PENTAENYL CATIONS FROM THE PHOTOLYSIS OF RETINYL ACETATE - SOLVENT EFFECTS ON THE LEAVING GROUP ABILITY AND RELATIVE NUCLEOPHILICITIES - AN UNEQUIVOCAL AND QUANTITATIVE DEMONSTRATION OF THE IMPORTANCE OF HYDROGEN-BONDING

The photolysis of retinyl acetate leads to carbon-oxygen bond heterolysis and yields the retinyl cation within 100 ps. This pentaenyl cation, which has an absorbance centered at 590 nm, is observed using picosecond and nanosecond laser transient absorption. This provides access to ion dynamics and affords the opportunity to study aspects of the S(N)1 mechanism, including the effect of hydrogen bonding cosolvents like water and alcohols. Contact ion pairs appear to be important in the early time regime, but free ions are the reactive species in polar solvents in the nanosecond experiment. Trapping of the free ions with nucleophiles yields a collection of rate data from Stern-Volmer quenching which in turn gives linear plots versus Pearson/Swain-Scott n values, measures of relative nucleophilicity based on S(N)2 reactions. These linear plots show a marked dependence on water content in the solvent acetonitrile with an increase in selectivity (i.e., slope of the line) with added water. For example, the second order rate constant for reaction of the retinyl cation with fluoride decreases by over four orders of magnitude in changing the solvent from dry acetonitrile to the same solvent with 11 M water. This is interpreted to be the result of hydrogen bonding between water and the nucleophiles. The special salt effect of Li+ is also demonstrated to arise from the strong interaction of this Lewis acid and the nucleophiles.