Hydrogen bonds, water rotation and proton mobility

Proton mobility is traditionally thought to be governed by water molecule rotation. Water rotation times from D₂O NMR spinlattice relaxation measurements are compared with proton hopping times from mobility data with and without subtraction of the estimated hydrodynamic mobility. In the latter case the two data agree nicely at high temperatures. It is concluded that the hydrodynamic proton mobility is considerably smaller than previously believed because H₃O⁺ is nearly immobilized by extra-strong hydrogen-bonds to first-shell water ligands, estimated to be about 2 kcal/mol stronger than bulk hydrogen-bonds. Water rotation is slower than proton hopping below 20°C and has a hydrodynamic component to its activation energy. Therefore, proton mobility is not governed by water rotation but rather both processes are controlled by hydrogen-bond dynamics. Comparison with hydrogen-bond lifetimes from depolarized light scattering suggests that two consecutive cleavage events of ordinary hydrogen bonds constitute a single proton hop. This agrees with a recent molecular model for the Grotthuss mechanism.