Vibrational spectroscopy of tolane; Coriolis coupling between Raman-active modes of g symmetry

Abstract

ABSTRACTVibrational spectroscopy of tolane (diphenylacetylene), which has 66 normal modes, has been advanced. Anharmonic wavenumber predictions were made with the quartic potential energy surface obtained with B3LYP/cc-pVTZ model and the second-order perturbation theory (VPT2). Infrared (IR) intensity and Raman activities were computed at the harmonic level. The IR spectrum of the crystal and Raman spectra of the liquid and the crystal tolane were newly recorded. The lingering problem of an excess of polarised Raman bands at wavenumbers appropriate for fundamentals, other than ag modes, has now been attributed to Coriolis coupling within modes of g symmetry species. Consequently, D2h point symmetry group has been confirmed for a planar tolane molecule. Assignments for almost all fundamentals of tolane are now secure. The assignment for ν32 remains questionable. Remaining unassigned fundamentals are: ν34 and ν35, which, as au symmetry species, are IR- and Raman-inactive transitions, and ν59(b2u), which is predicted to have a very low wavenumber.