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

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 CF 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 CF 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 CF 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.