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 bridged forms of the 2-phenylethyl radical are reported here together with activation energies and calculated rate constants for rearrangement of the bridged to the open radical. In addition, the effect of substituents on the aryl ring upon the relative energies, activation energies, and rate constants for rearrangement from the bridged to open forms are presented. Para-substituents include CH3, CF3.CN, CHO.OH, SH, O-, S-, and NO. The parent bridged 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 is 3.96 kcal/mol. Para-substitution by CN or CHO significantly lowers the difference in energy between the species, while substitution by NO renders the bridged form more stable. Para-substitution by CN or CHO coupled with substitution with CN in the ortho-positions makes the open and bridged radical approximately equivalent in energy.