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

Deuterium broad-line NMR experiments performed on crystalline tetragonal urea over a temperature range of 30-120-degrees-C allowed us to measure the rate of cis-trans isomerization as well as a previously described ''whole body'' two-site flip about the carbonyl. To follow the cis-trans rate, we found it useful to employ a selective inversion-recovery 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 barriers of 15.5 +/- 1.2 and 18.5 +/- 1.6 kcal/mol for the whole body motion and the cis-trans isomerization, respectively, are of the same order as previously suggested values and much higher than the corresponding solution values measured in protic and aprotic media. Both motions display modest or undetectable isotope effects when deuterons are replaced with protons and the residual deuterons are probed. We also estimate 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 solution. Thus, we demonstrate the viability of this approach in addressing very slow motions even for samples characterized by long spin-lattice relaxation times and poor signal to noise. The significant difference 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.