title: Evaluation of weak interactions in [2]pseudorotaxanes.
abstract: Viologens readily thread bis-p-phenylene crown ethers to form [2]pseudorotaxanes. However, the binding of sterically hindered 3,3'-dimethylviologens is very weak. Density functional theory (DFT) calculations indicated that the additional energy cost of "flattening" is substantial, 55 kJ mol(-1), and prevents the formation of a stable host-guest complex. The structures of [2]pseudorotaxanes determined by X-ray crystallography are in good agreement with the NMR characterisation and DFT results. Their association constants and thermodynamic parameters in solution were measured by using a dilution method and, for the first time, by host-guest nuclear Overhauser effect (NOE) correlations. The NOE approach was subsequently applied to study the sterically hindered analogues and it was shown that the binding in 3,3'-dimethyl-N,N-dibenzyl [2]pseudorotaxane is by 8.5 kJ mol(-1) weaker than in its regular analogue. The proposed technique helps to quantify weak interactions in [2]pseudorotaxanes and can be applied to other host-guest complexes.
authors.affiliation: School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin 4 (Ireland), Fax: (+353) 01 7162127. kirill.nikitin@ucd.ie.
authors.forename: Kirill
authors.initials: K
authors.lastname: Nikitin
authors.forename: Helge
authors.initials: H
authors.lastname: Müller-Bunz
authors.forename: Jimmy
authors.initials: J
authors.lastname: Muldoon
keywords: NMR spectroscopy; host-guest complex; molecular recognition; supramolecular chemistry