DFT study of the thermochemistry of gas-phase 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (β-HMX)

Enthalpies of formation for the important 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane, β-HMX, molecule and for the HMXR radical which results from the fission of an N-NO2 bond in β-HMX have been determined. For this, hybrid Hartree-Fock/density functional theory at the B3LYP, B1LYP, B3PW91, mPW1PW91 and PBE1PBE levels in conjunction with basis sets ranging from 6-31G(d) to 6-311++G(3df,3pd) were employed. The enthalpies of formation were derived from enthalpies computed for selected isodesmic and isogyric reactions. The values for β-HMX and HMXR determined at the highest level of calculation are 67.9 and 106.4 kcal mol−1. The resulting dissociation enthalpy for the β-HMXR-NO2 bond of 46.7 kcal mol−1 is about 7 kcal mol−1 higher than previous B3LYP results obtained from direct energy calculations. The influence of the electronic correlation energy on the N-N bond strength in β-HMX is discussed. A comparison of the computed thermodynamics data with other reported estimations is made.