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Abstract:
A previous 1H, 2H and 13C NMR study of dimethylmalonic acid DMMA revealed an intriguing motional process in the hydrogen bonded CO*OH-[Htilde]ÕÕ*[Ctilde] units [1]. Considering just the hydrogens it consists of an exchange of H and [Htilde]. Two models, I and II, were proposed: (I) a mere exchange of H and [Htilde]; (II) a 180°-flip of the whole CO*OH-[Htilde]ÕÕ*[Ctilde] unit followed by a rapid readjustment of H and [Htilde] along the hydrogen bonds. To distinguish between the two models, we report here on 17O NMR spectra and rotation patterns recorded at -55°C, where the motion is frozen out, and at 20°C, where the exchange rate is 147 kHz, from an orientated, 17O enriched single crystal of DMMA. Apart from central transitions, a host of quadrupolar split satellites appears in the -55°C spectra. At 20°C we still observe central transitions, but virtually no satellites. By way of simulations of spectra/rotation patterns, we show that the absence of satellites in the 20°C spectra is not consistent with a mere exchange of H and [Htilde]. Model II leads, on the other hand, to such severe exchange broadening of the satellites at 20°C that they become unobservable in a real experiment. Model II is therefore consistent with our experiments and is thought to apply to DMMA. This conclusion is checked by recording spectra up to 70°C from a second crystal. As model II requires, coalesced and exchange narrowed satellites indeed reappear at elevated temperatures.
