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Zusammenfassung:
The proton and carbon shielding and the carboxyl deuteron quadrupole coupling tensors were measured in single crystals of dimethylmalonic acid. For the first time sufficient resolution has been achieved in solid-state multiple-pulse proton NMR to allow tracing out a methyl group proton shielding tensor. Its anisotropy is only 1.6 ppm. At room temperature only half as many carboxyl proton and deuteron resonances are observed as crystal symmetry predicts. This is the consequence of a novel hydrogen exchange process within dimeric units, -COOH-HOOC-. Lineshape analysis of deuteron spectra yields the temperature dependence of the rate k of the exchange process. Its activation energy is 66 kJ/mol; k(T = 281 K) = 5 × 104, s−1. A flip of the whole dimeric unit followed by a rapid concerted jump of the hydrogens along strongly asymmetric hydrogen bonds is thought to be the most plausible sequence of events. This hypothesis might be subjected to a test by 17O NMR.
