Computational and experimental infrared spectra of 1,4-dinitropiperazine and vibrational mode assignment

The IR spectra of both solid and matrix-isolated 1,4-dinitropiperazine (p-D NP) were obtained. The results indicate that 1,4-dinitropiperazine at 11 K is a mixture of three chair conformer structures and a single twisted-boat conformer. Vibrational modes of each conformer were computed using the B3LY P and BP86 density functional methods. The computed frequencies are used to assign vibrational mode symmetries to the spectra obtained experimentally. The conformer populations obtained by fitting experimental data with a wei ghted-sum spectrum generated from individual calculated conformer spectra w ere compared with calculated thermodynamic conformer populations at the exp erimental temperature. The energies of transition states between conformers were also calculated to clarify that the energy barriers to transformation between the three chair conformers are small enough to allow interconversi on at the experimental temperature. Finally, calculated bond lengths and an gles are compared to structural parameters obtained from single-crystal X-r ay diffraction of p-DNP.