Nuclear magnetic resonance and molecular orbital studies of the conformational preferences of the fluoromethyl group in some benzylfluoride derivatives

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Abstract
The proton magnetic resonance spectra of some benzylfluoride derivatives yield long-range spin–spin coupling constants between ring protons and protons or fluorine nuclei in the fluoromethyl group. In conjunction with the eigenfunctions for a hindered twofold rotor, the couplings over six bonds are used to show that in 3,5-dichlorobenzylfluoride in solution the C—F bond prefers the benzene plane by 260 ± 50 cal/mol; in close agreement with ab initia and MINDO/3 molecular orbital calculations. The latter method suggests that in a conformation in which the C—F bond lies in a plane perpendicular to the benzene ring, the C – C – F angle reduces to 107.2° and the C – C – H angles become 116.1°, perhaps due to increased conjugation of the C—F bond or fluorine atom with the π electrons of the ring. The observed barrier is presumably a delicate balance between steric interactions, hyperconjugation or p–p conjugation effects, and dipole–dipole interactions between polarized bonds.

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