Hydronium ion complex of 18-crown-6: where are the protons? A density functional study of static and dynamic properties.

A quantum-chemical study employing the BLYP density functional is reported for the complex of H3O+ with 18-crown-6. According to a Car-Parrinello molecular dynamics (CPMD) study at 340 K, the complex is quite flexible, and is characterized by three quasi-linear (two-center) hydrogen-bond interactions for most of the time. On a time scale of 10 ps, frequent inversions of H3O+ are observed, as well as two 120 degrees rotations switching the hydrogen bonds from one set of crown-ether O atoms to the other. These results are consistent with density-functional studies of stationary points on the potential energy surface, which show how the crown "catalyzes" the guest's inversion. Two close-lying minima are characterized, as well as two distinct transition states connecting them, either via H3O+ inversion or rotation, with barriers of 1.0 and 4.6 kcal/mol, respectively, at the BLYP/II'//BLYP/6-31G level. Orbital interactions between lone pairs on ether O atoms and hydronium sigma(OH) antibonding orbitals are important factors for the directionality of the hydrogen bonds.