Characterization of a conformationally flexible system: NMR spectroscopic study of 1,5-diphenylpentane-2,4-dione

Conformational flexibility of the keto and enol tautomers of 1,5-diphenylpe ntane-2,4-dione was characterized using the solvent and temperature depende nce of the spin-spin coupling constants, after the conformational space of the keto form had been mapped with molecular mechanical grid search calcula tions. The results indicate that the keto form spans a very large conformat ional space and that the motions of the aromatic rings and the skeleton are significantly correlated; the rotational barrier about the benzylic bond i s up to 3 kJ mol(-1) higher for the flexible keto tautomer than for the eno l form having a rigid skeleton. Copyright (C) 1999 John Wiley & Sons, Ltd.