Abstract
High-resolutionsolid-state‘HNMRspectrawere obtainedbycombiningdeuteriumspin-dilutionandhigh- speed magic-angle spinning (MAS). The effective proton relaxation time 7), which is long in these magnetically dilute samples, was reduced by reverse cross-polarization (RCP) of protons from deuterons. Two-dimensional 'H NOESY
spectra obtained with RCP were used to investigate proton exchange in solids, and the rapid proton exchange observed in this way was linked to hydrogen bonds. A quantitative analysis of the dependence of the 2D spectral intensities on mixing time, as a function of temperature, was employed to determine the rates, and activation energies, for proton/ deuteron exchange between the carboxyl groups of the oxalate moiety and the water of hydration in oxalic acid dihydrate. We find rate constants of 55 and 58 s_1 at 17 °C and activation energies of 8.5 and 8.9 kcal/mol over the temperature range -26 to 28 °C.