C-13 NMR IN LIQUID-CRYSTAL SOLUTIONS - HINDERED ROTATION IN 4-(DIMETHYLAMINO)PYRIMIDINE

The hindered rotation of the dimethylamino group in 4-(dimethylamino)pyrimidine in a liquid-crystal solution (Merck ZLI 2142) was studied from 232 to 263 K by natural-abundance carbon-13 NMR. The enthalpy of activation ( DELTA H** DOUB DAG ) in the liquid-crystal solution is 53 plus or minus 1 kJ mol** minus **1, which is 50% higher than that in CD//2Cl//2(35 plus or minus 1 kJ mol** minus **1) and about the same as that in CD//3OD (52 plus or minus 1 kJ mol** minus **1). The ordering forces of the liquid-crystal solvent may favor the planar ground state of the solute more than the bulkier transition state, causing the barrier of rotation to be unusually high.