O-17 NMR EVIDENCE FOR 180-DEGREES-FLIPS OF (COOH)2 UNITS IN DIMETHYLMALONIC ACID

A previous H-1, H-2 and C-13 NMR study of dimethylmalonic acid DMMA revealed an intriguing motional process in the hydrogen bonded CO*OH-HOO*C units [1]. Considering just the hydrogens it consists of an exchange of H and H. Two models, I and II, were proposed: (I) a mere exchange of H and H; (II) a 180-degrees-flip of the whole CO*OH-HOO*C unit followed by a rapid readjustment of H and H along the hydrogen bonds. To distinguish between the two models, we report here on O-17 NMR spectra and rotation patterns recorded at -55-degrees-C, where the motion is frozen out, and at 20-degrees-C, where the exchange rate is 147 kHz, from an orientated, O-17 enriched single crystal of DMMA. Apart from central transitions, a host of quadrupolar split satellites appears in the -55-degrees-C spectra. At 20-degrees-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-degrees-C spectra is not consistent with a mere exchange of H and H. Model II leads, on the other hand, to such severe exchange broadening of the satellites at 20-degrees-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-degrees-C from a second crystal. As model II requires, coalesced and exchange narrowed satellites indeed reappear at elevated temperatures.