DFT calculations of triethyl and trimethyl orthoacetate elimination kinetics in the gas phase

The reaction paths for the gas-phase molecular elimination of triethyl and trimethyl orthoesters were examined at B3LYP/6-31G(d,p), B3LYP/6-31G++(d,p), B3PW91/6-31G(d,p), B3PW91++G(d,p), MPW1PW91/6- 31G(d,p), and MPWlPW91/6-31++G(d,p) levels of theory. The thermal decomposition of ethyl and methyl orthoesters involves similar transition state configurations in a four-membered ring arrangement. Products formed are ethanol and the corresponding unsaturated ketal for ethyl orthoesters, while in methyl orthoesters are methanol and the corresponding unsaturated ketal. Calculated thermodynamic and kinetic parameters from B3LYP calculations were found to be in good agreement with the experimental values. The calculated data imply the polarization of the C₃-O₄, in the direction C₃ ^δ+ 0₄ ^δ-, is rate determining. The NBO charges, bond indexes, and synchronicity parameters suggest the elimination reactions of ethyl orthoesters occur through a more polar asynchronic mechanism compared to methyl orthoesters.