High-level ab initio and density functional theory evaluation of combustion reaction energetics: NO2 and HONO elimination from dimethylnitramine

Dimethylnitramine (DMNA) is used as a model system for investigating accura te and efficient electronic structure methods for nitramines. Critical poin ts on the potential energy surfaces of DMNA, CH3NCH3, CH3NCH2, NO2, MONO, a nd the transition state to HONO elimination were located through geometry o ptimizations using the B1LYP, B3LYP, MPW1PW91, and BH&HLYP density function al methods, in addition to MP2, G2(MP2), and QCISD abinitio theories using the cc-pVDZ basis set. For cost-effective determination of nitramine reacti on energetics, highly correlated single-point calculations at DFT geometrie s are recommended. Our best estimated reaction enthalpies for N-N bond scis sion and HONO elimination are 41.6 and -0.9 kcal/mol, respectively, determi ned 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 s aving considerable computational cost without sacrificing accuracy. Using t he same strategy, the transition state energy for HONO elimination can be m odeled to within 0.1 kcal/mol of the QCISD(T)//QCISD result.