Ring Orientation in βIonone and Retinals

The ring orientation in beta-ionone, all-trans retinal, and 11-cis retinal, relative to that of the polyene chain, has been determined by means of semi-empirical calculations and magnetic resonance measurements of the nuclear Overhauser effect and long-range coupling constants. The experimental results yield a distorted s-cis conformation about the C(6)-C(7) "single bond", with the torsional angle in the range 30 degrees to 70 degrees . This agrees well with the semi-empirical potential function, which has a broad, rather flat minimum for angles from 40 degrees to 120 degrees . The temperature dependence of the NMR results provide confirmation for the form of the torsional potential.