Preferred Geometries and Energies of Sulfur Sulfur Interactions in Crystal Structures

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 sigma-hole interaction, where the positive potential on one sulfur atom (sigma-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.