The computed force constants and vibrational spectra of cubane

Abstract

The geometry, complete harmonic force field, and dipole moment derivatives of cubane, C 8H 8, have been calculated at the Hartree-Fock level using a 4–21 Gaussian basis set. The infrared and Raman spectra of cubane and four deuterated derivatives were calculated and compared with previously observed spectra. A set of five scale factors for the calculated force constants was then derived by least-squares fitting of the fundamental vibrational frequencies calculated from the scaled force field to the frequencies obtained by direct experimental measurement. The resulting scaled quantum-mechanical (SQM) force field, containing 73 unique elements, is believed to give an accurate representation of the harmonic vibrational potential of cubane. In most cases, the spectral assignments previously made from purely empirical considerations were confirmed, but a few corrections are proposed. The only major alteration is for an A 2 u mode revised to appear at 1030 cm −1 in the undeuterated molecule. Coriolis constants and approximate infrared intensities are also calculated.