EPR-ENDOR OF THE ELECTRONIC-STRUCTURE FROM 2 NITROGENOUSLY LIGATED BIS(MU-OXO)-MNIII-MNIV MODEL COMPLEXES SPECTROSCOPICALLY RELEVANT TO THE MULTI-MANGANESE CENTER OF PHOTOSYSTEM-II

ENDOR (electron nuclear double resonance) gave hyperfine couplings to nitrogen(s) on the bis(mu-oxo)-Mn(III)-Mn(IV) complexes of CYCLAM (1,4,8,11-tetraazacyclotetradecane) and TMPA (tris(2-methylpyridyl)amine). PFSEPR (pulse field-sweep EPR) gave nitrogen hyperfine couplings consistent with the ENDOR results. The nitrogen hyperfine couplings from ENDOR were 9.2 MHz for CYCLAM and 11.2 MHz for TMPA. We provide justification that these couplings are due to a Fermi contact interaction with nitrogen 2s electron spin density at the axial nitrogens on the Mn(III), where this electron spin is contained in an antibonding e(g) orbital along the Mn(III)-nitrogen bond. The relevant axial nitrogen ligands are secondary amine nitrogens in the case of CYCLAM and pyridyl nitrogens in the case of TMPA. Compared to quadrupolar couplings from unligated secondary amine or pyridine, the quadrupolar interaction for both types of nitrogen indicated a significant decrease in electric field gradient at the nitrogen nucleus due to interaction of the nitrogen lone pair with the manganese. Proton ENDOR showed exchangeable protons associated with the CYCLAM complex but not with the TMPA complex. We assign these exchangeable CYCLAM protons on the basis of their large 6.7-MHz hyperfine coupling as exchangeable protons on the secondary amine nitrogens at the Mn(III).