ENDOR on vanadium d⁵-sandwich compounds

Abstract

Low temperature ENDOR measurements on dibenzene vanadium diluted in single crystals of (2,2)-paracyclophane and ruthenocene are reported. The proton hyperfine tensors of the rigid benzene rings of the paramagnetic probe yield information on site symmetry and, indirectly, on the space group of the matrix crystals at 10 K. The principal axes systems of the proton hyperfine tensors of dibenzene vanadium in (2,2)-paracyclophane essentially retain the D _6h symmetry of the molecule. However, the isotropic parts of the coupling tensors clearly classify the protons into three sets of equivalent nuclei, whereas from the symmetry of the substitution site in (2,2)-paracyclophane only two sets were to be expected. A lowering of the space group of this matrix crystal is suggested to explain the observed ENDOR spectra. The proton hyperfine data of dibenzene vanadium are discussed in terms of a LCAO-Mo model. From the orthorhombicity of the tensors the spin density in the 2p_z -orbital of the adjacent carbon is calculated and compared with data from N.M.R. contact shift studies. Temperature dependence of the ENDOR spectra relating to reorientation processes of the aromatic rings will be reported in the subsequent paper.