Chromophore Systeme, 2. Konformation und Lichtabsorption von (2‐Alkoxyvinyl)ethandionen

Chromophoric Systems, 2. – Conformation and Absorption of Light in (2‐Alkoxyvinyl)ethanediones The unusual color properties of yellow 1,2‐bis(4,5‐dihydrofuran‐3‐yl)ethane‐1,2‐dione (1) and colorless 1,2‐bis(5,6‐dihydro‐4H‐pyran‐3‐yl)ethane‐1,2‐dione (2) in the solid state and in solution are investigated by crystal structure analysis, UV‐Vis, PE, and 13C‐NMR spectroscopy as well as by reparameterized force‐field calculations. Dione 1 takes an antiperiplaner CO/CO conformation of its chromophore in the solid state whereas dione 2 is twisted in this respect by 102°. In solution both chromophores are not planar, but 1 is less distorted (ca. 142°) than dione 2 (ca. 126°). The calculated rotational barrier for CO/CO twisting amounts to 0.9 kcal mol−1 for 1 and to 2.2 kcal mol−1 for 2.