ESR spectra and structure of the CF3Cl−, CF3Br−, and CF3I− radical anions

The anisotropic ESR spectra of the CF₃X^? radical anions (X = Cl, Br, I) have been observed following γ irradiation at 77 K of solid solutions containing up to 5 mole % of the CF₃X parent compound in tetramethylsilane (TMS), neopentane, and 2-methyltetrahydrofuran. The resolution of the line components in the TMS matrix allowed the spectra to be analyzed in detail, the parallel and perpendicular features showing clear evidence for axially symmetric hyperfine interactions with three equivalent fluorines and the unique halogen. On this basis, a matrix diagonalization program was used to calculate the line positions and the best-fit ESR parameters obtained. Confirmation of the CF₃X^? identification was achieved through chemical studies which showed that a similar ESR spectrum was generated by electron attachment to the parent molecule in a photoionization experiment. Also, the spectrum of the CF₃ radical was observed to grow in during the decay of the CF₃X^? spectrum in neopentane above 100 K. The spin density distributions calculated from the ESR parameters of these congeneric radical anions suggest that the unpaired electron resides in an a₁ (σ^*) antibonding orbital which is composed largely of the p orbitals from carbon and the unique halogen which lie along the C_3v symmetry axis of the radical anion. Consistent with this proposal, the spin densities in the s and p orbitals of the unique halogen increase along the series Cl, Br, I, which is the order expected for the effect of decreasing halogen electronegativity.