Abstract

The vibrational modes of the −NO₂ group in more than fifty energetic compounds containing the C-nitro and N-nitro functionalities were observed and then calculated in optimized structures using density functional theory (B3LYP/6-31+G^*). The trends in the symmetric and asymmetric stretches and scissor and out-of-plane deformations were explained by these calculations. A previously unreported correlation was found between the nitro group internal bonding angle and its asymmetric stretching frequency. The concept of <i>meta</i> and <i>ortho</i>/<i>para</i> directing groups was applicable to the trends in coupled motions in the nitroaromatic compounds. Both the scissor motion of C–NO₂ groups and the out-of-plane deformation of N–NO₂ groups were found to be virtually insensitive to the remainder of the molecule. These findings may be useful in analytical methods of explosive detection based on their infrared (IR) spectra.

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