Molecular dynamics simulations on the protonated 222. H+ and 222.2H+ cryptands in water: Endo versus exo conformations
Résumé
MD simulations on the 222 cryptand, monoprotonated 222.H+ and diprotonated 222.2H+ in the endo-endo and exo-exo conformations have been performed in vacuo and in a bath of water molecules. It is found that intrinsically endo protonation is favoured over exo protonation due to internal N--H+...O hydrogen bonding which makes the cage more rigid. On the other hand, endo protonated forms display 'hydrophobic' hydration compared to exo forms. For the monoprotonated 222. H + endo conformer, one water molecule is hydrogen bonded inside the cage thereby forming a 'water cryptate'. From the hydration pattern found previously for the neutral 222 cryptand and for its cation complexes, we suggest mechanisms (not involving exo to endo conversions) for the protonation of 222 and for acid catalysed decomplexation of cryptates in the endo-endo form.