Steve C. F. Au-Yeung

Chemical shift anisotropy and second-sphere hydrogen bondin in Cobalt (III) Complexes

The linewidths of the 59Co NMR resonances of 22 Co(III) complexes were measured at fields of 2.114, 5.872, and 9.395 T. The linewidths for octahedral complexes show no field dependence. Less symmetric compounds show differences in linewidth of as much as 10 kHz between the highest field and the lowest field. The linewidth changes parallel the chemical shift anisotropies calculated from optical absorption spectral data. For complexes with high symmetry the increase in width is proportional to the square of the field strength. For complexes of lower symmetry, plots against field squared show a pronounced curvature. If the increase in linewidth is calculated from the chemical shift anisotropies obtained from optical data (or from single-crystal measurements reported in the literature) and correlation times derived from quadrupole coupling constants the calculated increases in width are more than two orders of magnitude smaller than the observed increases. Correlation times of the order 10−9 to 10−10 sec rather than the accepted 10−11 sec for rotational correlation times are required to fit the data. It is suggested that these long correlation times are associated with the lifetimes of hydrogen bonds with second-sphere solvent molecules.

Spin-lattice relaxation in Co complexes of low symmetry

Abstract Spin-lattice relaxation times for 59 Co contained in several low symmetry cobalt complexes are reported. Measurements have been made at two field strengths and, in the case of one complex, in several solvents. Some complementary measurements of 14 N relaxation times on the same complexes are also reported. Linewidth measurements have been made to obtain T 2 values. The relaxation mechanisms depend on the solvent. In a “weakly hydrogen bonding” solvent (acetonitrile) quadrupolar and scalar relaxation through spin coupling to 14 N are the most important mechanisms. Reasonable agreements between experimental and calculated relaxation times at low magnetic field can be obtained using experimental solid state quadrupole coupling constants, spin-spin coupling constants from 15 N data, 14 N T 1 values, and correlation times consistent with those obtained from the Debye formula. In these cases T 1 for 59 Co is greater than T 2 . In hydrogen bonding solvents an additional field dependent term is necessary. Both T 1 and T 2 are affected eliminating the possibility of a chemical exchange process. The field dependent term is associated with chemical shift anisotropy involving the off diagonal (antisymmetric) terms in the chemical shielding tensor. This mechanism requires that T 1 be less than T 2 . The experimental data are consistent with this prediction.