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An unusual temperature dependence of the EPR parameters of copper(II) and oxovanadium(IV) bis(acetylacetonate) complexes
Author(s) -
Howes Barry D.,
Kühlmeyer Claudia,
Pogni Rebecca,
Basosi Riccardo
Publication year - 1999
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/(sici)1097-458x(199908)37:8<538::aid-mrc498>3.0.co;2-0
Subject(s) - chemistry , electron paramagnetic resonance , copper , paramagnetism , atmospheric temperature range , motional narrowing , hyperfine structure , spectral line , solvent , crystallography , analytical chemistry (journal) , nuclear magnetic resonance , condensed matter physics , thermodynamics , atomic physics , organic chemistry , physics , astronomy
A series of X‐band electron paramagnetic resonance (EPR) spectra were obtained in the temperature range 120–280 K at 10 K intervals for the bis(acetylacetonate) complexes of copper(II) and oxovanadium(IV) cations in dimethylformamide. The magnetic parameters and molecular rotational correlation time determined from simulations of the spectra revealed an unexpected temperature dependence of the isotropic hyperfine coupling constant ( A iso ). A small, but non‐negligible, abrupt increase in A iso is observed for both complexes at a temperature (215 K) corresponding to the change in molecular motional state from the rigid limit to slow motion. The change is approximately the same for both the Cu(II) and vanadyl complexes ( ca 2 × 10 −4 cm −1 ). No temperature dependence is observed in the isotropic g ‐factor for either complex within the experimental error. It is proposed that the data can be explained in terms of a change in the degree of distortion at the metal site coincident with the change in motional state and induced by a solvent (dimethylformamide) phase transition. Calculation of the bonding parameters shows that the in‐plane π‐bonding is more covalent in the vanadyl complex. Comparison with data from the literature reveals the importance of the solvent in determining the type of temperature dependence displayed by the magnetic parameters. Copyright © 1999 John Wiley & Sons, Ltd.