Ab Initio and Nonlocal Density Functional Study of 1,3,5-Trinitro-s-triazine (RDX) Conformers

Abstract

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 (α-RDX). The AAA and EEE conformers have C 3 v symmetry, a symmetry consistent with vapor and β-solid infrared spectra. The AAE and AAA conformers are studied at the MP2/6-31G*, B3LYP/6-31G*, and B3LYP/6-311+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 β-solid data, supporting the hypotheses derived from experiment that the AAA structure is the most probable conformer in vapor-phase and β-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.

An ab Initio Study of Solid Nitromethane, HMX, RDX, and CL20: Successes and Failures of DFT

“…Table 5: Mean, percent mean, absolute mean, standard deviation, and maximum error in bond distances versus experiment for PW91, MP2, and B3LYP for the gas phase RDX molecule. (Tables 5 and 6) (Rice and Chabalowski, 1997), we see that by 380 eV, PW91 with plane waves yields similar results to MP2/6-31G*. There is little improvement in going to a larger E cut , as the results have converged by 380 eV.…”

“…Density functional theories exhibit reasonable accuracies coupled with computational efficiency, and for such reasons have been used to study, both statically and dynamically, large molecular systems. Previous work for energetic species utilizing density functional theories has ranged from gas phase structure studies of RDX (Rice and Chabalowski, 1997) to shocked nitromethane calculations (Reed et al, 2000;Smith and Bharadwaj, 1999). Density functional theory has also been used in the charting of the unimolecular decomposition pathways for both RDX (Chakraborty et al, 2000) and HMX (Chakraborty et al, 2001).…”

Predicting heats of formation of energetic materials using quantum mechanical calculations

“…This modest level of theory was chosen due to the size of some of the molecules used in the study. The 6-31G* basis set has been shown to be reasonably accurate when used with the B3LYP density functional [16,30]. The B3LYP density functional has been shown to reproduce experimental properties and is commonly thought to be one of the most reliable of the available density functionals [1,8,31].…”

Recent Advances and Remaining Challenges for the Spectroscopic Detection of Explosive Threats

“…49,50 The Raman spectra and crystal structure have been studied in detail and vibrational mode assignments suggested. [50][51][52] The HMX is similar to RDX except that it contains an eight-membered ring instead of a six-membered ring. The most stable phase is denoted as b-HMX and is the one that is present in bulk explosives.…”