Systematic GVB study of harmonic vibrational frequencies and dipole moment derivatives; the vinyl radical C2H3 and other simple molecules

Structure, harmonic vibrational frequencies, and dipole moment derivatives of methane, ethylene, and acetylene have been obtained from generalized valence bond (GVB) wave function calculations. The results are compared to Hartree-Fock (HF) data, and other correlated wave function data. It is found that the GVB method consistently overemphasizes left-right electron correlation effects, and predicts bond lengths ∼0.01-0.02 Å longer than experiment. However, the calculated harmonic frequencies are within 4.2% of the observed fundamentals and within 3.5% of the experimental harmonic frequencies. Dipole moment derivatives are in semi-quantitative agreement with experiment. The method is used to predict the IR spectrum of the vinyl radical, including a very intense out of plane bending mode with frequency near 1000 cm^-1.