Solution dynamics of the osmium cluster [Os3(µ-H)2(CO)10] by 13C and 17O nuclear magnetic resonance. Estimation of the 13C chemical shift anisotropy and 17O quadrupole coupling constant

Abstract

Application of the saturation-transfer technique to ¹³C n.m.r. spectra of ¹³C-enriched [Os₃(µ-H)₂(CO)₁₀] shows that only localised ¹³C exchange occurs, involving two of the four types of carbonyl ligand. Equations are derived to describe the recovery of the exchanging AX₂ spin system towards equilibrium, after an initial perturbation. When the initial perturbation is a selective inversion of A or X₂, values for the ¹³C relaxation parameters and the A X₂ exchange rate constant can be determined. The rate constant for this localised process is 0.36 s^–1 at 300 K in CDCl₃ solution. An estimate of the quadrupole coupling constant for ¹⁷O in the complex (1.59 MHz), together with ¹⁷O spin-lattice relaxation time measurements, yielded a value for the correlation time for molecular reorientation of 22 ± 4 ps. The ¹³C spin-lattice relaxation rates are strongly field dependent and values for the ¹³C shielding anisotropy in the region of 383 ± 50 p. p. m. were obtained.