Molecular size and shape properties of diamondoid molecules occurring in crude oil

The molecular size and shape for 27 diamondoids molecules (adamantanes and alkyladamantanes, diamantine and alkyldiamantanes) were calculated by computational quantum mechanical modeling method of dispersion correction B3LYP-D3/6-311+G**, in which the family of adamantanes presents molecular sizes in terms of width of 6.8–7.9 Å, and for the family of diamantanes 6.8–7.4 Å, in terms of length the size for adamantanes are 7.6–9.5 Å and for diamantanes 9.3–10.0 Å and in terms of height are rounded from 7.4 to 9.6 Å for adamantanes and 7.4 to 8.5 Å for diamantanes. This size depends on the alkyl substitution either in CH₂ or CH bridgehead positions. A measure of spherical shape deviation in terms of ovality, (O = 1 for sphere shape) was calculated, in which for adamantane, methyladamantanes, dimethyladamantanes, and multi-substituted adamantanes, is 1.17, 1.21, 1.24, and 1.26–1.31, respectively and ovality value for diamantane is 1.20 and 1.22–1.27 for methyl substituted diamantanes. Ovality (shape) and molecular size differences between adamantane, methyladamantanes, dimethyladamantanes, multi-substituted adamantanes, and the corresponding diamantanes allow suggesting a dynamic model for separation from the linear alkanes in a mixture into a slit pore shape typical for microporous carbons.