CIS-TRANS ENERGETICS IN UREA AND ACETAMIDE STUDIED BY DEUTERIUM NMR

Deuterium broad-line NMR experiments performed on crystalline tetragon al urea over a temperature range of 30-120-degrees-C allowed us to mea sure the rate of cis-trans isomerization as well as a previously descr ibed ''whole body'' two-site flip about the carbonyl. To follow the ci s-trans rate, we found it useful to employ a selective inversion-recov ery experiment using a DANTE sequence applied at parallel edge of the powder pattern followed by a variable mixing time and quadrupolar echo detection. T1 anisotropy and line shape experiments were also used to measure the whole body flip and the cis-trans isomerization. The barr iers of 15.5 +/- 1.2 and 18.5 +/- 1.6 kcal/mol for the whole body moti on and the cis-trans isomerization, respectively, are of the same orde r as previously suggested values and much higher than the correspondin g solution values measured in protic and aprotic media. Both motions d isplay modest or undetectable isotope effects when deuterons are repla ced with protons and the residual deuterons are probed. We also estima te a barrier of 21.7 +/- 2 kcal/mol for the cis-trans isomerization in rhombohedral acetamide, which is a much higher than the value in solu tion. Thus, we demonstrate the viability of this approach in addressin g very slow motions even for samples characterized by long spin-lattic e relaxation times and poor signal to noise. The significant differenc e in the barrier heights between acetamide and urea in the solid state as compared with previous solution measurements is also discussed in relation to hydrogen bonding.