ELECTRON-PARAMAGNETIC-RESONANCE STUDIES OF METALLOPORPHYRIN ANION-RADICALS - EFFECTS OF SOLVENT, COUNTERION, TEMPERATURE, AND ISOTOPIC-SUBSTITUTION ON THE JAHN-TELLER ACTIVE(2)E(G) GROUND-STATE

Electron paramagnetic resonance (EPR) spectra are reported for a series of porphyrin anion radicals including H2TPP-, H2OEP-, MgTPP-, ZnTPP-, ZnOEP-, and CdTPP- (TPP = tetraphenylporphyrin, OEP = octaethylporphyrin). The anions were generated by both chemical and electrochemical reduction in a variety of solvents. EPR spectra are also reported for a number of isotopomers of the free bases and Zn(II) complexes. These isotopomers included TPP-beta-d(g), TPP-(meso-C-13)4, TPP-(N-15)4, and OEP-meso-d4. The temperature dependence of the EPR spectra was also investigated from 6 to 380 K. Contrary to previous reports, the spectra are not particularly sensitive to the solvent or counterion. The low-temperature spectra of all the anion radicals are anisotropic with g(parallel-to) < g(e). The anisotropy for H2TPP- (and H2OEP-) and MgTPP- is small (approximately 0.0003 and approximately 0.0023, respectively). On the other hand, the anisotropy is significant for ZnTPP- (and ZnOEP-) and CdTPP- (approximately 0.01 and approximately 0.02, respectively). The anisotropic EPR spectra are indicative of unquenched orbital angular momentum in the ground states of the anion radicals. The EPR spectra were modeled by using an harmonic potential function which is subject to both Jahn-Teller and strain distortions. The general characteristics of the spectra are well accounted for by a model that assumes distortions along a single vibronically active coordinate. However, the detailed characteristics of the EPR line shape indicate that more than one mode is involved. Simulations of the EPR spectra indicate that the Jahn-Teller energy is less than the zero point energy of the active vibrational modes; accordingly, the distortion is dynamic rather than static. The calculations, in conjunction with the spectral data, indicate that the strain energy is substantial (at least 100 cm-1) in both the solid and liquid states. The strain distortion alters the relative energies of the two wells in the Jahn-Teller distorted system. Accordingly, the dynamically averaged structure does not have rigorous 4-fold symmetry.