The¹⁷O Hyperfine Interaction in V¹⁷O(H₂¹⁷O)₅²⁺and Mn(H₂¹⁷O)₆²⁺Determined by High Field ENDOR Aided by DFT Calculations

Abstract

The 17O hyperfine interaction of the water ligands and the V=O oxygen in the vanadyl aquo complex and of the water ligands in the Mn2+ aquo complex in a frozen solution were determined by W-band (95 GHz) electron-nuclear double resonance (ENDOR). Orientation selective ENDOR spectra of the vanadyl complex exhibited two distinct signals assigned to the vanadyl oxygen and the water ligands. The assignment of the signals was done based on the orientation of the principal axis system of the hyperfine interaction and through comparison with the hyperfine interaction predicted by DFT calculations. The latter showed good agreement with the experimental values thus providing clear evidence that the vanadyl oxygen is exchangeable. The interaction of the vanadyl oxygen, especially its anisotropic part, was significantly larger than that of the water oxygens due to a relatively large negative spin density on the oxygen p orbitals. The 17O hyperfine interaction of the water ligand in the Mn2+ complex was found to be similar to that of the water ligand in the vanadyl complex and was in good agreement with earlier single-crystal data. Here, due to the large thermal polarization, it was also possible to determine the absolute sign of the hyperfine coupling by selecting different EPR transitions.

High‐Field Pulsed EPR Spectroscopy for the Speciation of the Reduced [PV2Mo10O40]6− Polyoxometalate Catalyst Used in Electron‐Transfer Oxidations

“…This assignment is consistent with an earlier W-band ENDOR study of the vanadyl aqua complex that gave A = 10 MHz for the oxo-oxygen and approximately 5 MHz for the equatorial oxygen atoms. [16] Earlier density functional theory (DFT) calculations on 6À gave a significantly higher spin density on the terminal oxygen, consistent with this assignment. [17] Both 17 O and 95 Mo X-and W-band HYSCORE signals had a significant width (2-3 MHz) that could be either due to anisotropic hyperfine couplings or due to a distribution of isotropic hyperfine couplings (or a combination of both).…”

“…Therefore we assign it to a VO 2 + species that originates from [PV O hyperfine couplings is attributed to fast exchange with the water solvent. [16] Since species II originates from isomers with distal vanadium atoms (Figure 1), it is therefore reasonable to assume that species I forms from the isomers with the proximal vanadium atoms.…”

Quantitative characterization of the Mn2+ complexes of ADP and ATPγS by W-band ENDOR

“…In this case IH, 2H and ~70 ENDOR can be applied. For the latter, however, the signal-to-noise ratio may require long accumulations times for accurate results [35].…”

Probing the Local Environment of Ti3+ Ions in TiO2 (Rutile) by 17O HYSCORE

“…Remarkably, a 17 O a iso value of the same order was recently observed by some of us [18] . [19] This comparison strongly suggests that excess electrons in the blue rutile sample are (at 4 K) largely localized over a single Ti ion, which bears strong similarities to a genuine Ti 3+ molecular cation. The smaller 17 O hfi responsible for the signal in the (+ , + ) quadrant of the HYSCORE spectrum ( 17 O-2) can be interpreted as arising from the interaction between the unpaired electron in the Ti 3d xy orbitals and the axially coordinated oxygen atoms.…”

“…The 17 O hfi parameters are expected to be highly dependent on the coordination position of the O ligand and in the case of molecular aqua vanadyl cations values of about 2 MHz have been predicted for axially coordinated water. [19] In summary, this initial study on the nature of reduced states in TiO 2 has shown that stable Ti 3+ ions can be generated in the bulk of TiO 2 by a direct and simple synthetic method. The 17 O HYSCORE spectra of such Ti 3+ ions allowed the spin [a] Unresolved.…”