Theoretical Study On RDX Thermal Decomposition In Gas Phase And Effect Of Solvents On RDX Decomposition

There are many different views on the decomposition mechanism of nitramine explosives due to the fact that experimental studies were largely affected by external factors. In order to clarify and verify the conclusions of experimental studies from the theoretical point of view we investigated the decomposition mechanism of RDX in gas phase and the effect of solvents on the decomposition of RDX in the present study.At first, we investigated the N-N bond dissociation energies of the simplest model compounds of nitramines, namely DMNA and NH2NO2, by employing various methods at different levels. By comparing the results obtained by B3LYP/6-311g(3df,p)//6-311g(d,p) and MP2/6-311g(3df,p)//B3LYP/6-311g(d,p) methods with high accuracy methods, the corrected method is proposed. According to the form of Morse function, we found that the PESs of N-N bond dissociation reaction in large molecule can be obtained by fitting Morse parameters using a polynomial function. By means of the two methods, namely the correction of N-N bond dissociation energies and the fitting of PES of N-N bond dissociation reaction, the accuracy of rate constant calculation can approach the results based on the PES of the much expensive method CCSD(T)/6-311g(3df,p), but the cost is the same cheap as the MP2/6-31 lg(3df,p) method.By employing the DFT and reaction kinetics methods, two initial decomposition channels HONO elimination and N-N bond dissociation in DMNA molecule were investigated. The results show that N-N bond dissociation is the dominant initial step of gas-phase decomposition in energy and kinetics. Using kinetics methods, based on the potential surfaces of N-N bond dissociation reaction in DMNA obtained by the high level ab initio method (CCSD(T)/6-311g(2df,p)) and the fitting method, the rate constants of N-N bond dissociation reaction were calculated and the results were both well agreed to the experimental values. This demonstrates that our fitting method has a comparative accuracy with high level ab initio methods and also supports the conclusion that N-N bond dissociation is the dominant initial steps of gas-phase DMNA decomposition.The phenomena that NO₂ can accelerate the decomposition of the gas-phase DMNA were observed in experiments, yet the mechanism is still not clear. We studied the reaction between NO₂ and DMNA and found that NO₂ can lower the barriers of the DMNA decomposition. The catalysis mechanism of NO₂ to the decomposition of DMNA, by whichthe autocatalysis phenomena in DMNA decomposition can be explained very well, was presented theoretically.The isomers of gas-phase RDX were investigated, and the calculated results support that there is a rapid transformation each other between AAA and AAE isomers. The possible two initial channels of thermal decomposition of the most stable isomer AAE-RDX, N-N bond dissociation and HONO elimination were calculated and found that the energy required in N-N bond dissociation was close to that of the channel HONO elimination. But the kinetic results obviously show that N-N bond dissociation is the main initial channel in gas-phase RDX decomposition. Using the potential surface of N-N bond dissociation given by our correction-fitting methods the rate constant was also calculated. The results are well consistent with the experimental values. This once again shows that N-N bond dissociation is the main reaction in the initial decomposition of RDX and demonstrates the correctness of fitting methods.The studies of the interactions of RDX molecule with methanol, acetone and benzene molecules explicated that the hydrogen bond formed between H in solvent molecules and O in RDX would affect the rate of RDX decomposition in the solvents. The stronger the hydrogen bond is, the larger decomposition rate of RDX in the solvent is. The X-H(X=C or O) bond dissociation energies in 29 solvent molecules were calculated and found that the rate constant of RDX decomposition in solvent is related to the bond dissociation energies of the weakest X-H bond in the solvent molecule: in the active solvent, all X-H bond dissociation energies is smaller than 420kJ/mol. The position-block effect and the numbers of the weakest X-H bond also have significant effects on the decomposition rate of RDX in solvent.In the final part, we studied the N-N bond dissociation in the other two important nitramines: HMX and CL-20. It was pointed out that the N-N bond is the easiest broken positions. The rate constants of N-N bond dissociation were calculated. For HMX the result is excellently agree with experimental values. The predicted value of rate constants for the N-N bond in CL-20 is much larger than that of HMX and RDX at 473K, which is consistent with the poor stability of CL-20 (among RDX, HMX and CL-20) obtained by experimental methods.