No abstract

A previous ~H, 2H and nC NMR study of dimethylmalonic acid DMMA revealed an intriguing motional process in the hydrogen bonded CO*OHI~t)O*t~ units [1]. Considering just the hydrogens it consists of an exchange of H and I:I. Two models, I and II, were proposed: (I) a mere exchange of H and I4; (II) a 180~ of the whole CO*OH-lqO0*(~ unit followed by a rapid readjustment of H and Iq along the hydrogen bonds. To distinguish between the two models, we report here on ~70 NMR spectra and rotation patterns recorded at -55~ where the motion is frozen out, and at 20~ where the exchange rate is 147 kHz, from an orientated, ~70 enriched single crystal of DMMA. Apart from central transitions, a host of quadrupolar split satellites appears in the -55~ spectra. At 20~ 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~ spectra is not consistent with a mere exchange of H and Iq. Model II leads, on the other hand, to such severe exchange broadening of the satellites at 20~ 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~ from a second crystal. As model II requires, coalesced and exchange narrowed satellites indeed reappear at elevated temperatures.