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Calculating NMR paramagnetic relaxation enhancements without adjustable parameters: the spin‐3/2 complex Cr(III)(AcAc) 3
Author(s) -
Miller Jeremy,
Schaefle Nathaniel,
Sharp Robert
Publication year - 2003
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1251
Subject(s) - chemistry , zeeman effect , relaxation (psychology) , paramagnetism , condensed matter physics , spin (aerodynamics) , magnetization , magnetic field , quantum mechanics , physics , thermodynamics , psychology , social psychology
NMR paramagnetic relaxation enhancement (NMR‐PRE) produced by the electron spin S = 3/2 complex Cr(III)(acac) 3 (acac = acetylacetonato) has been simulated by spin dynamic (SD) simulation methods in order to test current theory of NMR‐PRE. This system provides a particularly demanding test of theory, since the Zeeman and zero field splitting (zfs) contributions to the electron spin Hamiltonian are of comparable magnitude in the range of magnetic field variation of the data, and Brownian reorientation of both the zfs tensor and the interspin vector play important roles in the relaxation mechanism. For Cr(III)(acac) 3 , all of the sensitive parameters of theory were known from independent experiments, so that a calculation was possible without variation of parameters. The R 1 field dispersion profile (fdp), reported by Wang et al. ( J. Magn. Reson. 1987; 73 : 277), exhibit a single pronounced dispersive feature between 0.23 and 2.1 T. SD simulations showed that this feature results physically from the change in spatial quantization of the electron spin motion that occurs in the intermediate regime of field strengths where the Zeeman and zfs energies are comparable. Thus, the observed R 1 dispersion for Cr(III)(acac) 3 has a qualitatively different physical origin than the dispersions of Solomon–Bloembergen–Morgan theory, where R 1 dispersions result from Zeeman breaking of level degeneracies. The shape of the experimental fdp suggests that the point‐dipole approximation is invalid for Cr(III)(acac) 3 due to delocalization of electron spin density onto the π‐system of the acac ligand. After accounting for the altered geometry, good agreement with experiment was obtained. Copyright © 2003 John Wiley & Sons, Ltd.