Electron spin echo envelope modulation theory for high electron spin systems in weak crystal field

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Electron spin echo envelope modulation (ESEEM) experiments with aqueous complexes of Gd3+ and Mn2+ have shown that a common and unusual feature of the primary ESEEM spectra of such high spin/weak crystal field systems is an extremely low intensity of the sum combination line. Numerical simulations of the ESEEM spectra based on the existing theory [Coffino and Peisach, J. Chem. Phys. 97, 3072 (1992); Larsen and Singel, J. Chem. Phys. 98, 6704 (1993)] could not reproduce these ESEEM spectra. In this work the theoretical description of the ESEEM was revised and corrected, and new expressions were derived for the ESEEM from high electron spin (S>1/2) systems in a weak crystal field, interacting with a nuclear spin I=1/2. The corrections primarily affected the shape and intensity of the sum combination line, whose position was found to be sensitive to the product of the crystal field and anisotropic hyperfine interaction constants. These theoretical improvements resulted in a successful simulation of the primary ESEEM from a model system, Mn2+(H2O)6 complex in a frozen glassy water–methanol solution. The results of this work show that the shape and intensity of the sum combination line in ESEEM spectra may be used for evaluating the distribution of crystal field interactions in Mn2+, Gd3+, and similar complexes.