Structure and chemistry of retinylidene iminium salts and related systems

Abstract

<p>This thesis embodies an investigation into the structure and thermal isomerizations of retinylidene iminium salts and related systems. Over the past three decades there has been a tremendous amount of interest surrounding the structurally related natural pigments rhodopsin and bacteriohodopsin. Studies of these compounds have had a large impact in the fields of bioenergetics and visual photochemistry. Both of these proteins consist of a protonated Schiff base linkage between the retinal chromophore and the protein. A series of retinylidene iminium salts were synthesized and characterized both in solution and the solid state. A unique combination of x-ray crystallography, solid state $\sp{13}$C NMR, FTir and uv spectroscopic techniques were used in order to address the mechanism by which the protein can modify the electronic properties of the retinal chromophore in the ground state. One important conclusion of this study was that wavelength and positive charge delocalization in retinylidene iminium salts can primarily be modified by varying the distance between the anion and the proton bonded to the Schiff base nitrogen atom. It was also found that most of the positive charge in these systems is located on the nitrogen atom and the proton to which it is bonded. The thermal cis-trans isomerization about the C11-C12 bond of a series of retinylidene iminium salts was investigated by high field $\sp1$H NMR spectroscopy at ambient temperatures. It was found that the isomerizations proceeded via nucleophilic catalysis by the counterion present in the salt. No 11-cis isomer was detected at thermodynamic equilibrium for all of the salts examined. The thermal reactivity of the isoelectronic protonated poly-unsaturated aldehydes were also examined by high field $\sp1$H NMR spectroscopy. In contrast to the observed iminium salt chemistry, it was found that these species cyclized completely in superacidic media to form cyclopentene ring moieties. It was found that the rate determining step in these reactions involves additional protonation on the oxygen atom to form a dication.</p>