(13)C NMR study of reorientational dynamics in Ru3(Co)9(μ(3)-Co)(μ(3)-NPh). Relaxation time analysis in the non-motional narrowing regime

Article abstract of DOI:10.1016/0020-1693(95)04610-L

The NMR (13)C spin-lattice relaxation times of the phenyl group and equatorial carbonyl carbons in Ru3(CO)9(μ(3)-CO)(μ(3)-NPh) were measured as a function of temperature and resonance frequency in the solvent methylene chloride, and at several temperatures in chloroform. Phenyl T(1)s were used to calculate the diffusion coefficients characterizing molecular tumbling, D(.perp.), and spinning of the phenyl group, D(s); carbonyl relaxation times yielded the parallel diffusion coefficient, D(.paral.). Values of D(.perp.) calculated using the standard motional narrowing assumption were incorrectly observed to be frequency dependent, and inmarked error (by as much as a factor of two) from results obtained usin g the non-motionally narrowed equations. The spinning diffusion coefficients, D(s), were an order of magnitude greater than either D(.perp.) or D(.paral.), but somewhat lower than the rate of the equivalent rotation of free benzene in the same solvents, indicating the existence of a barrier to internal rotation of the phenyl group. Fenske-Hall and extended Hueckel calculations revealed that the barrier arises primarily from steric interactions between the ortho protons of the phenyl group and the carbon atom of the equatorial carbonyls.