Carotenoid Radical Cations, Dications And Radical Trications

The radical cation, dication and radical trication of the long chain polyenes known as carotenoids have been characterized by a combination of diverse techniques such as CV, EPR, ENDOR and simultaneous applications of electrolysis and EPR, optical and resonance Raman spectroscopy, and by AM1, RHF-INDO/SP and INDO/S molecular orbital calculations. In this paper, the AMI calculated structure and charge density distribution of the radical trication is reported and its optical spectrum is estimated based on INDO/S calculations. An iterative fit to the CV of the radical trication indicates that its acid strength (pK(a) similar to 2) is similar to that of the dication (pK(a) similar to 1), and that it deprotonates rapidly and is less stable than the dication.