Complete Determination of Nitrogen Quadrupole and Hyperfine Tensors in an Oxovanadium Complex by Simultaneous Fitting of Multifrequency ESEEM Powder Spectra

One-dimensional C- and X-band as well as two-dimensional X-band ESEEM experiments were performed on the complex oxobis(2-methylquinolin-8-olato) vanadium(IV) in frozen solution. A 14N ESEEM simulation strategy based on initial first- and second-order perturbation analysis of peak positions in orientationally selected ESEEM spectra is presented. The constraint parameters extracted enable one to reduce the number of free fitting parameters for each nitrogen from 10 to 4. These are the alpha, beta resp. the phi, theta Euler angles of the NQI and the HFI tensor defined in the coordinate system of the axial g tensor. The local symmetry of the complex allows one to reduce the number of free parameters to two angles only. Subsequently, a grid search in the remaining Euler space produced the starting parameters for the final fit of the 14N hyperfine and quadrupole tensors. The anisotropic nitrogen hyperfine interaction tensor was found to be strongly nonaxial (0.06, 0.51, -0.57) MHz with the components significantly smaller than the isotropic hyperfine constant -6.18 MHz. In contrast, the quadrupole tensor with K = 0.58 MHz is close to axial (eta = 0.13). These tensors share the principal axis normal to the ligand plane (as imposed by the local symmetry). The axes in the ligand plane are, however, rotated 50 degrees with respect to each other. The orientation of the quadrupole tensor axes correlate within 10 degrees with the orientation of the ligand plane following from the X-ray structure. Copyright 1998 Academic Press.