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

Journal Article · Thu May 30 00:00:00 EDT 1991 · Journal of Physical Chemistry; (United States)

DOI:https://doi.org/10.1021/j100164a024· OSTI ID:5954112

Sandusky, P O; Oertling, W A; Chang, C K; Babcock, G T ^[1]

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1. Michigan State Univ., East Lansing (United States)

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.

• OSTI ID: 5954112
• Journal Information: Journal of Physical Chemistry; (United States), Vol. 95:11; ISSN 0022-3654
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
14 SOLAR ENERGY
PORPHYRINS
ELECTRON SPIN RESONANCE
ENDOR
ANIONS
CATIONS
CHELATES
COBALT COMPLEXES
CONFORMATIONAL CHANGES
EXPERIMENTAL DATA
HYPERFINE STRUCTURE
MAGNESIUM COMPLEXES
MATHEMATICAL MODELS
RADICALS
RAMAN SPECTROSCOPY
ALKALINE EARTH METAL COMPLEXES
CARBOXYLIC ACIDS
CHARGED PARTICLES
COMPLEXES
DATA
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
INFORMATION
IONS
LASER SPECTROSCOPY
MAGNETIC RESONANCE
NUMERICAL DATA
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
RESONANCE
SPECTROSCOPY
TRANSITION ELEMENT COMPLEXES
400201* - Chemical & Physicochemical Properties
140505 - Solar Energy Conversion- Photochemical
Photobiological
& Thermochemical Conversion- (1980-)