Quadrupole effects in the EPR spectra of bis [1, 3-di (p-n-octyloxyphenyl propane-1, 3 dionato] Cu(II]: Role in phase transitions

Restricted Access.X band studies of alkyl/alkloxy phenyl substituted Cu complexes indicate that only C80-Cu exhibits a complex hyperfine structure (h.f.s.) at 300K. It also undergoes a first-order phase transition around 213K. However, the complex h.f .s. could not be delineated at X band. In the Q band, in addition to the four permitted h.f. lines, 10 for- bidden lines due to quadrupole effects are obser- ved. Further, the forbidden lines disappear in the discophase but the permitted h.f. lines remain. This difference between C80-Cu and C8Cu could be explained from their crystal structures. In C80-Cu, the repeating unit is a Cu dimer, as oppo- sed to a single Cu atom placed at the inversion centre in C Cu. Distortion in the dimer, possibly n accounts for the observed quadrupole effects