Enthalpy and entropy of conjugative interaction in a nearly coplanar styrene and cinnamyl radical

Conjugative interaction in styrenes designed to constrain the dihedral angl e between the planes of the phenyl and olefinic groups near to coplanarity (21 degrees by X-ray) is greater by 0.7 kcal mol(-1) than that in a styrene in which the phenyl group is unconstrained (ad libitum) (-35 degrees by MM 2 molecular mechanical calculation). Conjugative interaction in a cinnamyl radical similarly constrained to maintain the phenyl and allyl moieties nea rly coplanar (15 degrees by MM2) is greater by 1.1 kcal mol(-1) than in a c innamyl radical in which the phenyl group is ad libitum. Efforts to separat e the contribution of pi-electron delocalization from the nonbonded steric factor by employing MM2 calculations proved quantitatively unsatisfactory.