Ad. Bain et al., THE BARRIER TO INTERNAL-ROTATION AND CHEMICAL-EXCHANGE IN N-ACETYLPYRROLE - A STUDY BASED ON NMR METHODS AND MOLECULAR MODELING, Journal of physical chemistry, 98(31), 1994, pp. 7458-7463
The rate of exchange governed by the rotation about the C-N single bon
d 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, an
d the entropy, Delta S, of activation. The calculation of errors in t
he activation parameters is also discussed. The enthalpy is measured t
o 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 t
wo standard deviations. Tn the Arrhenius formulation, the energy of ac
tivation, Delta E, is 55 +/- 0.6 kJ mol-L. The barrier is also calcul
ated 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 v
alue is lower than in most simple amides and indicates substantial int
eraction between the nitrogen lone pair and the pi system in the ring.