The solvent dependence of the small internal barrier to rotation in 3,5-dichlorobenzyl fluoride

The 1H and 19F nuclear magnetic resonance spectra of 3,5-dichlorobenzyl fluoride, dissolved in solvents of varying polarity, are analyzed to yield precise values of the long-range coupling constants between ring protons and the nuclei in the fluoromethyl group. The conformational properties of these coupling constants, dependent on the angle by which the C—H and C—F bonds of the side chain twist out of the aromatic plane, are used to determine the solvent dependence of the predominantly twofold barrier to rotation about the Csp2—Csp3 bond. In all solvents, the conformer in which the C—F bond lies in the aromatic plane represents the minimum in the internal rotational potential. The magnitude of the twofold potential changes from 3.2(2) kJ/mol in the nonpolar medium, C6D12, to 0.6(2) kJ/mol in the polar solvent, CD3CN. Intermediate magnitudes occur for solvents of intermediate dielectric constant. Comparisons are made with the parent molecule, benzyl fluoride, for which the perpendicular conformer is most stable in the same solvents. Such comparisons indicate that for the free molecules, the in-plane conformer of the dichloro derivative is also more stable than the same conformer of benzyl fluoride. This indication is in qualitative agreement with molecular orbital predictions.