Vibrational spectrum, torsional potential, and bonding of gaseous N2O4

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Abstract
The infrared spectrum of gaseous N2O4 was obtained under conditions which allowed study of both strongly and weakly absorbing features. These spectral features were assigned with the aid of vibration‐rotation band shape theory. Weakly absorbing sequence bands were identified which contain torsional vibrations. The rocking frequency of the NO2 group, the only mode which has not been observed directly, was estimated from a Third Law entropy calculation. The frequencies obtained from this research together with previous Raman work provide, for the first time, a reliable determination of all the vibrational frequencies of N2O4. The observation of the torsional mode frequencies has allowed an internal rotation potential function to be estimated which has a barrier height of 2 to 3 kcal/mole. From this potential function a picture of the bonding in N2O4 and other nitrogen oxides is presented. It is suggested that in addition to the N–N bond, weak σ bonds form between the cis oxygens which stabilize the eclipsed oxygen configurations and tend to lock the N2O4 molecule into a planar configuration. The planarity of N2O3 and the stability of cis (NO)2 can also be explained by these weak oxygen σ bonds.

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