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 C-13-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-degrees. In solution both chromophores are not planar, but 1 is less distorted (ca. 142-degrees) than dione 2 (ca. 126-degrees). 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.