Interplay Between π-Stacking and Hydrogen Bonding in the Self-Association of Different Isomers of Naphthalenedicarboxylic Acid.

Using proton and carbon chemical shifts, we investigated the self-association of three isomers of naphthalenedicarboxylic acid, a model for the aggregation of asphaltenes. Experimental proton chemical shifts of each isomer were measured as a function of concentration in an aprotic solvent. Several potential structures of the monomer and dimer of each naphthalenedicarboxylic acid were considered, and calculated proton chemical shifts for the potential monomer and dimer structures were compared to the experimental chemical shifts to find the weighted average structure that best fit the experimental shifts. Calculated carbon chemical shifts were also compared to experimental values. The chemical shift comparison and calculated energies indicate that π-stacked dimers are not likely to contribute significantly to the dimer structure of any of the three naphthalenedicarboxylic acid isomers studied.