Nuclear-Electron Overhauser Effect in MC800 Liquid Asphalt Solutions

Experimental results on the extrapolated ultimate enhancement factors of o-, m-, and p-xylene protons at 1.53 mT are obtained for MC800 asphalt solutions. The ultimate enhancement factors are found such as ?26.9, ?25.7, and ?11.7 for o-, m-, and p-xylene, respectively. These results show that the solvent proton Overhauser effect cannot reach the extrapolated enhancement of ?330 in the extreme narrowing case because of occurrence of small scalar interactions in addition to the dipole–dipole interactions between solvent protons and asphalt electrons. The ortho, meta, and para positions of the –CH₃ group change the nature of the interactions. The nuclear magnetic resonance (NMR) signal enhancements exhibit a sensitive behavior depending on the chemical environment differing from isomer to isomer. The solvation or association of asphalt in xylene isomers at room temperature is revealed. Quantum chemical calculations for the xylene isomers with the electronic and optical properties; absorption wavelengths, excitation energy, atomic charges, dipole moment and frontier molecular orbital energies, molecular electrostatic potential; are carried out using the density functional theory (DFT) method (B3LYP) with the 6-311G(d,p) basis set by the standard Gaussian 09 software package program. The relative importance of scalar and translational dipolar interaction parameters determined in dynamic nuclear polarization experiments is explained by the electronic structure of HOMO–LUMO of the xylene isomers.