Vibrational Behavior of the −NO 2 Group in Energetic Compounds

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 meta and ortho/para 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.