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 R, 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 2V 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 R-, 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 2V symmetry conformation and the experimental geometry of molecules in the Rand δ-phase lattices of HMX.

A far- and mid-infrared study of HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) under high pressure

“…The three lines observed near 380, 420, and 440 cm À1 all agree with ambient pressure measurements on polycrystalline b-HMX [24] (the last two lines were also observed in Ref. [6]) and were assigned to delocalized ring vibrations [24]. The first of three lines centered around 630 cm À1 is saturated.…”

“…It is created from the b phase by heating between 159-165°C or above 190°C [3] and at ambient pressure. In both of the a and d phases, the molecules adopt a 'boat' conformation and have very similar vibrational spectra [6]. The stability of the polymorphs is as follows: b > a > c > d [3,4], which is also the order of density.…”

“…Though the phase stability diagram of HMX is poorly-understood, the b phase is generally considered to be the most stable polymorph up to 10 GPa [4,5]. In this phase, the molecules adopt a 'chair' conformation with two NO 2 groups situated on one side of the C 4 N 4 ring and the remaining two NO 2 groups are on the other side giving the molecule inversion symmetry [6].…”

DFT Investigation of Nuclear 14N Quadrupole Coupling Constants for Four HMX Polymorphs

“…In this paper, we will use the theorem to investigate the NQCC and η of HMX polymorphs theoretically, due to the importance of HMX in munitions. Although many spectrum properties of HMX have been found in literature, [9][10][11][12] to our knowledge, the NQR spectrum of 14 N in HMX polymorph is still not available except the NQCC of 14 N in β-HMX. [13][14][15] Its theoretical value of η for 14 N in NO 2 group on the basis of single molecule model deviates greatly from experiment.…”

A first-principles investigation into the hydrogen bond interaction in β-HMX

“…Thus, a systematic investigation into the basic properties of β-HMX as a solid energetic material appears very important in understanding its explosive properties. β-HMX has been intensively studied from both experimental and theoretical sides [6][7][8][9][10][11][12][13][14][15][16][17], including crystal properties, elastic properties, equations of states, and vibrational spectra. Lewis et al [8] examined the electronic structure of the β-HMX using density functional theory (DFT) and predicted the bulk modulus.…”