EPR and ENDOR studies of the metallooctaethylporphyrin .pi.-cation radical

• Published in: Journal of physical chemistry (1952) 1991-05, Vol.95 (11), p.4300-4307
• Main Authors: Sandusky, P. O, Oertling, W. A, Chang, C. K, Babcock, G. T
• Format: Article
• Language: English
• Subjects: 140505 - Solar Energy Conversion- Photochemical, Photobiological, & Thermochemical Conversion- (1980-); 400201 - Chemical & Physicochemical Properties; ALKALINE EARTH METAL COMPLEXES; ANIONS; Atomic and molecular physics; CARBOXYLIC ACIDS; CATIONS; CHARGED PARTICLES; CHELATES; COBALT COMPLEXES; COMPLEXES; CONFORMATIONAL CHANGES; DATA; ELECTRON SPIN RESONANCE; ENDOR; Exact sciences and technology; EXPERIMENTAL DATA; HETEROCYCLIC ACIDS; HETEROCYCLIC COMPOUNDS; HYPERFINE STRUCTURE; INFORMATION; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; IONS; LASER SPECTROSCOPY; MAGNESIUM COMPLEXES; MAGNETIC RESONANCE; MATHEMATICAL MODELS; Molecular properties and interactions with photons; Multiple resonances (including double and higher-order resonance processes, such as double nuclear magnetic resonance, electron double resonance and microwave optical double resonance); NUMERICAL DATA; ORGANIC ACIDS; ORGANIC COMPOUNDS; ORGANIC NITROGEN COMPOUNDS; Physics; PORPHYRINS; RADICALS; RAMAN SPECTROSCOPY; RESONANCE; SOLAR ENERGY; SPECTROSCOPY; TRANSITION ELEMENT COMPLEXES
• ISSN: 0022-3654; 1541-5740
• DOI: 10.1021/j100164a024

Two classes of {beta}-substituted metalloporphyrin {pi}-cation radicals are distinguishable by their visible spectra: the green form complexes typified by Mg{sup II}OEP{sup {sm bullet}+}, and the gray form complexes represented by Co{sup III}OEP{sup {sm bullet}+}2ClO{sub 4}{sup {minus}}. The present consensus in the literature now holds that both classes have predominately {sup 2}A{sub 1u} ground states. Despite this, however, the authors find the gray form and green form complexes differ markedly in terms of their magnetic resonance properties. The gray form complexes have isotropic g values atypically high for simple S = 1/2 metalloporphyrin radical species. Further, application of high-resolution techniques including Q-band EPR and ENDOR spectroscopy reveals that Co{sup III}OEP{sup {sm bullet}+}2ClO{sub 4}{sup {minus}} differs from Mg{sup II}OEP{sup {sm bullet}+} in having a larger axial g anisotropy, a much larger {sup 15}N hyperfine coupling, and a significantly smaller meso-proton coupling. Analysis of the gray form g tensor indicates that the larger nitrogen coupling derives from a small contribution from molecular orbitals of e{sub u} symmetry that are formed from the nitrogen lone-pair orbitals. The contribution of this nitrogen lone-pair centered excited state is much smaller in the green form complexes than in the gray form species. The authors interpret this behavior to indicate a different metal-nitrogen lone-pair interaction and a different ring conformation in the green form and gray form complexes. This conclusion is further supported by evident differences in the meso-proton hyperfine coupling mechanism and by variations in the Raman-active skeletal model vibrational frequencies. The intrinsic difference between the gray form and green form complexes, therefore, appears to be a matter of ring conformation, rather than a profound difference in the ground-state electronic configuration.