Vibrational spectroscopy of tolane; Coriolis coupling between Raman-active modes of <i>g</i> symmetry

<p>Vibrational 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 <i>a_g</i> modes, has now been attributed to Coriolis coupling within modes of <i>g</i> symmetry species. Consequently, <i>D_2h</i> 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 <i>a_u</i> symmetry species, are IR- and Raman-inactive transitions, and ν₅₉(<i>b_2u</i>), which is predicted to have a very low wavenumber.