Vibrational Spectra and Structure of Azomethane and Azomethane-d6

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Abstract
The infrared spectra of gaseous and polycrystalline CH3NNCH3 and CD3NNCD3 have been recorded from 33 to 4000 cm−1. The corresponding Raman spectra of the liquid and solid samples have been measured from 100 to 4000 cm−1. The vibrational data provide definitive verification of a trans structure with C 2h symmetry. A vibrational assignment is proposed for the 24 fundamental modes based upon the observed band contours, isotopic shift ratios, and group frequency correlations. The au and bg torsional modes were assigned to bands at 222 and 223 cm−1, respectively, and the internal rotational barrier of the methyl group was calculated to be ∼ 2.7 kcal / mole . A number of the internal fundamentals were observed to be split into doublets in the spectra of the solids which suggests that there are at least four molecules per unit cell in azomethane. The inversion center is apparently retained in the crystal since the rule of mutual exclusion was found to be operative.