Assessing the dependence of (51)V A(z) value on the aromatic ring orientation of V(IV)O(2+) pyridine complexes.

Characterization of V(IV) biomolecules relies strongly on electron paramagnetic resonance (EPR) spectroscopy, particularly the application of the additivity relationship of A(z) values. It has been shown experimentally that the A(z) values of V(IV)O(2+) imidazole species have a critical angular dependence. Density-functional theory (DFT) calculations elucidate the dependence of (51)V A(z) value on the orientation of the aromatic ring in V(IV)O(2+) pyridine complexes, following closely the functional dependence observed for V(IV)O(2+) imidazole species, [A(z)(pyr) = 42.23 + 1.80 x sin(2theta - 90)], with A(z) measured in 10(4) cm(-1). A DFT re-examination of V(IV)O(2+) imidazole complexes gives an equation very similar [A(z)(imid) = 42.35 + 2.34 x sin(2theta - 90)] to that experimentally found. These results generalize the application of the additivity relationship for V(IV)O(2+) complexes containing aromatic nitrogen ligands such as pyridine or imidazole. The increase of the absolute value of A(z), |A(z)|, when the dihedral angle theta between the V=O and N(pyr)-C or N(imid)-C bonds varies from a parallel to a perpendicular orientation is due to an increase of the d vanadium orbital contribution and to a decrease of the pi aromatic system participation in the singly occupied molecular orbital.