Study 5.1 Mechanistic photochemistry adiabatic proton transfer reactions of 2-naphthol and 4-hydroxyacetophenone

Case Study 5.1 Mechanistic photochemistry - adiabatic proton transfer reactions of 2-naphthol and 4-hydroxyacetophenone 

The wavenumbers of the 0 0 transitions for the acid, AH, and the base, A, estimated from the intersection points of the corresponding absorption and fluorescence spectra, are Vo o(AH) = 2.95 pm 1 and Vo o(A ) = 2.65 pm By inserting these data into Equation 5.10 we obtain pA a = 3.2 for the acidity constant of 10 in the excited singlet state. Thus the acidity of 10 is predicted to increase by over six orders of magnitude upon excitation to the lowest singlet state. The prediction was tested by a fluorescence titration The relative fluorescence intensity at the emission 

The sharply increased acidity of phenols in the excited state can be used to lower the pH of aqueous solutions by a pulsed light source within nanoseconds (photoacid). However, the equilibrium is rapidly re-established in the ground state by diffusion-controlled recombination of the released protons with the basic phenolates. 

Adiabatic protolytic equilibria in the triplet state are generally fully established due to the intrinsically longer lifetimes of triplets. Soon after Forster s work, Jackson and Porter determined the acidity of 2-naphthol (10) in the triplet state by flash photolytic titration.382 The triplet triplet absorption of 10 changes from 2max = 432 to 460 nm as the pH is moved above the triplet p/Ta (10) of 8.1. Triplet state acidity can also be predicted using the Forster cycle. The triplet excitation energies ET of the acid and its conjugate base are determined from the 0 0 bands of their phosphorescence spectra. 

Isotopes play a multifaceted role in mechanistic chemistry. Isotopic labelling at specific sites of a reagent is often used to support or exclude a hypothetical reaction mechanism based on the location of the isotopic label in the products. Kinetic isotope effects, that is, changes in rate constants that are induced by isotopic substitution, may be seen in time-resolved experiments or may express themselves as changes in product quantum yields. They hold important information regarding the nature of the rate-determining step that limits the overall rate in a sequence of reactions. 

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