Phenyl-bridging in the 2-phenylethyl radical. A molecular orbital study

Density functional theory at the UB3PW91/6-31G(d.p) level on the open and b ridged forms of the 2-phenylethyl radical are reported here together with a ctivation energies and calculated rate constants for rearrangement of the b ridged to the open radical. In addition, the effect of substituents on the aryl ring upon the relative energies, activation energies, and rate constan ts for rearrangement from the bridged to open forms are presented. Para-sub stituents include,CH3, CF3, CN, CHO. OH, SH, O-, S-, and NO. The parent bri dged radical is found to be 10.1 kcal/mol higher in enthalpy than the open form. The activation energy for conversion of the bridged to open radical i s 3.96 kcal/mol. Para-substitution by CN or CHO significantly lowers the di fference in energy between the species, while substitution by NO renders th e bridged form more stable. Para-substitution by CN or CHO coupled with sub stitution with CN in the ortho-positions makes the open and bridged radical approximately equivalent in energy.