DFT and NMR parameterized conformation of valeranone

A Monte Carlo random search using molecular mechanics, followed by geometry optimization of each minimum energy structure employing density functional theory (DFT) calculations at the B3LYP/6–31G* level and a Boltzmann analysis of the total energies, generated accurate molecular models which describe the conformational behavior of the antispasmodic bicyclic sesquiterpene valeranone ( 1 ). The theoretical HCCH dihedral angles gave the corresponding 1 H, 1 H vicinal coupling constants using a generalized Karplus‐type equation. In turn, the 3 J (H,H) values were used as initial input data for the spectral simulation of 1 , which after iteration provided an excellent correlation with the experimental 1 H NMR spectrum. The calculated 3 J (H,H) values closely predicted the experimental values, excepting the coupling constant between the axial hydrogen α to the carbonyl group and the equatorial hydrogen β to the carbonyl group ( J 2β, 3β ). The difference is explained in terms of the electron density distribution found in the highest occupied molecular orbital (HOMO) of 1 . The simulated spectrum, together with 2D NMR experiments, allowed the total assignment of the 1 H and 13 C NMR spectra of 1 . Copyright © 2004 John Wiley & Sons, Ltd.