The ¹³C Dipole-dipole Relaxation and Pseudorotation of Cyclododecanone

Abstract

¹³C dipole-dipole relaxation time of the carbon atoms of macrocyclic ketone, cyclododecanone, have been determined at various temperatures in 15.0 MHz in CDC1³. T₁ measurements of the methylene bridges were conducted with inversion recovery method and the measured NOE values at similar conditions gave the T^DD ₁ of the particular carbon moiety. The linear relationship between the inverse of absolute temperature and the LnT^DD ₁ strongly indicated the mechanism of the relaxation was purely dipolar. Therefore, the contribution of conformation on the internal motion was discussed depending on the relaxation time of pseudorotating sites. The selectivity of observed relaxation times of carbons of “anti” and “gouche” conformers, T_1a and T_1g indicated the role of segmental motion affected from the carbonyl group. Accordingly, the activation energy of the pseudorotation of the carbons were obtained by this way. The results were found to be inconsistent with earlier reports of DNMR and strain energy calculations.