Proton–Radical Interaction in Crude Oil—A Combined NMR and EPR Study

Abstract

We present a detailed study of electron/nuclear interaction in a specific crude oil by continuous-wave and pulsed EPR, electron–nuclear double resonance (ENDOR) at W-band (94 GHz), and fast field-cycling dynamic nuclear polarization (FFC-DNP) at X-band. A perceptible non-Overhauser (solid) effect is found at room temperature as a result of the polarization transfer from the intrinsic oil “free” radicals to the 1H nuclei with different dynamics. On the basis of the analysis of the longitudinal nuclear relaxation times, three dynamical components described by different electron–proton coupling parameters were found, which in combination with ENDOR provides information about the distribution of the radicals in the high-molecular oil components.

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

“…(see [46] and references therein) and are not in the scope of the present study. Assuming a nearly planar skeleton structure of a single VP molecule (VO 2+ , S = 1/2, Figure 3), the EPR spectra could be satisfactory described by the g-tensor of axial symmetry with g ∥ ≈ 1 96, g ⊥ ≈ 1 98, and anisotropic hyperfine interaction with a magnetic moment I = 7/2 (therefore, 8 projections of I with the quantum numbers m I = ±7/2; ±5/2; ±3/2 and ±1/2 are possible) for 51 V nuclei (natural abundance of 99.75%) with hyperfine structure constants A ∥ ≈ 480 MHz, A ⊥ ≈ 157 MHz (Table 2).…”

“…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].…”

Qualitative and Quantitative Analysis of Heavy Crude Oil Samples and Their SARA Fractions with 13C Nuclear Magnetic Resonance

“…Use of radiospectroscopic techniques such as nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) [4], and their double (NMR and EPR) resonance combinations [5,6] in high magnetic fields (B 0 > 1.5 T) have greatly expanded the possibilities for the selective detection of oil structural fragments and functional groups mainly due to the higher spectral resolution. NMR spectroscopy is based on the phenomenon of resonance absorption of the energy of a radiofrequency (in the frequency range of about f RF = 1-1000 MHz) electromagnetic field due to atomic nuclei having a non-zero magnetic moment.…”

“…Interest in the determination of SARA (saturates, aromatics, resins, and asphaltenes) components directly from the 13 C NMR experiments is also governed by possibilities to enhance the NMR signal in heavy oils through dynamic nuclear polarization (DNP) by applying EPR to the native for oil [6,[34][35][36][37] or artificially introduced [38] paramagnetic centers. It was shown that 1 H NMR signal enhancement depends not only on the type, electronic properties, concentration of paramagnetic species, and viscosity of oils but also on the SARA fraction [6,34] that can be selected in situ by choosing an appropriate repetition time of NMR experiment. To the best of the authors' knowledge, all of the oil DNP measurements to date have been conducted on 1 H nuclei.…”