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The gas-phase pyrolysis of methyl azidoformate in the absence and presence of water: a theoretical study

ABSTRACT The potential energy surface profiles for the gas-phase pyrolysis of methyl azidoformate (MA, CH3OC(O)N3) in the absence and presence of one water molecule have been investigated by ab initio methods at CCSD(T)/6-311++G(2df,2pd)//MP2(full)/6-311++G(d,p)+0.95×ZPE levels of theory. Three types of mechanisms are discussed for the gas-phase decomposition of CH3OC(O)N3. Ab initio calculations show that a four-membered-ring intermediate can be formed by the stepwise routes. The resulting intermediate can undergo two competitive decomposition channels to generate the major products CO2 + CH2 = NH and HNCO + HC(O)H. The calculated results are in qualitative agreement with the observed experimental data. However, CH3ONCO can be produced from the Curtius-type rearrangement route. This is an intriguing finding in this study. Moreover, the effect of one water molecule on the gas-phase pyrolysis of MA has been also explored. We find that the relative energy of the hydrated transition states is effectively lowered when water is added to the reaction. However, the estimated rate constant at 625 K for the naked reaction is about 30 times faster than the reaction with water. Thus, a single water molecule cannot play an important role in the thermal decomposition of MA. GRAPHICAL ABSTRACT