Conformational Studies of Dodecanolactone by Dynamic NMR Spectroscopy and Computational Methods

Abstract

AbstractLow‐temperature carbon NMR spectra of dodecanolactone (1), the parent 13‐membered lactone, showed the presence of two conformations, with populations of 55.5 and 44.5 % at –164.2 °C. Populations were estimated at the coalescence temperature of –151.9 °C for the carbonyl carbon atoms, and rate constants of 172.1 and 141.1 s–1 were obtained by lineshape matching. The corresponding free‐energy barriers were 5.63 and 5.68 kcal/mol. A search of conformational space was made with Allinger's MM4 program, and the conformations found were numbered 1a–m in order of increasing steric energies. Ab initio calculations (HF/6‐311G* level) of chemical shifts and of free energies at –150 °C predict that the major conformation is 1b and the minor conformation is 1a.