Solid- and Solution-State Nuclear Magnetic Resonance Analyses of Ecuadorian Asphaltenes: Quantitative Solid-State Aromaticity Determination Supporting the 'Island' Structural Model. Aliphatic Structural Information from Solution-State H-1-C-13 Heteronuclear Single-Quantum Coherence Experiments

摘要

Solid- and solution-state nuclear magnetic resonance (NMR) studies of six Ecuadorian asphaltenes have provided insight into using NMR to characterize asphaltenes. For these asphaltenes, free radicals prevent obtaining quantitatively meaningful H-1-C-13 cross-polarization magic angle spinning (CPMAS) spectra but facilitate obtaining meaningful carbon aromaticity values in direct polarization with C-13 pulse excitation. The benefit of studying any asphaltene by obtaining a series of CPMAS spectra with multiple contact times, rather than just a single contact time, is demonstrated. Analyzing the CPMAS and direct C-13 pulse spectra indicates that, on average, condensed aromatic ring systems become larger as the aromaticity increases, which seems more consistent with "island" than "archipelago" aromatic structures, just as recently proposed for asphaltenes. Solution-state two-dimensional (2D) H-1-C-13 heteronuclear single-quantum coherence (HSQC) spectra of the asphaltenes provide a wealth of structural information and are particularly useful for demonstrating that isolated methyl branches on alkyl chains are much more abundant than aromatic methyl groups. The prospect of using cryoprobes to obtain one-dimensional (ID) and 2D spectra with a much higher signal-to-noise ratio (S/N) allowing for a more detailed analysis of the less abundant structural environments is clearly attractive.