UV absorption spectra of 1,3-diphenylpropane-1,3-dione (1), its three methoxy derivatives (2-4) and its six dimethoxy derivatives (5-10) in various solvents dissolved were collected. The keto-enol tautomerism equilibrium constant was calculated with (1)H NMR. The position of the methoxy group in 1,3-diphenylpropane-1,3-dione was shown to have an influence on the molecule's UV absorption spectrum and the keto-enol tautomerism equilibrium constant. The methoxy group in the para position increases the absorption of radiation in the UV-A range. A shift to the keto form in the keto-enol tautomerism equilibrium is experienced by compounds with methoxy groups in ortho position. When two methoxy groups are present, the influence of their position is cumulative.
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