SINGLE-CRYSTAL COBALT-59 NMR-STUDY OF TRIS(2,4-PENTANEDIONATO-O,O')COBALT(III)

Cobalt-59 NMR spectra of a single crystal of tris(2,4-pentanedionato-O ,O') cobalt(III) were obtained as a function of crystal orientation in an applied magnetic field of 9.40 T. The analysis provides the magnit udes and orientations of the Co-59 nuclear quadrupole coupling and che mical shift tensors for each of the two magnetically distinct but crys tallographically equivalent cobalt sites. The cobalt electric field gr adient and chemical shift tensors are not coincident, but their unique components are close to the approximate C-3 axis of the complex. Smal l deviations from perfect octahedral symmetry at the cobalt nucleus re sult in a significant nuclear electric field gradient and highly aniso tropic chemical shift. The cobalt nuclear quadrupolar coupling constan t is 5.53 +/- 0.10 MHz with an asymmetry of 0.219 +/- 0.005, while the chemical shift tensor has a span of 1174 +/- 25 ppm. The principal co mponents relative to the isotropic chemical shift, delta(iso) = 12498 +/- 5 ppm, are delta(11) = 698 +/- 22 ppm, delta(22) = -222 +/- 12 ppm , and delta(33) = -476 +/- 5 ppm. The quadrupolar tensor was character ized by examining splittings between the satellite transitions, while the chemical shift tensor was characterized by analyzing the central t ransition and correcting for the second-order quadrupolar interaction. The results obtained in this study are compared with those of previou s Co-59 NMR studies.