Anharmonic vibrational spectra of hydroxylamine and its 15N, 18O, and deuterium substituted analogs

Abstract

Vibrational self-consistent field (VSCF) and correlation-corrected vibrational self-consistent field (CC-VSCF) methods were used to compute the anharmonic frequencies of fundamentals, overtones, and combination transitions of natural abundance hydroxylamine, 15NH 2OH, NH 2 18OH, ND 2OD, ND 2OH, and NH 2OD isotopomers at second order Møller–Plesset perturbation theory (MP2) in basis sets of triple- ζ quality. Frequencies of the fundamental transitions observed in the gas phase spectrum were reproduced by CC-VSCF treatment within 20 cm −1 in TZV(d,p) and TZV(2d,2p) basis sets, and the change of basis set composition had only minor effect on the frequencies of the computed fundamentals. CC-VSCF computed wave numbers of overtone and combination transitions were typically within 1–40 cm −1 of the gas phase band positions, except for those resulting from multiple excitations of ν 2, ν 3, and ν 7 fundamentals, because of a strong mutual coupling between these modes. Integral transition intensities calculated at MP2 level closely followed those of experimental spectrum, including intensity decrease in ν 1, 2 ν 1, 3 ν 1 progression, and 30-fold intensity increase of 2 ν 8 in respect to that of ν 8 fundamental. The frequency of the OH torsional fundamental was found to be strongly dependent on the mode–mode interaction potential among ν 9 and ν 1, ν 7, ν 2, ν 4, ν 5 modes. Band shifts resulting from 15N, 18O and complete 2H substitutions were reproduced almost quantitatively by CC-VSCF computation in TZV(d,p) basis. Computed anharmonic isotope frequency shifts were different from those obtained in the harmonic approximation and no scaling procedure seemed capable of performing their interchange.