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Effects of entropy on the gas‐phase pyrolysis of ethyl N,N‐dimethylcarbamate

In this study, we examined the gas‐phase pyrolysis of ethyl N,N‐dimethylcarbamate theoretically at various theoretical levels. The reaction consists of a two‐step mechanism, with N,N‐dimethylcarbamic acid and ethylene as reaction intermediates. In the first step, the reaction proceeds via a six‐membered cyclic transition state (TS), which is more favorable than that via a four‐membered cyclic TS. Here, the contribution of entropy to the overall potential energy surface was found to play an important role in determining the rate‐limiting step, which was found to be the second step when viewed in terms of the enthalpy of activation (ΔH≠), but the first step when entropy changes (−TΔS≠) were considered. These results are consistent with experimental findings. Moreover, the experimental activation entropy can be reproduced by using the hindered rotor approximation, which converts some low vibration frequencies that correspond to internal rotational modes into hindered rotors. © 2006 Wiley Periodicals, Inc. J Comput Chem 28: 625–631, 2007