Asphaltene deposition is a well-known problem in the petroleum industry. Nevertheless, there seems to be a lack of information on the processes involved in asphaltene association and its relationship to asphaltene solubility under certain conditions. Molecular mechanics and molecular dynamics have had an important role in the investigation of these phenomena. To better understand the role of solvents in fractionating asphaltenes extracted from vacuum residues and evaluate their tendency to dissociate under different conditions, we modeled the effect of toluene, n-butane, isobutane, and n-heptane on an aggregate formed by two asphaltene molecules that would have a tendency to associate (not average structures commonly used in similar studies). Molecular dynamics simulations were performed on an asphaltene dimer after minimizing the conformation of each molecule and verifying the most stable position for docking. They reveal the extent to which these solvents are able to separate the aggregate at different temperatures after a given period of time. As expected, toluene is the most effective and n-heptane affects the aggregate the least, with n-butane and isobutane falling between these two bounds.

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Substituent effects are widely used to investigate the factors affecting carbon‐13 chemical shifts. Adamantane and its derivatives are convenient probes for relative contributions to these factors owing to their symmetry and relative absence of ring strain. We have extended our studies on NMR chemical shifts of 1‐ and 2‐methyladamantanes to hydroxy, bromo, methoxy and acetamide substituents and also certain disubstituted (one of the groups being methyl) analogs. DFT/GIAO calculations at the B3LYP/6–31G(d,p) level show that, except for α‐effects, steric interactions are mainly responsible for substituent effects on chemical shifts. The CHARGE program is particularly well suited for localizing these effects and estimating their approximate shape and range.