EPR STUDY OF THE ANISOTROPIC INTERACTION PARAMETER OF DICHLOROBIS(ETA-CYCLOPENTADIENYL)VANADIUM IN CHLOROFORM

The temperature dependent EPR line widths of a paramagnetic species, [ dichlorobis(eta-cyclopentadienyl)vanadium] or [V(eta(5)-C5H5)(2)Cl-2], abbreviated as Cp2VCl2, were measured in chloroform at X band. An ana lysis of the temperature dependent EPR line widths of Cp2VCl2 in chlor oform in the motional narrowing region was carried out, using the four sets of the magnetic parameters available in the literature. It was f ound that only those from Petersen and Dahl [J. L. Petersen and L. F. Dahl, J. Am. Chem. Sec., 97 (1975), p. 6422.] could explain our line w idth data. Our analysis in chloroform is in agreement with our results in toluene and suggests that Cp2VCl2 exhibits anisotropic rotational diffusion with N = 2.7 +/- 0.7 at an axis z' = y, where N is the ratio R-parallel to/R-perpendicular to;R-parallel to is the rotational diff usion constant along the molecular z axis; and R-perpendicular to, is the rotational diffusion constant perpendicular to the molecular z axi s. This value is consistent with the N value obtained from the Stokes- Einstein model (N = 3.3) and the allowed-values equation, AVE, with rh o(x) = 3.02 rho(y) - 7.18. The AVE is not consistent with isotropic re orientation and is only consistent with axial symmetry if rho(x) = 1, rho(y) = 2.71. The anisotropic interaction parameter kappa, which is a measure of the anisotropy of intermolecular interactions, was found t o be 0.156. The kappa value of 0.156 showed that Cp2VCl2 in chloroform is less slip than Cp2VCl2 in toluene (its kappa is 0.044). Therefore, it seems that the kappa value increases with increasing solute-solven t interaction. The stickiness factor (S) was calculated for Cp2VCl2 in chloroform and was found to be close to the slip limit (S approximate to 0.1), indicating that there is very slight tendency for the chloro form solvent molecules to stick to the surface of the diffusing Cp2VCl 2 particles.