INDO and CNDO/2 calculations for substituted benzene cations

Abstract

Radical cations of PhCH₂F and PhCF₃ have been prepared by exposing dilute (1:1000) solutions of these compounds to ⁶⁰Co γ-rays at 77 K. For PhCH₂F⁺ the e.s.r. spectra showed two strongly coupled equivalent protons (29 G) and no ¹⁹F coupling. For PhCF⁺₃ a five-line spectrum (A= 6.5 G) was assigned to four equivalent protons with again no coupling to ¹⁹F. E.s.r. results for other aromatic cations are taken from the literature. INDO and / or CNDO/2 calculations were performed for these cations and those of benzene, toluene, ethyl benzene, 1,4-diethylbenzene and benzyl chloride. The result for the benzene cation gave a distorted D₂h structure as the lowest-energy state, in accord with recent e.s.r. studies at 4 K. For the substituted benzene cations (except PhCF⁺₃), the calculations show that this distorted structure is retained. The bond distance between the substituted ring carbon and the carbon of the substituents is considerably reduced relative to those for the parent molecules so that rotation of the substituting group about this bond is hindered. Calculations suggest that the conformation for the ethyl benzene cation with an out-of-plane methyl group is slightly more stable than that with methyl in-plane. However, e.s.r. studies show that the latter is more stable. For diethyl benzene and benzyl fluoride and chloride cations, the calculations gave conformations with out-of-plane methyl and with in-plane fluorine and chlorine atoms, respectively, all of which are in good accord with e.s.r. experiments. E.s.r. results suggest that the cation of PhCF₃ has switched the SOMO, so that the nodal plane passes through the CF₃ group (²B_2g). INDO calculations also predict a drastic change in the SOMO for PhCF⁺₃ radical cations, but not to the alternative π orbital required to the e.s.r. data.