| Abstract
| - The electron paramagnetic resonance (EPR) spectra of VO2+-exchanged zeolites were interpreted by comparingtheoretical calculations of the EPR parameters for VO2+ model complexes with experimental EPR data. Thisis the first report in which density functional theory (DFT) has been used to calculate the EPR parameters inorder to reproduce an empirical correlation between the electronic g-factor and the nuclear hyperfine couplingconstant, A. In this study, a series of VO2+-exchanged zeolites (ZSM5, mordenite, Beta and Y) were preparedby a standard aqueous ion exchange procedure. EPR spectra of the samples were obtained before and afterdehydration and in the presence of ammonia. The EPR parameters were determined by applying a least-squares fitting routine to the data. The EPR spectra of the hydrated VO2+-exchanged zeolites exhibited similarEPR spectral features that were independent of the identity of the parent zeolite. After dehydration, the EPRspectra were broad relative to the EPR spectra of the hydrated VO2+-exchanged zeolites, presumably due tosite heterogeneity, but otherwise exhibited similar EPR parameters. Upon adsorption of ammonia, the EPRparameters systematically changed; g∥ increased and A∥(51V) decreased relative to the EPR parameters for thehydrated VO2+-exchanged zeolites. To further understand ligand binding in this system, the g and A tensorsfor several vanadyl model complexes, VO(H2O)42+, VO(H2O)52+, cis and trans VO(NH3)2(H2O)22+, VO(NH3)42+, and VO(NH3)4H2O2+ were calculated using the Amsterdam density functional theory (ADF) program.The calculated g values were in good agreement with experimental g values, but the calculated A valueswere systematically too small. Significantly, the trends in g and A with ligand substitution were reproducedvery well by the calculations and were used to interpret the EPR data. The EPR parameters for the modelcomplexes can be correlated to the VO bond lengths.
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