromo derivatives in solution. The results are based on an assumed relationship between the four-bond and the fictitious six-bond couplings to the sulfhydryl proton. The conclusions are consistent with the observed magnitudes of the couplings oper six and seven bonds, respectively, between the sulfhydryl proton and the fluorine nucleus and the methyl protons in the appropriate derivatives; as well as with the coupling between the sulfhydryl and methyl protons in 4-bromo-3-methylbenzenethiol. The experimental barriers are compared with ab initio molecular orbital calculations of their substituent dependence.TED SCHAEFER et WILLIAM J. E. PARR. Can. J. Chem. 55, 552 (1977).En se basant sur les constantes de couplage spin-spin observees entre les protons sulfhydryles et les protons du cycle et en traitant comme un rotor empCchC la barriere binaire la rotation interne dans une serie de benzenethiols substituts enpam, on suggtre que V2 est pratiquement Cgal a zero dans lep-aminobenzenethiol et a 2.5 i 0.2 kcal/mol dans lep-nitrobenzenethiol; en solution, les valeurs sont intermediaires pour les derives methoxy-, fluoro-, methyl-et bromo-. Ces resultats sont bases sur une relation hypothktique entre les couplages a travers quatre liaisons et sur les couplages fictifs a travers six liaisons pour le proton sulfhydryle. Les conclusions sont en accord avec les ordres de g r a n d e~~r observes respectivement pour les couplages a travers six et sept liaisons entre le proton sulfhydryle et le noyau de fluor et les protons des methyles dans des derives appropries: ces conclusions sont aussi en accord avec le couplage entre les protons sulfhydryles et methyles du bromo-4 methyl-3 benzenethiol. On compare les barrieres experimentales avec les rtsultats de calculs ab inirio d'orbitales moltculaires predisant leur dependance sur les substituants.[Traduit par le journal]

Comparison of the intramolecular hydrogen bonds and of the internal barrier to rotation the hydroxyl and sulfhydryl groups in 2-methoxyphenol and 2-methoxythiophenol

“…This trend was reasonably explained by a change in conjugation of the SH group with the aromatic nucleus. Such substituent induced changes in V2 are supported by STO-3G MO calculations (25) and are also found in analogous phenol derivatives (26).…”

“…In 2-nitrothit suggesting a b; than 3 kcal/mo Now, in 2-n ence between c than 0.2 kcall much smaller hindered rotor (25,27,28) electron acceptors entailed an increase in V2. For example, V2 in 4-nitrothiophenol in solution was over 2 kcal/mol and was near zero in 4-methoxythiophenol.…”

“…In the para derivatives the barrier decreases relative to the parent compounds by about 1 kcal/mol (25,26). In the ortho derivatives, the V2 component increases by about 2 kcal/mol relative to the parent compounds.…”

“…in that event a hindered rotor treatment based on a twofold barrier (25,27,28) becomes meaningful. 6~p S H * H is -0.14 + 0.02 Hz.…”

Concerning the nonexistence of the intramolecular hydrogen bond in 2-nitrothiophenol

“…If nonplanar forms of 2-nitrothiophenol can be ignored, the o-n mechanism does not contribute to 5J,H3SH (16). If this coupling displays the stereospecificity of 5J,H,0H, then 5JcH3SH is 0.0 Hz, i.e., 5JH33SH is zero in l a and 5JH53sH vanishes in lb.…”