Theoretical study of anharmonic vibrational spectra of HNO₃, HNO₃-H₂O, HNO₄: Fundamental, overtone and combination excitations

Vibrational frequencies are computed for the fundamental, OH stretching overtone and combination transitions of HNO₃, HNO₄ and the HNO₃-H₂O complex. The frequencies are computed directly from ab initio MP2 potential surface points, using the correlation corrected vibrational self-consistent field (CC-VSCF) method, which includes anharmonic effects. The results are compared with experimental data. The computed fundamental transitions are in accord with experiment. The main improvement over the harmonic approximation is for the OH stretching frequencies. The OH overtone excitations (up to the 3rd overtone) of HNO₃, HNO ₄ are also in good accord with experiment. For overtone levels near the dissociation threshold, the deviations from experiment are larger, as the VSCF method is unsatisfactory for the extremely large anharmonicities in these cases. Finally, very satisfactory results are obtained for the combination mode transitions. The main conclusions are (1) CC-VSCF seems to work well also for low overtone excitations and for combination transitions. (2) The MP2/TZP potential surfaces, used in the CC-VSCF calculations, are by the test of spectroscopy adequate for these species. The results are encouraging for VSCF calculations of larger, related systems such as HNO₃-(H ₂O)_n, n > 1.