HYDROGEN-BONDS, WATER ROTATION AND PROTON MOBILITY

Proton mobility is traditionally thought to be governed by water molec ule rotation. Water rotation times from D2O NMR spin-lattice relaxatio n measurements are compared with proton hopping times from mobility da ta with and without subtraction of the estimated hydrodynamic mobility . In the latter case the two data agree nicely at high temperatures. I t is concluded that the hydrodynamic proton mobility is considerably s maller 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 degrees C and has a hydrodynam ic component to its activation energy. Therefore, proton mobility is n ot governed by water rotation but rather both processes are controlled by hydrogen-bond dynamics. Comparison with hydrogen-bond lifetimes fr om depolarized light scattering suggests that two consecutive cleavage events of ordinary hydrogen bonds constitute a single proton hop. Thi s agrees with a recent molecular model for the Grotthuss mechanism.