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 e xperiments were performed on the complex oxobis( 2-methylquinolin-8-ol ato) vanadium(IV) in fro ten solution. A N-14 ESEEM simulation strateg y based on initial first-and second-order perturbation analysis of pea k positions in orientationally selected ESEEM spectra is presented. Th e constraint parameters extracted enable one to reduce the number of f ree fitting parameters for each nitrogen from 10 to 4, These are the a lpha, beta resp. the phi, theta Euler angles of the NQI and the HFI te nsor defined in the coordinate system of the axial g tenser. The local symmetry of the complex allows one to reduce the number of free param eters to two angles only. Subsequently, a grid search in the remaining Euler space produced the starting parameters for the final fit of the N-14 hyperfine and quadrupole tensors. The anisotropic nitrogen hyper fine interaction tensor was found to be strongly nonaxial (0.06, 0.51, -0.57) MHz with the components significantly smaller than the isotrop ic hyperfine constant -6.18 MHz, In contrast, the quadrupole tensor wi th K = 0.58 MHz is close to axial (eta = 0.13), These tensors share th e principal axis normal to the ligand plane las imposed by the local s ymmetry). The axes in the ligand plane are, however, rotated 50 degree s with respect to each other. The orientation of the quadrupole tensor axes correlate within 10 degrees with the orientation of the ligand p lane following from the X-ray structure. (C) 1998 Academic Press.