Molecular Structure in Solution: An ab Initio Vibrational Spectroscopy Study of Phenyloxirane

We report a study of the structure of phenyloxirane ( 1 ) in solution using ab initio vibrational spectroscopy. Vibrational unpolarized absorption and circular dichroism spectra of 1 in CCl 4 and CS 2 solutions are measured. Harmonic potential energy surfaces and, thence, harmonic frequencies, dipole strengths, and rotational strengths of the fundamental modes of 1 are calculated ab initio using Hartree−Fock/self-consistent field (HF/SCF), MP2 and density functional theory (DFT) methods. DFT/B3LYP/TZ2P vibrational spectra are in excellent agreement with experimental spectra; alternative combinations of functional and basis set yield less good agreement. HF/SCF and MP2 calculations yield spectra in even poorer agreement with experiment. The DFT/B3LYP/TZ2P equilibrium structure is therefore the most accurate. The plane of the phenyl ring is tilted from perpendicular to the plane of the oxirane ring by 14.5°. The direction of tilt brings a phenyl ortho H closer to the oxirane O. The direction and magnitude of tilt can be understood in terms of the competing forces of pseudoconjugation, C−H···O hydrogen bonding and H···H van der Waals repulsion. The DFT/B3LYP/TZ2P dipole moment agrees with experiment within experimental error.