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, beta-HMX, molecule and for the HMXR radical which results from the fission of an N-NO2 bond in beta-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 fort-nation were derived from enthalpies computed for selected isodesmic and isogyric reactions. The values for beta-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 beta-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 beta-HMX is discussed. A comparison of the computed thermodynamics data with other reported estimations is made. (C) 2004 Elsevier B.V. All rights reserved.