Theoretical and Experimental Study of the Vibrational Spectra of the α, β, and δ Phases of Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)

Abstract

The vibrational spectra of the α, β, and δ phases of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) have been investigated by complimentary theoretical and experimental approaches. Density functional theory and the scaled quantum mechanical force-field method have been used to calculate the vibrational spectra of the H 8 C 4 N 8 O 8 molecule in C i and C 2 v symmetry conformations in the gas phase. By comparing the calculated molecular vibrational frequencies of the H 8 C 4 N 8 O 8 molecule with the experimental infrared and Raman spectra of HMX crystalline samples, we attempt to assign the intramolecular motions that correspond to the measured bands in the experimental spectra of HMX in α-, β-, and δ-phase crystal lattices. Our analysis of the vibrational spectra verifies that the particular conformation of the H 8 C 4 N 8 O 8 molecule in each crystal lattice of HMX determines, to a great extent, the general pattern of the vibrational spectrum of the crystal lattice. We also compare our detailed motion assignments with the general assignments proposed in previous experimental work. Good agreement is found between the calculated geometry of the H 8 C 4 N 8 O 8 molecule in the C i symmetry conformation and the experimental geometry of molecules in the β-phase lattice of HMX and between the calculated geometry of the H 8 C 4 N 8 O 8 molecule in the C 2 v symmetry conformation and the experimental geometry of molecules in the α- and δ-phase lattices of HMX.