Charge-Transfer Character in the Intramolecular Hydrogen Bond: Vacuum Ultraviolet Spectra of Acetylacetone and Its Fluoro Derivatives

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Abstract
Near and vacuum UV absorption spectra of trifluoroacetylacetone (TFAC) and hexafluoroacetylacetone (HFAC) were measured at room temperature in the vapor phase and in perfluorohexane in the 30000–80000 cm−1 region, together with a vacuum UV absorption spectrum of acetylacetone in the 60000–80000 cm−1 region. By the aid of the modified CNDO-CI calculation, each of the observed valence-shell transition bands was assigned to the π–π**, π–σ*, or σ–σ* transition. The third σ–σ* bands observed at 65200 cm−1 for acetylacetone, at 78000 cm−1 for TFAC, and at 70400 cm−1 for HFAC were concluded to be rich in the CT character pertinent to hydrogen bond. In particular, the third σ–σ* band of HFAC was found to be the CT band pertinent to hydrogen bonding in the sense that the contribution of the CT configuration to the excited state amounts to 67.4%.