Calculation of non-fundamental IR frequencies and intensities at the anharmonic level. I. The overtone, combination and difference bands of diazomethane, H2CN2

The experimental assignment of IR non-fundamental bands can be assisted by calculation of both frequencies and intensities, as shown in this work on d iazomethane. The ab initio B3LYP method is used to obtain the anharmonic fo rce fields up to the fourth order. The anharmonic vibrational wave function s have been calculated using a variation-perturbation algorithm. The dipole moment expansion needed in the evaluation of absolute intensities is limit ed to the first derivatives. The results, including those for overtone, com bination and difference bands disagree with some experimental attributions and complement the available experimental data. (C) 2001 Elsevier Science B N. All rights reserved.