High Field Electron Paramagnetic Resonance Characterization of Electronic and Structural Environments for Paramagnetic Metal Ions and Organic Free Radicals in Deepwater Horizon Oil Spill Tar Balls

Abstract

In the first use of high-field electron paramagnetic resonance (EPR) spectroscopy to characterize paramagnetic metal-organic and free radical species from tar balls and weathered crude oil samples from the Gulf of Mexico (collected after the Deepwater Horizon oil spill) and an asphalt volcano sample collected off the coast of Santa Barbara, CA, we are able to identify for the first time the various paramagnetic species present in the native state of these samples and understand their molecular structures and bonding. The two tar ball and one asphalt volcano samples contain three distinct paramagnetic species: (i) an organic free radical, (ii) a [VO](2+) containing porphyrin, and (iii) a Mn(2+) containing complex. The organic free radical was found to have a disc-shaped or flat structure, based on its axially symmetric spectrum. The characteristic spectral features of the vanadyl species closely resemble those of pure vanadyl porphyrin; hence, its nuclear framework around the vanadyl ion must be similar to that of vanadyl octaethyl porphyrin (VOOEP). The Mn(2+) ion, essentially undetected by low-field EPR, yields a high-field EPR spectrum with well-resolved hyperfine features devoid of zero-field splitting, characteristic of tetrahedral or octahedral Mn-O bonding. Although the lower-field EPR signals from the organic free radicals in fossil fuel samples have been investigated over the last 5 decades, the observed signal was featureless. In contrast, high-field EPR (up to 240 GHz) reveals that the species is a disc-shaped hydrocarbon molecule in which the unpaired electron is extensively delocalized. We envisage that the measured g-value components will serve as a sensitive basis for electronic structure calculations. High-field electron nuclear double resonance experiments should provide an accurate picture of the spin density distribution for both the vanadyl-porphyrin and Mn(2+) complexes, as well as the organic free radical, and will be the focus of follow-up studies.

Nondestructive Analysis of Mummification Balms in Ancient Egypt Based on EPR of Vanadyl and Organic Radical Markers of Bitumen

“…First, Electron Nuclear Double Resonance (ENDOR) spectroscopy will provide additional information on the evolution of VO-P complexes, by probing hyperfine interactions with 1 H nuclei of metallic complexes and radicals. 46 Second, pulse-EPR methods will allow direct measurements of relaxation times T1 and T2, and measurement of small hyperfine interactions with 14 N, 13 C, 31 P and 33 S nuclei. 17,32,47 However, the ease of use of standard continuous wave X-band EPR spectroscopy (recording a spectrum takes only a few minutes) and the ubiquity of vanadyl geoporphyrins VO-P and radicals in bitumen, open the door to the easy characterization of bitumen in all types of archaeological artefacts.…”

High-Field (3.4 T) ENDOR Investigation of Asphaltenes in Native Oil and Vanadyl Complexes by Asphaltene Adsorption on Alumina Surface

“…In details, the nature and simulations of the W-band spectra are discussed in refs. [45,50,51]. For clarity, Figure 5 presents the calculated electronic-nuclear energy levels of the VO 2+ paramagnetic complex, EPR transitions, and the corresponding EPR absorption line with the values of the components of g and A close to those obtained in the experiment.…”

“…Different aspects of application of various electron paramagnetic resonance (EPR) and double resonance (ENDOR) techniques for the elucidation of the asphaltene architecture and the oil PC nature are covered in refs. [2,19,[32][33][34][35][36][37][38][39][40][41][42][43][44][45][46].…”

“…The effectiveness of the use of the VP as a native probe for the investigation of asphaltenes is determined by the resolved anisotropic spectral parameters (g-factor and hyperfine components, zero-field splitting, quadrupole tensors) in the EPR/ENDOR spectra, which makes VP to be orientationselective even in disordered media [40][41][42][43][44][45][46][47][48][49][50][51]. Using X-band (microwave frequency ν MW of 9 GHz, magnetic field B 0 of 0.34 T) and high-frequency (ν MW of 95 GHz, B 0 of 3.4 T, W-band) conventional and pulsed EPR/ENDOR techniques, we have shown that, on the one hand, the EPR and ENDOR spectra for VP in the initial oil samples of different genesis are similar while, on other hand, the relaxation parameters of VP (the electron longitudinal (T 1e ) and transverse (T 2e ) relaxation times) and the temperature dependences of the EPR spectra depend strongly on the nature of the sample and the method of the fractionation procedure [49][50][51][52].…”

Core‐crown Quantum Nanoplatelets with Favorable Type‐II Heterojunctions Boost Charge Separation and Photocatalytic NO Oxidation on TiO2

“…Due to the corresponding g-factor of the detected EPR signal, it is likely that this is new signal which is occurring only in the presence of UV irradiation is due to photo-induced electron transfer from the CdSeTe crown to the CdSe core of the quantum well system, where electron-trapping occurs in the CdSe core. While trapped electrons and trapped holes yield similar g-values (g e = 2.0023 [76] ) in EPR, detection of trapped hole states in EPR is difficult due to the significantly smaller EPR intensities of hole states. [77] It should be noted that control EPR experiments performed on P25 under UV irradiation and under dark conditions did not yield any evidence for such charge carriers, suggesting that most of the photo-induced charge carriers originated from NPL ( Figure S4).…”