Theoretical Study of Binding Site Preference in [2]Rotaxanes.

Rotaxanes that can be switched between co-conformations by some external stimulus are of interest because the switching mechanism might be used to create molecular devices capable of producing useful work. Probably the most common approach to create a switchable rotaxane is to start with a rotaxane where the ring interacts more strongly with one of two possible binding sites along the shaft and then apply an external stimulus that weakens the binding interaction between the ring and the shaft at this site, thereby changing the binding site preference. We have investigated binding site preference in two rotaxanes and two pseudorotaxanes with electronic structure calculations at several levels of theory. To gain insight into the origins of the intercomponent binding, empirical approximations were applied to estimate the electrostatic and dispersion contributions. Dispersion has been thought to make an important contribution to the intercomponent interaction in the presence of π-π stacking interactions between the components, but the role of dispersion interaction has been a controversial issue because many computational methods neglect this interaction. For example, AM1 semiempirical calculations neglect dispersion but often predict correct co-conformational preferences. This suggests that inclusion of the dispersion interaction is required for correct quantitative, but not qualitative, description of the intercomponent binding, a result that is supported by the analytic partitioning of the binding interactions. The origins of this result are investigated.