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-CH₂SH) in the crystal structures from the CSD indicates that in the sulfur–sulfur interactions the preferred is the 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 interaction in methanethiol dimer (−2.20 kcal/mol) is the geometry with the σ-hole interaction, where the positive potential on one sulfur atom (σ-hole) interacts with negative potential on the sulfur atom of the second molecule. SAPT decomposition of the interaction energies was performed in order to explain the nature of the interactions. This study points out the importance of parallel interactions of cysteine residues and can be useful for recognizing the sulfur–sulfur interactions in the crystal structures and biomolecules.

Stacking interactions of organometallic sandwich and half-sandwich compounds with cyclopentadienyl (Cp) were studied by searching and observing the crystal structures in the Cambridge Structural Database and performing density functional calculations. The strongest calculated interactions are at an offset of 1.5 Å with energies for sandwich and half-sandwich dimers of −3.37 and −2.87 kcal mol⁻¹, respectively, somewhat stronger than the stacking interaction between two benzene molecules, −2.73 kcal mol⁻¹. At large offsets of 5.0 Å, 74% of the strongest energy is preserved for the sandwich dimer and only 29% for the half-sandwich dimer. In crystal structures, for sandwich compounds, the stacking at large offsets is dominant (73%), since the interaction at large offsets is relatively strong, and the geometries enable additional simultaneous interactions with Cp faces. The stacking at large offsets between half-sandwich compounds is less dominant, since the interaction is weaker. However, Cp half-sandwich compounds stack at large offsets unexpectedly often (almost 60%), since the branching of their other ligands in the compound favors more simultaneous interactions with Cp faces. Strong interaction at large offsets for sandwich compounds is the consequence of favorable electrostatic interaction, which is not the feature of stacking between half-sandwich compounds.

Positive values of electrostatic potentials above the central regions of the molecular surface are strongly related to the high sensitivities of highly energetic molecules. The influence of aromatic system size on the positive values of electrostatic potentials and bond dissociation energies of C–NO₂ bonds was studied by Density Functional Theory (DFT) calculations on a series of polycyclic nitroaromatic molecules. Calculations performed at PBE/6-311G** level showed that with the increase of the aromatic system size, values of positive electrostatic potential above the central areas of selected energetic molecules decrease from 32.78 kcal mol⁻¹ (1,2,4,5-tetranitrobenzene) to 15.28 kcal mol⁻¹ (2,3,9,10-tetranitropentacene) leading to the decrease in the sensitivities of these molecules towards detonation. Results of the analysis of electrostatic potential maps were in agreement with the trends in bond dissociation energies calculated for C–NO₂ bonds of studied nitroaromatic molecules. Bond dissociation energies values indicate that the C–NO₂ bond in the molecule of 1,2,4,5-tetranitrobenzene (56.72 kcal mol⁻¹) is weaker compared to the nitroaromatic molecules with the additional condensed aromatic rings and with a similar arrangement of –NO₂ groups (59.75 kcal mol⁻¹ in the case of 2,3,9,10-tetranitropentacene). The influence of the mutual arrangement of –NO₂ groups on the sensitivity of nitroaromatic molecules was also analyzed. Results obtained within this study could be of great importance for the development of new classes of highly energetic molecules with lower sensitivity towards detonation.

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 C ₂ 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.

Non-covalent selenium–selenium interactions between selenium-containing organic molecules were studied in crystal structures from the Cambridge Structural Database and by high-level quantum chemical calculations. Se⋯Se contacts in the crystal structures were analyzed, and the most frequent patterns were identified and used to design the model systems for quantum chemical calculations. The strongest calculated Se⋯Se interaction (ΔE_CCSD(T)/CBS = −2.31 kcal mol⁻¹) was identified in the model system with a mutual parallel orientation of interacting molecules. In the crystal structures, this orientation of molecules is predominant. In the geometry with the σ-hole bonding, the interaction is somewhat weaker (ΔE_CCSD(T)/CBS = −2.13 kcal mol⁻¹). NCI analysis showed that Se⋯Se interaction in the most stable geometries is further enhanced by hydrogen bonding of Se–H⋯Se or C–H⋯Se type. The results of energy decomposition analysis (SAPT) calculations revealed that the nature of the Se⋯Se interaction is predominantly dispersive with a strong electrostatic contribution. The results of the energy decomposition analysis also suggest that the electrostatic component has a crucial role in defining the geometry of selenium–selenium interactions due to their directional nature.

Non-covalent interactions between disulfide fragments and sulfur atoms were studied in crystal structures of small molecules and by quantum chemical calculations. Statistical analysis of the geometrical data from the Cambridge Structural Database (CSD) reveals that in most cases, interactions between sulfur and disulfide bonds are bifurcated. Quantum chemical calculations are in agreement with those findings. A strong interaction energy was calculated for bifurcated interactions (E_CCSD(T)/CBS = −2.83 kcal mol⁻¹) considering the region along the disulfide bond. Non-bifurcated interactions are weaker except in cases where σ-hole interaction is possible or in cases where S⋯S interaction is accompanied by additional hydrogen bonds (E_CCSD(T)/CBS = −3.26 kcal mol⁻¹). SAPT decomposition analysis shows that dispersion is the main attractive force in the studied systems while electrostatics plays a crucial role in defining the geometry of interactions.