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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‐diphenylpentane‐2,4‐dione was characterized using the solvent and temperature dependence of the spin–spin coupling constants, after the conformational space of the keto form had been mapped with molecular mechanical grid search calculations. The results indicate that the keto form spans a very large conformational space and that the motions of the aromatic rings and the skeleton are significantly correlated; the rotational barrier about the benzylic bond is up to 3 kJ mol−1 higher for the flexible keto tautomer than for the enol form having a rigid skeleton. Copyright © 1999 John Wiley & Sons, Ltd.