Anharmonic potential-energy surfaces, vibrational frequencies and infrared intensities calculated from highly correlated wavefunctions

Abstract

Using potential-energy and dipole-moment surfaces calculated by the coupled electron-pair approximation (CEPA) variational calculations have been performed to obtain vibrational term energies, vibrational wavefunctions and infrared intensities for a number of small polyatomic molecules, ions and hydrogen-bonded complexes. Calculated high-overtone intensities of stretching vibrations of HCN are found to be in good agreement with recent experimental values. The astrophysically important C₃O molecule show unusual behaviour with respect to the i.r. intensities of the stretching vibrations. A new potential has been calculated for H₂Cl⁺ and used to obtain significantly improved values for the vibrational frequencies. Predictions are made for the negative ions NH^– ₂, PH^– ₂ and AsH^– ₂ as well as for protonated methyl cyanide. In the latter case, very good agreement was found between the calculated and experimental values for the ν₁ band origin (NH stretch). Stretching vibrational frequencies were calculated for OCHF and OCHCl. In the former case, the computed bathochromic shift in the donor frequency is in excellent agreement with precise experimental values.