Combining Theory and Experiment to Interpret the EPR Spectra of VO²⁺-Exchanged Zeolites

Abstract

The electron paramagnetic resonance (EPR) spectra of VO 2+ -exchanged zeolites were interpreted by comparing theoretical calculations of the EPR parameters for VO 2+ model complexes with experimental EPR data. This is the first report in which density functional theory (DFT) has been used to calculate the EPR parameters in order to reproduce an empirical correlation between the electronic g -factor and the nuclear hyperfine coupling constant, A . In this study, a series of VO 2+ -exchanged zeolites (ZSM5, mordenite, Beta and Y) were prepared by a standard aqueous ion exchange procedure. EPR spectra of the samples were obtained before and after dehydration 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 VO 2+ -exchanged zeolites exhibited similar EPR spectral features that were independent of the identity of the parent zeolite. After dehydration, the EPR spectra were broad relative to the EPR spectra of the hydrated VO 2+ -exchanged zeolites, presumably due to site heterogeneity, but otherwise exhibited similar EPR parameters. Upon adsorption of ammonia, the EPR parameters systematically changed; g ∥ increased and A ∥ ( 51 V) decreased relative to the EPR parameters for the hydrated VO 2+ -exchanged zeolites. To further understand ligand binding in this system, the g and A tensors for several vanadyl model complexes, VO(H 2 O) 4 2+, VO(H 2 O) 5 2+, cis and trans VO(NH 3 ) 2 (H 2 O) 2 2+, VO(NH 3 ) 4 2+, and VO(NH 3 ) 4 H 2 O 2+ 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 values were systematically too small. Significantly, the trends in g and A with ligand substitution were reproduced very well by the calculations and were used to interpret the EPR data. The EPR parameters for the model complexes can be correlated to the V O bond lengths.