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00925njm 22002777a 4500 dc Barbero San Juan, Héctor author Meunier, Antoine author Kotturi, Kondalarao author Smith, Ashton author Kyritsakas, Nathalie author Killmeyer, Adam author Rabbani, Ramin author Nazimuddin, Md author Masson, Eric author 2020 The torsional barriers along the Caryl–Caryl axis of a pair of isosteric disubstituted biphenyls were determined by variable temperature 1H NMR spectroscopy in three solvents with contrasted hydrogen bond accepting abilities (1,1,2,2-tetrachloroethane-d2, nitrobenzene-d5 and dimethyl sulfoxide-d6). One of the biphenyl scaffolds was substituted at its ortho and ortho′ positions with N′-acylcarbohydrazide groups that could engage in a pair of intramolecular N–H⋯O=C hydrogen bonding interactions at the ground state, but not at the transition state of the torsional isomerization pathway. The torsional barrier of this biphenyl was exceedingly low despite the presence of the hydrogen bonds (16.1, 15.6 and 13.4 kcal mol−1 in the three aforementioned solvents), compared to the barrier of the reference biphenyl (15.3 ± 0.1 kcal mol−1 on average). Density functional theory and the solvation model developed by Hunter were used to decipher the various forces at play. They highlighted the strong stabilization of hydrogen bond donating solutes not only by hydrogen bond accepting solvents, but also by weakly polar, yet polarizable solvents. As fast exchanges on the NMR time scale were observed above the melting point of dimethyl sulfoxide-d6, a simple but accurate model was also proposed to extrapolate low free activation energies in a pure solvent (dimethyl sulfoxide-d6) from higher ones determined in mixtures of solvents (dimethyl sulfoxide-d6 in nitrobenzene-d5). Phys.Chem.Chem.Phys., 2020, 22, 20602-20611 1463-9076 https://uvadoc.uva.es/handle/10324/66384 10.1039/D0CP03285A 20602 36 20611 Physical Chemistry Chemical Physics 22 1463-9084 Counterintuitive torsional barriers controlled by hydrogen bonding