EPR Properties of the Vanadyl Ion

Vanadium(IV), almost always found as the highly stable oxoeation VO, is a Kramers doublet. S = 54 system that is very well suited to EPR studies. Following Cu(II), it is the second most-studied S = 54 system. Vanadium(TV) is rarely found without an oxo ligand, and some notable exeeptions do oeeur [26,30], but sinee all currently reported insulin-enhancing vanadium(IV) compounds are based on the vanadyl ion, the naked vanadium(IV) struetural motif will not be diseussed. 

The near-square pyramidal strueture of most eomplexes yields an orbi-tally nondegenerate dxy ground state wifli no low-lying exeited states. This leads to slow relaxation behavior in EPR and allows for observation of vanadyl signals at ambient temperatures. Sueh speetra provide additional struetural and/or dynamie information this spectral property of vanadium eompounds has been exploited in studies of protein interactions and cellular uptake of vanadium. The typical room-temperature EPR spectrum of [VO(H20)5] in acidic solution has eight equally 

In frozen solution, perpendicular A values are small (55-80 x 10 cm ) and largely insensitive to ehanges in ligand environment. The parallel g and A values, however, show a large range (1.92 gy 1.96 130 220 x 10 cm ) and 

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