The conformation and internal rotation barrier in benzyl fluoride from its microwave spectrum

Microwave spectra of benzyl fluoride have been observed on a pulsed-jet Fourier transform microwave spectrometer in the range 4-25 GHz. Many of the rotational transitions display tunneling splittings from internal rotation between the phenyl and CH2F groups. Rotational and centrifugal distortion constants were determined for the ground and first excited torsional states which differ in energy by 3.426(2) MHz. That tunneling splitting is consistent with a twofold torsional barrier of 58.2(4) cm(-1). The minimum energy configuration has the C-C-F plane orthogonal to the plane of the phenyl ring. This is proved by the observed planar moment, P-CC, which is 16.88 u-Angstrom(2), a value compatible with only this geometry, and supported by the fact that only a-and c-type (not b-type) transitions were observed. (C) 1997 Academic Press.