Substituent Effects on the Gas-Phase Pyrolyses of 2-Substituted Ethyl N,N-Dialkylcarbamates: A Theoretical Study

The Rand Z-substituent effects for the gas-phase thermal decompositions of carbamates, R2NC(=O)OCH2CH2Z, have been investigated theoretically at B3LYP level with 6-31G(d) and 6-31++G(d,p) basis sets. Both the Zand R-substituent effects on reactivity (ΔH≠) were well consistent with experimental results, although the R-substituent effect was underestimated theoretically. No correlations were found between activation enthalpies and reaction enthalpies. The substituent effects on reactivity seemed to be complicated at a glance, but were understandable by concurrent electronic and steric factors. Variations of bond lengths at TS structures were well correlated with the Taft’s σ values and the TS structures became tighter as the Zsubstituent became a stronger electron-acceptor (δσ > 0). However the effects of R-substituents on the TS structures were much smaller when compared to those of Z-substituents.