ESE-ENDOR study and DFT calculations on oxovanadium compounds:: Effect of axial anionic ligands on the 51V nuclear quadrupolar coupling constant

High-frequency electron spin echo-electron nuclear double resonance (ESE-ENDOR) spectroscopy is applied to oxovanadium VO2+ complexes of Hcapca in the form of trans-[VOX(capca)] and to H(2)capcah in the form of cis-[VOX(Hcapcah)], where X = Cl- or SCN-. Nuclear quadrupolar coupling constants (nqcc), which are unobtainable by conventional continuous-wave electron paramagnetic resonance (CW-EPR), were measured and reported in terms of P-parallel to (P-parallel to = 3e(2)qQ/84 for I = 7/2). P-parallel to values for trans-[VOCl(capca)] and trans-[VOSCN(capca)] were calculated to be -0.18 and -0.21 MHz, respectively. In the cases of cis-[VOCl(Hcapcah)] and cis-[VOSCN(Hcapcah), P-parallel to values were calculated to be -0.35 and -0.45 MHz, respectively. The experimental results are supported by DFT calculations of quadrupolar and hyperfine couplings for various oxovanadium compounds, including the cis and trans complexes studied by ESE-ENDOR. The charged ligands, coordinated axially trans to the oxo bond, reduce the electric field gradient along the V=O bond, thereby decreasing the observed magnitude of the nuclear quadrupolar coupling constants relative to those of the comparable cis compounds. This experimental finding is confirmed by quantum mechanical calculations. Although the absolute values of quadrupolar splittings cannot be calculated with acceptable accuracy, the observed experimental trends are very well reproduced. Thus, the complementary use of DFT and pulsed-ENDOR is a promising methodology for the study of biologically relevant vanadyl compounds.