34 GHz e.p.r. study of vanadyl complexes in various asphaltenes: Statistical correlative model of the coordinating ligands

High precision 34 GHz e.p.r. measurements were performed to study the coordination of vanadyl complexes in various asphaltenes. The spin-Hamiltonian parameter analysis indicates that the asphaltenes can be classified into those which have g- and A-parameters nearly identical to those for the vanadyl tetraphenyl porphyrins doped in carbonaceous materials and those for which these parameters are nearly identical to those for the vanadyl etioporphyrins. Quantitative statistical analysis of the parameters for model compounds with coordination N₄, N₂S₂, NS₃, S₄, S₂O₂, N₂O₂ and O₄ shows that these parameters cannot be used to predict the coordination for various asphaltenes. It is found that g₀ is a better parameter than A₀ to predict the coordination, although it cannot resolve the N₄, S₄ and S₂O₂ coordinations. However, the spin-Hamiltonian parameter resolution was sufficient to distinguish between those vanadyl complexes present in porphyrins, humates and minerals. The importance of high precision e.p.r. measurement at 34 GH_z or higher frequencies for model compounds and chromatographically-separated fractions is discussed.