Asphaltene aggregation causes several problems in the petroleum industry, and an understanding of the aggregation behavior is needed to solve the problems. In this study, the effects of asphaltene concentration on aggregate size were investigated to elucidate the aggregation mechanism of asphaltene. Diffusion coefficients (D) of three asphaltenes from the vacuum residue (VR) of Khafji, Iranian Light, and Maya crude oils and one resin were determined in deuterated chloroform solution using pulsed-field gradient spin-echo 1 H NMR. The pulsed-field gradient spin-echo 1 H NMR of asphaltenes and resin showed different diffusion behaviors from well-characterized reference compounds such as polystyrene because of structural and compositional irregularities; that is, they are a complex mixture of molecules of various molecular weights and structures. From the D values, their average hydrodynamic radii were estimated. The concentration dependency (0.1-30 g/L) of the D values and the hydrodynamic radii support the widely accepted stepwise aggregation mechanism (i.e., monomer < small aggregates < medium-size aggregates < large aggregates (precipitate)). At low concentrations (0.1 and 1 g/L), the D values corresponding to the range of small to medium aggregates were observed, and at higher concentrations (10 and 30 g/L), only medium aggregates were detected. Similar D values were obtained for the three asphaltenes, although Maya asphaltene gave slightly lower D values. For the resin, higher D values (lower hydrodynamic radii) were obtained than the asphaltenes.

Three-dimensional aromaticity arising from the close stacking of two antiaromatic π-conjugated macrocycles has recently received considerable attention. Here, a cyclophane consisting of two antiaromatic Ni­(II) norcorrole units tethered with two flexible alkyl chains was synthesized. The norcorrole cyclophane showed crystal polymorphism providing three different solid-state structures. Surprisingly, one of them adopted an aligned face-to-face stacking arrangement with negligible displacement along the slipping axis. Although the exchange repulsion between two π-clouds should be maximized in this orientation, the π–π distance is remarkably close (3.258 Å). Three-dimensional aromaticity in this conformation has been supported experimentally and theoretically as evidenced by small bond length alternations as well as the presence of a diatropic ring current. An analogous cyclophane with two aromatic Ni­(II) porphyrin units was prepared for comparison. The porphyrin cyclophane exhibited a slipped-stacking conformation with a larger displacement (2.9 Å) and a larger interplanar distance (3.402 Å) without noticeable change of the aromaticity of each porphyrin unit. In solution, the norcorrole cyclophane forms a twist stacking arrangement with effective interplanar orbital overlap and exists in an equilibrium between stacked and nonstacked structures. Thermodynamic parameters of the stacking process were estimated, revealing an inherently large attractive interaction operating between two norcorrole units, which has been further supported by energy decomposition analysis.

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