The conformational properties of some phenyl esters. Molecular orbital and nuclear magnetic resonance studies

Extensive, geometry-optimized, STO-3G MO computations on phenyl formate imply a strongly nonplanar Z conformer (C=O bond cis to the phenyl group) at ambient temperatures. The internal barrier to rotation about the C(1)—O bond in this conformer is computed as V/kJ mol−1 = (−5.17 ± 0.27) sin2 θ − (2.42 ± 0.27) sin2 2θ, θ being zero for the planar conformer; the twofold is nearly twice as large as the fourfold component. The expectation value of θ is 58° at 300 K. The spin–spin coupling constants over six bonds between 13C and I9F nuclei in 4-fluorophenyl formate, acetate, propionate, and isobutyrate, as well as in the 2,6-dichloro-4-fluorophenyl acetate, are adduced as evidence for nonplanar conformers of these molecules. The magnitudes of these six-bond coupling constants are consistent with internal barriers to rotation about the C(1)—O bonds, which are similar in magnitude to those given by the computations on the Z conformer of phenyl formate. The energies of the planar and nonplanar E conformers, as well as the interconversion energies for [Formula: see text] isomerization, are computed. Small amounts of the nonplanar E conformer are predicted at ambient temperatures. The 13C chemical shifts and the one-bond 13C, 19F coupling constants are consistent, respectively, with only minor variations in the conformational behavior of the ester moieties caused by the fluorine substituent and by changes in the structures of these moieties themselves.