The molecular structure of benzyl fluoride — a combined theoretical and experimental study

Abstract

The molecular geometry and conformation of benzyl fluoride have been studied by ab initio and molecular mechanics calculations, by NMR and by gas electron diffraction. The CPh dihedral angle, θ, was shown by the latter method to be 52.3°. The observed vibrational amplitudes indicate mean oscillations of ca. 7° about the equilibrium CPh dihedral angle. The NMR study gave coupling constants for benzyl fluoride consistent with either a single conformation with the CPh dihedral angle equal to 54° or a preferred non-planar conformation with a small rotational barrier. The molecular mechanics calculations gave a minimum energy conformation at θ ≅ 60° and a barrier to rotation of ca. 0.4 kcal mol −1. The ab initio calculations were also carried out for several values of CPh dihedral angle, θ. The energy profile as a function of θ was found to be very flat with the planar conformer being most stable with an energy barrier of 0.24 kcal mol −1.