Scaled quantum mechanical (SQM) force field and theoretical vibrational spectrum for benzonitrile

Abstract

The complete harmonic force field of benzonitrile has been determined by ab initio Hartree—Fock calculations using a 4–21 Gaussian basis set. As force constants are systematically over-estimated at this level, the directly calculated force field was scaled by empirical factors previously optimized for benzene and HCN. Frequencies calculated from this scaled quantum mechanical (SQM) force field confirm the published experimental assignments for benzonitrile, benzonitrile- p- d and benzonitrile- d 5. Aside from the CH (and CD) stretching frequencies, which are strongly affected by anharmonicity, the mean deviation between the observed and calculated frequencies is below 9 cm −1 for each isotopomer. Theoretical i.r. intensities reproduce the main features of the spectra semiquantitatively.