Vibrational spectra of β-cyclotetramethylene tetranitramine and some of its isotopic isomers

The infrared and Raman spectra of the β-polymorph of cyclotetramethylene tetranitramine (commonly known as "HMX" in the explosives field) and its isotopic isomers, HMX-¹⁵NO₂, HMX-¹⁵N(NO₂), HMX-¹³C(50%), and HUX-d ₈, have been recorded in the spectral region between 30 and 3500 cm^-1. Single crystal polarized Raman measurements on β-HMX were also made in order to separate out modes of A _g symmetry from those of B _g symmetry. Sizable correlation field induced splitting of some of the (NO₂) modes was observed. Probable assignments have been proposed for the complex internal vibrations of β-HMX based mainly on the observed isotopic shifts. The spectra of HMX and HMX-d₈ in solutions of dimethyl sulfoxide and acetonitrile have also been recorded. The results indicate that the molecular conformation in solution is centrosymmetric and probably similar to that in the β-HMX crystal.