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The experimental assignment of IR non-fundamental bands can be assisted by calculation of both frequencies and intensities, as shown in this work on diazomethane. The ab initio B3LYP method is used to obtain the anharmonic force ®elds up to the fourth order. The anharmonic vibrational wave functions have been calculated using a variation± perturbation algorithm. The dipole moment expansion needed in the evaluation of absolute intensities is limited to the ®rst derivatives. The results, including those for overtone, combination and dierence bands disagree with some experimental attributions and complement the available experimental data.

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