Structure and isomerization of α-phenylcinnamic acid stereoisomers — reaction pathway and transition-state structure by semiempirical methods

The isomerization reaction of E- and Z-α-phenylcinnamic acid molecules was studied at the level of semiempirical quantum chemical methods (MNDO, AM1 and PM3). Calculations revealed that Z-α-phenylcinnamic acid is slightly more stable than the E isomer. The activation enthalpy for isomerization and the transition-state structure were also determined. The difference in the acidities of the two stereoisomers of α-phenylcinnamic acid and cinnamic acid (this is the basis of their separation experimentally) was rationalized by computing the dipole moments of the ions.