MAGNETIC-FIELD (G-VALUE) DEPENDENCE OF PROTON HYPERFINE COUPLINGS OBTAINED FROM ESEEM MEASUREMENTS - DETERMINATION OF THE ORIENTATION OF THE MAGNETIC AXES OF MODEL HEME COMPLEXES IN GLASSY MEDIA

Electron spin echo envelope modulation (ESEEM) studies were utilized t o characterize the coupling between protons of axially bound pyrazole ligands (PzH) and the unpaired electron of low-spin tetraphenylporphyr inatoiron(III) chloride. Samples were prepared in mixed-solvent glasse s to maximize the resolution of the electron paramagnetic resonance (E PR) signals. X-band two-pulse ESEEM experiments at 4.2 K in deuterated solvent glasses demonstrated that this coupling results in 0.2-0.7 MH z shifts of the nu(alpha) + nu(beta) proton sum combination peak from twice the Larmor frequency. These shifts have been investigated across most of the EPR absorption spectrum of [TPPFe(PzH)(2)]Cl-+(-). Two-pu lse ESEEM spectra were simulated at different magnetic field positions . Combination peaks were observed from both distant protons (DP) (beta -pyrrole, ortho-phenyl, and beta-pyrazole) and near protons (NP) (alph a-H of the pyrazole ligands). For the simulations, the orientation of the nearest four protons of the pyrazole ligands with respect to the g -tensor of the complex, the Fe-III-proton distance, and g-strain were taken as input parameters. Comparison of the experimental data and com puter simulations, in terms of magnetic field dependence of both frequ ency and intensity data, allows for the determination of the orientati on of the hyperfine coupling tensor of the protons in coordinates of t he g-tensor principal axes. For the DP peak, the magnetic field depend ence clearly shows that the maximum g-value is aligned with or close t o the Fe-N-ax vector perpendicular to the plane of the porphyrinate. F or the NP doublet, the results show that the alpha-H atoms of the axia l pyrazole ligands, and thus the planes of those ligands, are aligned with the g(min) or g(xx) magnetic axis of the metal, and hence, the p( pi) orbital of the axial ligands are aligned with the g(yy) magnetic a xis of low-spin Fe-III. Thus, in spite of the fact that the ''rhombici ty'' defined by Blumberg and Peisach is much greater than the theoreti cal value of 2/3, the magnetic axes of this model heme complex corresp ond to a ''proper axis system'', with g(zz) > g(yy) > g(xx).