BALANCE OF STERIC AND CONJUGATIVE EFFECTS IN PHENYL-SUBSTITUTED CATIONS, RADICALS, AND ANIONS

The delicate balance of steric and conjugative effects which determines the equilibrium geometry of phenyl-substituted carbonium, allyl, pentadienyl, cyclopropenyl, cyclopentadienyl, and cycloheptatrienyl cations, radicals, and anions is studied by means of extended Hückel calculations. Despite some doubts as to the applicability of this method to such species, the resulting potential energy curves are reasonable. In triphenylcarbonium ion there is found no evidence for a local minimum other than that in the propeller geometry. In diphenylcarbonium ion the transition from the equilibrium geometry to its enantiomer is most easily accomplished through a transition state in which one phenyl group is coplanar, the other perpendicular. In the triphenylcarbonium ion the corresponding conversion is estimated to proceed through a transition state in which all phenyl rings are perpendicular. In all cases the anions are predicted to be conformationally more stable than the cations. Hyperconjugative charge transfer occurs to ortho carbons of a phenyl ring as it is twisted out of conjugation. The potential energy curve for twisting the phenylcyclopropenium cation out of planarity is calculated to be very flat. This insensitivity of energy to rotation is not matched by absence of charge transfer. Several theoretical explanations for the calculated insensitivity of phenylcyclopropenium energy to twist of phenyl group are discussed, but none is completely satisfactory.