NMR Spectra of Porphyrins: 31: Ring Currents in Hydroporphyrins

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The dipole-dipole network model of the porphyrin macrocyclic ring current is used to investigate the ring currents in reduced (hydro)porphyrins derived from phylloerythrin methyl ester. Reduction of ring D to afford the corresponding chlorin results in a decrease of the inner loop and pyrrole subunit ring currents by about 10%. Further reduction of another pyrrole ring gives very different results depending upon the pyrrole ring reduced. In bacteriochlorophyll a, in which the opposite pyrrole subunits (B and D) are reduced, there is a further 10% reduction in the pyrrole ring current and a 20% reduction of the inner loop ring current. In the isobacteriochlorin system (rings A and D reduced) there is a much larger decrease in the inner loop ring current, to about 45% of the chlorin value. In the pyrrocorphin (hexahydroporphyrin) system (rings A, B, and D reduced) there is a further 10% decrease in the pyrrole ring current and the inner loop ring current is only 25% of the chlorin value. These results are discussed in terms of steric effects in the macrocycle. The ring current models thus obtained are used to investigate structural and conformational differences in the various isobacteriochlorin isomers. © 1987, American Chemical Society. All rights reserved.

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Journal of the American Chemical Society

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