THE BARRIER TO INTERNAL-ROTATION AND CHEMICAL-EXCHANGE IN N-ACETYLPYRROLE - A STUDY BASED ON NMR METHODS AND MOLECULAR MODELING

The rate of exchange governed by the rotation about the C-N single bond joining the acetyl group to the ring in N-acetylpyrrole is measured using a novel combination of NMR techniques in the fast, intermediate, and slow exchange regimes. This gives a very wide range of rates and permits the determination of good values of the enthalpy, Delta H*, and the entropy, Delta S*, of activation. The calculation of errors in the activation parameters is also discussed. The enthalpy is measured to be 53 +/- 0.6 kJ mol(-1), and the entropy of activation is 5 +/- 3 J K-1 mol(-1). These errors are 95% confidence regions, approximately two standard deviations. Tn the Arrhenius formulation, the energy of activation, Delta E*, is 55 +/- 0.6 kJ mol-L. The barrier is also calculated using ab initio calculations at the HF/6-31G** level of theory to give a value which corresponds to Delta H* of 52.8 kJ mol(-1). This value is lower than in most simple amides and indicates substantial interaction between the nitrogen lone pair and the pi system in the ring.