Monatshefte fuÈr Chemie 133, 1109±1113 2002) Rotational Energy Barrier of the Polarized Carbon±Carbon Double Bond in Quinophthalone Issa Yavari 1; , Mehdi Adib 1 , Hamid R. Bijanzadeh 1 , Majid M. M. Sadegi 2 , Hossein Logmani-Khouzani 2 , and Javad Safari 2 1 Department of Chemistry, University of Tarbiat Modarres, Tehran, Iran 2 Department of Chemistry, University of Isfahan, Isfahan, Iran Summary. A dynamic NMR effect is observed in the 13 C NMR spectra of anhydrous quin- ophthalone quinoline yellow) and its monohydrate in the vicinity of 47  C and 0  C, respectively, and is attributed to a restricted rotation around the polarized carbon±carbon double bond. The free energy of activation for this process in anhydrous quinophthalone and the monohydrate is 652 and 552 kJ  mol 1 , respectively, in CDCl 3 . Keywords. Restricted rotation; Quinoline yellow; Dynamic NMR; Push-pull ole®nes. Introduction Quinophthalone dyes are fascinating for their color brilliance and light fastness [1] and are among the most important yellow dyes. Quinoline yellow quinophtha- lone), which was ®rst discovered by Jacobsen in 1882, is probably the most widely used yellow compound for trichromatic dyeing of bulk textile goods made from polyester ®bers. Its structure was a matter of debate for several decades. Infrared and NMR data given by Kehler et al. [2] veri®ed structure 1 2-21H)-quin- olinyliden)-1H-indene-1,32H)-dione) and excluded 2. Thus, compound 1 possesses a highly polarized carbon±carbon double bond in its enaminone moiety and is therefore expected to exhibit restricted rotation around this bond.  Corresponding author. E-mail: isayavar@yahoo.com