ELECTROCHEMICAL, EPR AND AM1 STUDIES OF ACETYLENIC AND ETHYLENIC CAROTENOIDS

It has been shown, by cyclic voltammetry (CV) and simultaneous electro chemical and electron paramagnetic resonance (SEEPR) measurements of b eta-carotene (I) and 15,15'-didehydro-beta-carotene (II) in dichlorome thane solution, that the central triple bond increases the oxidation p otentials by 237 +/- 5 mV and decreases the solution EPR peak-to-peak linewidth(Delta H-app) by 1.7 G. Two synthetic carotenoids, 7,7'-diphe nyl-7,7'-diapocarotene (II) and 7,7'-diphenyl-15,15'-didehydro-7,7'-di apocarotene (IV), close analogues of I and II, were also studied by CV and SEEPR methods. In contrast to compound I, two separate oxidation waves are observed for III, a feature previously found only for carote noids containing electron-withdrawing substituents. The increase in ox idation potential of IV compared to m was similar to that of II vs. I, but the decrease in Delta H-pp (2.7 G) was considerably larger. Becau se the radical cation of III is adsorbed strongly on the platinum elec trode in dichloromethane, complete analysis using the DigiSim (R) prog ram was not possible; however, the oxidation potentials could be estim ated. Structural parameters of neutral I-IV and their dications were c alculated using the AM1 method, and the bond lengths and bond angles o f I and II are in good agreement with crystallographic data. The magni tude of the EPR parameters, Delta H-pp and the g factors, for the caro tenoid radical cations of I-IV indicate a polyene pi-radical cation st ructure with little unpaired electron density occurring near the termi nal groups. The radical cation and dication deprotonate by loss of a p roton from the chain methyl groups, with the former forming a polyene pi-radical.