Ab initio and experimental investigation of the vibrational energy pattern in N 2O 4: the mid and near infrared ranges

Abstract

Extensive ab initio calculations are performed concerning N 2O 4 in the ground electronic state, at the MP4 and CMRCI levels of theory, using the 6-31G* and cc-pVTZ basis sets. Special care is devoted to internal degrees of freedom characterizing the quasi van der Waals NN bond, namely the torsion and the NN stretch vibrational modes. The role of the methodology is investigated with the help of previous literature results. It is shown that most properties are significantly better described by MP4 than by MP2, but that a variational multireference method, like CMRCI, is however required for all properties linked to the NN bond. The ab initio vibrational analysis is merged with the one of absorption spectra recorded at medium resolution between 700 and 7000 cm −1, using a Fourier transform interferometer. The resulting set of effective vibrational frequencies and anharmonicities is provided. The height of the torsional potential energy barrier is calculated to be 2000 cm −1, at the best level of theory (CMRCI/cc-pVTZ), in agreement with the highest experimental value available.