On the properties of S center dot center dot center dot O and S center dot center dot center dot pi noncovalent interactions: the analysis of geometry, interaction energy and electron density

Computational studies have been carried out to investigate the origin and magnitude of chemically and biologically important S center dot center dot center dot O and S center dot center dot center dot pi interactions at the MP2/aug-cc-pVDZ level. All the model complexes involving S center dot center dot center dot O and S center dot center dot center dot pi interactions exhibit similar structures in which the O atom or the pi-system approaches the S atom from the backside of the F-S bond (in the sigma(star)(S) direction). The decomposition of interaction energy shows that induction energy plays a dominant role in stabilizing the complexes, indicating that the primary source of such interaction is the charge transfer from the O lone pair or pi-electrons to the S sigma(star) antibonding orbital. The topological analysis of electron density at the bond critical points (BCPs) reveals that S center dot center dot center dot O and S center dot center dot center dot pi interactions in these complexes are pure closed-shell interactions in nature. Our results suggest that accurate quantum chemical calculations on models of noncovalent interactions may be helpful in understanding the structures of proteins and other complex systems.