Structure and E-Z isomerization of alpha-pyridylcinnamic acids studied by ab initio and semiempirical methods

Cinnamic acids containing a pyridyl group with variously positioned nitroge n in the position a relative to the carboxylic group were studied at the le vel of semiempirical quantum chemical and ab initio molecular orbital metho ds. Comparison of the total energies or standard enthalpies of formation da ta in the fully optimized structures of the stereoisomer pairs revealed tha t their thermodynamic stabilities are not dramatically different at the HF/ 3-21 G(*) level and negligible at the level of semiempirical quantum chemic al methods (AM1, MNDO, PM3). Structures computed at ab initio level are rep orted. The E-Z (and Z-E) isomerization reactions of the neutral molecules i n the gas phase are investigated at the semiempirical quantum chemical leve l of theory (AM1, MNDO and PM3). Reaction and activation enthalpies for the configurational isomerization reaction were computed and the transition-st ate structures were determined.