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 a b initio vibrational spectroscopy. Vibrational unpolarized absorption and c ircular dichroism spectra of 1 in CCl4 and CS2 solutions are measured. Harm onic potential energy surfaces and, thence, harmonic frequencies, dipole st rengths, and rotational strengths of the fundamental modes of 1 are calcula ted ab initio using Hartree-Fock/self-consistent field (HF/SCF). MP2 and de nsity functional theory (DFT) methods. DFT/B3LYP/TZ2P vibrational spectra a n in excellent agreement with experimental spectra; alternative combination s of functional and basis set yield less good agreement. HF/SCF and MP2 cal culations yield spectra in even poorer agreement with experiment. The DFT/B 3LYP/TZ2P equilibrium structure is therefore the most accurate. The plane o f the phenyl ring is tilted from perpendicular to the plane of the oxirane ring by 14.5 degrees. 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 bond ing and H ... H van der Waals repulsion. The DFT/B3LYP/TZ2P dipole moment a grees with experiment within experimental error.