H+-catalyzed symmetric proton exchange in neat liquids with a network of N-H•••N and O-H•••O hydrogen bonds and molecular mechanism of Grotthuss proton migration

Proton exchange rates provide direct information about the mean lifetime tau(H)(+) and Grotthuss mobility 1/tau(H)(+) of solvated protons in liquids. Here we will analyze our own data of NMR studies on liquid hydrazine and its methyl derivatives as well as the available data on liquid NH3, methanol, and water with special emphasis on relationship between 1/tau(H)(+) and (a) orientational mobility of molecules, (b) structure of hydrogen-bonded liquids, and (c) structure of proton H solvates. Variation in 1/tau(H)(+), different temperature behavior, and unexpectedly high proton mobility in some hydrazines H can be rationalized in terms of the so-called structural diffusion, that is, diffusion of structural defects formed by excess protons in hydrogen-bonded liquids. Also discussed is the effect of hydrophobic substituents. (C) 2002 Elsevier Science B.V. All rights reserved.