The conformation and internal rotational barrier of benzyl fluoride

The twofold barrier for internal rotation between the phenyl and CH2F moieties in the benzyl fluoride molecule was recently experimentally estimated at just 0.7 kJ/mol, with the minimum energy conformation having the C(sp2)–C(sp3)–F plane orthogonal to the phenyl ring. Earlier theoretical studies do not reproduce this behaviour, with medium-sized basis set MP2 calculations instead predicting the minimum energy conformation to have the C–F bond in the ring plane. By improving the basis set through the introduction of diffuse functions, we show that MP2 does predict the orthogonal conformation to be lower in energy, in line with the experiment. Analogous observations are made using density functional theory, where our best calculation predicts an orthogonal conformation, with an internal rotational barrier of 1.6 kJ/mol.