Theoretical Studies of Methyleneamino (CH2N) Radical Reactions. 1. Rate Constants and Product Branching Ratios for the CH2N + N2O Process by ab Initio Molecular Orbital/Statistical Theory Calculations

Abstract

High level molecular orbital calculations have been performed in the framework of the G2M method to study the kinetics and mechanism for the bimolecular reaction of CH2N with N2O, one of the key reactions considered in the RDX combustion modeling. Three different reaction channels have been identified for this reaction. The direct abstraction reaction, which occurs via transition state TS1 producing CH2NO and N2, has an activation barrier of 42.4 kcal/mol. A transition state theory calculation employing the predicted energies and molecular parameters gave rise to rate constant k(CH2NO) = 2.84 × 10-11 exp(−24900/T) cm3/(molecule·s) in the temperature range 1000−3000 K. The CH2N + N2O reaction can also produce CH2N2 and NO either by the formation and decomposition of CH2NN2O intermediate LM1 via TS2 and TS3 or from the cyclic intermediate LM3 via transition state TS7. LM1 can also be formed from its isomer LM2 via TS5, which appears as an additional reaction path for the formation of CH2N2 and NO through the same TS3 transition state. LM3 can be formed by direct side-on addition of N2O to CH2N via a five-membered ring transition state (TS6) with a barrier of 37.3 kcal/mol. The RRKM theory predicted the total rate constant for the formation of CH2N2 by these three channels: k(CH2N2) = 9.93 × 10-12 exp(−22000/T) in the same temperature range. The cyclic intermediate (LM3) can also undergo stepwise decomposition to form endothermic products CH2O and N3 via TS8, intermediate LM4 and TS9; the predicted maximum barrier of this path is 32.9 kcal/mol at TS8 with respect to the reactants. The results of RRKM calculations carried out at various temperature and pressure conditions indicate that the CH2O + N3 channel is least competitive, whereas the CH2N2 + NO channel dominates the reaction up to 2500 K.