Perhaps unsurprisingly, the optimised structure of [1H + •9] was found to be a threaded complex, in which enolate 9 H-bonds to the protonated bipyridinium, which was predicted to be 7.1 kcalmol -1 more stable than the π-stacked structure (Figure ). The competition between the threaded structure and the stacked structure may explain the lower selectivity for 12 observed in reactions catalysed by 1H + . The same calculations for rotaxane 6H + , which lacks the NDI moiety but in which the cavity of the macrocycle is blocked, show that the binding of 9 is around 3 kcal/mol less favourable than for rotaxanes 4H + , which is in keeping with the smaller MEP of 1H + . Based on these results, regardless of which -surface mediates anion-π catalysis in the case of rotaxanes 4, the pyridinium moiety appears to be responsible for the behaviour of rotaxane 6 and may well play a role for macrocycle 1, albeit in competition with a threaded, hydrogen bonded structure.