ESR studies of the coupling of translational and reorientational motions of dichlorobis(η-cyclopentadienyl)vanadium in solution

The translational diffusion constant of a paramagnetic species, dichlorobis(η-cyclopentadienyl)vanadium (V(η⁵-C₅H₅)₂Cl₂), abbreviated as CP₂VCl₂, in chloroform was measured at room temperature (25 ± 1°C) by a capillary diffusion method. Using the Stokes-Einstein equation for translational diffusion, the diffusion coefficient and the molecular hydrodynamic radius of Cp₂VCl₂ in chloroform were found to be 5.20 × 10^-6 cm²/s and 5.86 Å, respectively. An analysis of the temperature dependent ESR line widths of Cp₂VCl₂ in toluene in the motional-narrowing region was carried out using the four sets of the magnetic parameters available in the literature. It was found that only those by Peterson and Dahl could explain our line width data. Our analysis suggests that Cp₂VCl₂ exhibits anisotropic rotational diffusion with N = 2.7 ± 0.7 at an axis z′ = Y, where N is the ratio of R_∥/R_⊥, R_∥ is the rotational diffusion constant along the molecular Z axis, and R_⊥ is the rotational diffusion constant perpendicular to the molecular Z axis. This value is consistent with the N value obtained from the Stokes-Einstein model (N = 3.3) and the allowed-values equation, AVE, with ρ_X = 3.02ρ_Y -7.18. The AVE is not consistent with isotropic reorientation and only consistent with axial symmetry if ρ_X = 1, ρ_Y = 2.71. In addition, the rotational correlation times as a function of temperature were determined from the line width data. The anisotropic interaction parameter κ, which is a measure of the anisotropy of intermolecular interactions, was found to be 0.044. The κ value showed that Cp₂VCl₂ in toluene is more slip than VOAA in toluene, which is more stick (its κ is 0.58). Therefore, it seems that the κ value decreases with decreasing solute corrugation. The stickiness factor (S) was calculated for Cp₂VCl₂ in toluene and was found to be close to the slip limit (S ≈ 0), indicating that there is no tendency for the solvent (toluene) to stick at the surface of the diffusing Cp₂VCl₂ particles.