Vibrational behavior of the -NO2 group in energetic compounds

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.