  Structural characterization of metalloporphyrins in complex systems such as
native hydrocarbons is in the focus of scientific and industrial interests
since many years. We describe electron-nuclear double resonance (ENDOR) of
crude oil from the well without any additional sample treatment (i.e., in the
native environment) in the magnetic field of about 3.4 T and temperature of 50
K by applying microwave pulses at 94 GHz (W-band) and radiofrequency pulses at
near the proton Larmor frequencies of 144 MHz to probe the paramagnetic
vanadyls. By means of density functional theory (DFT) calculations, ENDOR
features are explained and ascribed to certain vanadyl porhyrin structural
forms known to be present in crude oil.
