It has been demonstrated that sulfur-sulfur interactions can exist in various molecular systems. In this work we investigated sulfur-sulfur interactions in crystal structures of small molecules by analyzing geometric data from the Cambridge Structural Database (CSD) and by quantum chemical calculations. The analysis of cysteine residues (R-CH 2 SH) in the crystal structures from the CSD indicates that in the sulfur-sulfur interactions the preferred is parallel orientation of two C-S-H planes. Quantum chemical calculations were performed on model systems of methanethiol dimers. The most stable geometry of methanethiol dimer with parallel orientation of C-S-H planes is significantly strong; the interaction energy is -1.80 kcal/mol calculated at the very accurate CCSD(T)/CBS level. However, the strongest sulfur-sulfur Author Contributions: I.S.A. performed quantum chemical calculations, interpreted the data, and prepared the manuscript.; G.V.J. proposed topic and performed CSD search.; M.K.M. has done SAPT, QTAIM and NCI index method calculations and analyzed data.; S.D.Z. analyzed the results, supervised, wrote and edited the manuscript.

73% of stacking of Cp sandwiches in the CSD is at large offsets, since these interactions are relatively strong. Much weaker stacking between Cp half-sandwiches is surprising 60% of all stacks, due to more simultaneous interactions.

Edge-to-face interactions between two pyridine molecules and the influence of simultaneous hydrogen bonding of one or both of the pyridines to water on those interactions were studied by analyzing data from ab initio calculations. The results show that the edge-to-face interactions of pyridine dimers that are hydrogen bonded to water are generally stronger than those of non-H-bonded pyridine dimers, especially when the donor pyridine forms a hydrogen bond. The binding energy of the most stable edge-to-face interacting H-bonded pyridine dimer is -5.05 kcal/mol, while that for the most stable edge-to-face interacting non-H-bonded pyridine dimer is -3.64 kcal/mol. The interaction energy data obtained in this study cannot be explained solely by the differences in electrostatic potential between pyridine and the pyridine-water dimer. However, the calculated cooperative effect can be predicted using electrostatic potential maps.

Tetrathiafulvalene (TTF) and its derivatives are very well known as electron donors with widespread use in the field of organic conductors and superconductors. Stacking interactions between two neutral TTF fragments were studied by analysing data from Cambridge Structural Database crystal structures and by quantum chemical calculations. Analysis of the contacts found in crystal structures shows high occurrence of parallel displaced orientations of TTF molecules. In the majority of the contacts, two TTF molecules are displaced along their longer C2 axis. The most frequent geometry has the strongest TTF–TTF stacking interaction, with CCSD(T)/CBS energy of −9.96 kcal mol−1. All the other frequent geometries in crystal structures are similar to geometries of the minima on the calculated potential energy surface.

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