Ab initio and nonlocal density functional study of 1,3,5-trinitro-s-triazine (RDX) conformers

Geometry optimizations and normal-mode analyses of three conformers of 1,3,5-trinitro-s-triazine (RDX) are performed using second-order Moller-Plesset (MP2) and nonlocal density functional theory (DFT) methods. The density functional used in this study is B3LYP. The three conformers of RDX are distinguished mainly by the arrangement of the nitro groups relative to the ring atoms of the RDX molecule. NO2 groups arranged in either pseudo-equatorial or axial positions are denoted with (E) or (A), respectively. The AAE conformer has C-s symmetry and is the structure in the room-temperature stable crystal(alpha-RDX). The AAA and EEE conformers have C-3v symmetry, a symmetry consistent with vapor and beta-solid infrared spectra. The AAE and AAA conformers are studied at the MP2/6-31G*, B3LYP/6-31G*, and B3LYP/6-31l+G** levels, and the EEE conformer is studied using the B3LYP density functional and the 6-31G* and 6-311+G** basis sets. The geometric parameters and infrared spectra of the AAA conformer are in good agreement with experimental gas-phase and beta-solid data, supporting the hypotheses derived from experiment that the AAA structure is the most probable conformer in vapor-phase and beta-solid RDX. The B3LYP/6-311+G** structures and simulated infrared spectra are in closest overall agreement with experimental data. The MP2/6-31G* structures and spectra are in poorest overall agreement with experiment.