Vanadyl Porphyrin Speciation Based On sub-MHz Ligand Proton Hyperfine Couplings

The speciation of vanadyl (VO2+) porphyrins in crude oil (vanadyl petroporphyrins) is an area of ongoing interest in petroleomics. In this paper, we describe a method for the speciation of vanadyl porphyrins that uses Electron Nuclear DOuble Resonance (ENDOR), a high-resolution Electron Paramagnetic Resonance (EPR) spectroscopic technique. We use 1H ENDOR to measure hyperfine couplings between ligand protons and the paramagnetic vanadyl ion as small as about 0.1 MHz. From the measured hyperfine couplings, we directly determine all vanadium–ligand proton distances up to 8 A. This information differentiates porphyrin ligands by their ring substitution pattern and substituent nature. We demonstrate this using a series of vanadyl porphyrin model compounds. Additionally, we demonstrate that the composition of binary vanadyl porphyrins mixtures can be determined. The ability of ENDOR to differentiate types of ligand protons in vanadyl porphyrin mixtures provides a basis for analyzing more complex mixtures of va...