Wv. Doering et al., Enthalpy and entropy of conjugative interaction in a nearly coplanar styrene and cinnamyl radical, J AM CHEM S, 121(5), 1999, pp. 962-968
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.