RESONANCE RAMAN STRUCTURAL CHARACTERIZATION OF BETA-SUBSTITUTED METALLOPORPHYRIN PI-ANION RADICALS - NATURE OF THE JAHN-TELLER EFFECT

Variable-wavelength excitation resonance Raman (RR) spectra are report ed for the electrochemically reduced monoanion radicals of vanadyl com plexes of octaethylporphyrin and etioporphyrin-I as well as their meso -d(4) and pyrrole-N-15 isotopomers. Drastic alterations in spectral pr operties are found for the anion radicals: (1) the depolarization rati o (rho) of skeletal modes are scrambled, i.e., rho > 1/8 for A(1g) mod es and <(3/4) for B-1g modes; (2) several B-1g modes gain dominant int ensity; (3) pairs of A(1g) and B-1g modes of the same local coordinate exhibit disparate frequency shift patterns, most notably, nu(2) and n u(11) (C-beta-C-beta stretching), which are shifted down 11 and 53 cm( -1), respectively; and (4) the isotope shifts of several modes are sig nificantly altered, e.g., nu(4) acquired 5-cm(-1) meso-d(4) sensitivit y. These changes imply lowering of the porphyrin symmetry in the anion radical ground state, which is attributed to a static Jahn-Teller eff ect. The observed frequencies and isotope shifts for the high-frequenc y skeletal modes of the anion radicals are satisfactorily reproduced i n a normal-mode calculation by adjusting the bond stretch force consta nts of the neutral parent according to the nodal pattern of the e(g)( ) orbitals. The relative intensities of high-frequency RR bands are fo und to vary markedly as the excitation wavelengths are tuned throughou t the absorption band centered at 456 nm, implying the presence of mul tiple electronic transitions.