High-Level ab Initio and Density Functional Theory Evaluation of Combustion Reaction Energetics: NO 2 and HONO Elimination from Dimethylnitramine Michael A. Johnson and Thanh N. Truong* Henry Eyring Center for Theoretical Chemistry, Department of Chemistry, UniVersity of Utah, 315 S 1400 E, Room Dock, Salt Lake City, Utah 84112 ReceiVed: July 20, 1999 Dimethylnitramine (DMNA) is used as a model system for investigating accurate and efficient electronic structure methods for nitramines. Critical points on the potential energy surfaces of DMNA, CH 3 NCH 3 , CH 3 - NCH 2 , NO 2 , HONO, and the transition state to HONO elimination were located through geometry optimizations using the B1LYP, B3LYP, MPW1PW91, and BH&HLYP density functional methods, in addition to MP2, G2(MP2), and QCISD ab initio theories using the cc-pVDZ basis set. For cost-effective determination of nitramine reaction energetics, highly correlated single-point calculations at DFT geometries are recommended. Our best estimated reaction enthalpies for N-N bond scission and HONO elimination are 41.6 and -0.9 kcal/mol, respectively, determined at the QCISD(T)//QCISD level of theory. These numbers can be reproduced to within 1.3 kcal/mol for the N-N bond and to within 0.5 kcal/mol for the HONO reaction by calculating QCISD(T) energies at B1LYP geometries, thus saving considerable computational cost without sacrificing accuracy. Using the same strategy, the transition state energy for HONO elimination can be modeled to within 0.1 kcal/mol of the QCISD(T)//QCISD result. 1. Introduction Accurate numerical models for combustion of high-energy (HE) materials have safety and educational benefits and are viable alternatives to dangerous experimental tests. Indeed, modeling an explosion is a challenging problem that requires extensive atomic-scale information such as detailed chemical reaction mechanisms with associated thermodynamic and kinetic parameters, many of which are still not known. Understanding the decomposition of HE molecules is a key element in the simulation of explosions and combustion of propellants. The cyclic nitramines HMX ((CH 2 NNO 2 ) 4 ) and RDX ((CH 2 - NNO 2 ) 3 ) are universal HE ingredients in the manufacture of propellants and explosives. Although HMX and RDX have unique decomposition pathways that may involve several hundred fundamental chemical reactions, NO 2 and HONO are believed to be common intermediates in their decomposition. 1-3 To illustrate this, Figure 1 shows a mechanism proposed by Zhao et al. for the unimolecular decomposition of RDX that is consistent with IRMPD experiments. 4 Symmetric triple dis- sociation was identified as the dominant primary reaction channel accounting for two-thirds of the decomposition. The remaining one-third undergoes N-N bond scission, and 77% of that one-third continues to decompose through HONO elimination; therefore, approximately one-fourth of RDX de- composition in these experiments proceeds via N-N bond scission followed by HONO elimination. Such information may be combined with the associated energetics and used directly as input data for combustion models 2 or used in conjunction with transition state theory to estimate reaction rate constants for use in future simulations. 5 In the absence of experimental data, thermodynamic and kinetic parameters must be calculated. Because of the large number of reactions involved, it is important to employ a methodology in building a database of reaction parameters that provides both efficiency and accuracy. The main limitation of accurate ab initio molecular orbital (MO) methods is their cost, scaling as N 4 or greater, where N is the number of basis functions. Consequently, the use of large basis sets and high levels of correlation needed to generate accurate chemical kinetics data for many reactions involving large molecules is not presently feasible in the context of an ab initio MO methodology. Alternatively, density functional theory (DFT), with a formal scaling of N 3 , provides cost-effective access to reasonably accurate ground-state chemical properties. However, current DFT methods do not perform as well in the * Corresponding author. Figure 1. Initial steps in the unimolecular decomposition of RDX proposed on the basis of IRMPD experiments. 8840 J. Phys. Chem. A 1999, 103, 8840-8846 10.1021/jp9925029 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/15/1999