An ab initio calculation of the intramolecular stretching spectra for the HF dimer and its D-substituted isotopic species

We have carried out an ab initio calculation of the intramolecular stretching spectra (wave numbers and transition moments) of (HF)₂, (DF)₂, and HFDF involving υ₁, + υ₂ ≤ 3, where υ₁ and υ₂ are the local mode quantum numbers for the two intramolecular (HF or DF) stretches. The ab initio surface used as a basis for these calculations has already been published [M. Kofranek, H. Lischka, and A. Karpfen, Chem. Phys. 121, 137 (1988); P. R. Bunker, P. Jensen, A. Karpfen, M. Kofranek, and H. Lischka, J. Chem. Phys. 92, 7432 (1990)], but in the present work we have extended the 1520 nuclear geometry points previously available with 198 points in order to explore further the variation of the intramolecular stretching energies and the dipole moment along the minimum energy (trans tunneling) path. We compute the intramolecular stretching energies and transition moments by making an adiabatic separation of the intramolecular stretching motion and the other vibrational motions of the molecules, and we use the semirigid bender Hamiltonian to average over the trans-tunneling motion. For HFHF, we obtain the fundamental level corresponding to the "free-H" stretch ν₁ at 3925 cm-¹ and that corresponding to the "bound-H" stretch ν₂ at 3874 cm^-1, in very good agreement with the experimental results of 3930.9 and 3868.1 cm^-1, respectively [A. S. Pine, W. J. Lafferty, and B. J. Howard, J. Chem. Phys. 81, 2939 (1984)]. For the higher excited states, we obtain the 2ν₁ energy level at 7674 cm^-1 (7700 ± 20 cm^-1), 2ν₂ at 7570 cm^-1 (7555 ± 15 cm^-1), 3ν₁ at 11 259 cm^-1 (11 260 cm ^-1),and3v2 at 11 085 cm^-1 (11 060 cm^-1), where the experimental values [K. von Puttkamer and M. Quack, Chem. Phys. 139, 31 (1989)] are given in parentheses.