Importance of the consideration of anharmonic motion in charge-density studies: a comparison of variable-temperature studies on two explosives, RDX and HMX.

Extremely accurate X-ray data were obtained for the explosive RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) at three different temperatures (20, 120 and 298 K). Collected reflections were integrated using the latest version of the program VIIPP which uses separate Kα(1)/Kα(2) contributions to the profile fitting during integration. For each temperature both anharmonic and harmonic descriptions of the atomic thermal motion were utilized in the model refinements along with the multipole expansion of the electron density. H atoms were refined anisotropically and agree well with a previous neutron study. Topological analysis [Bader (1990). Atoms in Molecules: A Quantum Theory. The International Series of Monographs of Chemistry, edited by J. Halpern & M. L. H. Green. Oxford: Clarendon Press] of the attained electron density followed. For 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX), old data collected at 20 and 120 K were re-integrated with the new version of VIIPP and refined in the same manner as for RDX. In both cases theoretical structure factors were also calculated based on the 20 K structures, and employed in comparison multipole refinements for the atoms at rest. Limiting the refinement to a harmonic model of the atomic displacements may result in a biased and erroneous electron density, especially when atomic vibrations are significant (as in RDX) and at temperatures higher than obtained by using liquid helium. Given the similarity of the two compounds the effects of anharmonic motion are strikingly more severe in the case of RDX. Our study reinforces the conclusion of Meindl et al. [Acta Cryst. (2010), A66, 362-371] that in certain cases it is necessary to include anharmonic term(s) of the probability density function (or temperature factor) in order to obtain a meaningful electron density suitable for topological analysis, even for compact (high-density) light-atom structures. For RDX it was observed that the oxygen lone-pair concentrations of electrons are located close to perpendicular to the N-O bond vectors, which is typical for explosive materials. Conjugation of the electron density in the -N-NO(2) fragment has been established based on the topological bond orders. Nine moderately strong hydrogen bonds and nine N-N, O-N and O-O bonding interactions were found and described. The RDX molecular electronic energy per mole is 4.02-4.04 a.u., very close to the reported value for HMX.