Proton transfer dynamics and N?H bond lengthening in N?H���N model systems: a solid-state NMR study

Abstract

Hydrogen-bonded pairs of the 2-methylimidazolium cation and 2-methylimidazole are expected to have matched pKa values, owing to chemical symmetry. These structures serve as models for enzyme active sites. Several crystalline salts of this system were prepared and characterized by crystallography and solid-state NMR. Short N···N distances are observed (∼2.65 Å), and fast (>105 s−1) proton transfer through the N—H···N bridge was suggested by NMR lineshape measurements at temperatures from 200 to 320 K. The equilibrium constants for these transfer processes were found to differ from unity, and to be strongly temperature and counter-ion dependent. For the perchlorate salt, the proton is observed to be mainly attached to one of the bridge nitrogens across this temperature range, for the iodide salt, the proton is substantially delocalized on to both bridging nitrogens across the temperature range. Interestingly, for the chloride and bromide salts, a temperature-dependent equilibrium constant is observed, with equal populations of the two isomers in rapid exchange at room temperature and an effectively trapped proton (or very strong population of one isomer) at 200 K. This temperature-dependent equilibrium constant indicates that the proton transfer is associated with an enthalpy of the order the 20 kJ mol−1. This study underscores the power of NMR spectroscopy to account for protons, and the important influence of remote ionic interactions on proton transfer coordinates. Copyright © 2001 John Wiley & Sons, Ltd.