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PHOTOREACTION OF VALEROPHENONE IN AQUEOUS SOLUTION
Citation:
Zepp, R., M. M. Gumz, W. L. Miller, AND H. Gao. PHOTOREACTION OF VALEROPHENONE IN AQUEOUS SOLUTION. Journal of Physical Chemistry A 102(28):5716-5723, (1998).
Description:
Kinetics and products of the photoreaction of the phenyl ketone valerophenone were investigated as a function of temperature, pH, and wavelength in aqueous solution. Under these conditions (<10-4M), the photoreactions are pseudo-first-order with respect to valerophenone concentration. Type II quantum yields for photoreaction were close to unity throughout the 290-330 nm spectral region and in the temperature range from 10 to 40 C. The quantum yields for the photoproducts were 0.65 0.04 for cleavage to acetophenone and propene and an overall yield of 0.32 0.03 for cyclization to two cyclobutanols at 20 C. A small amount of 1-phenylcyclopentanol (~2% yield) also was formed. These photoreactions were linear up to at least 80% quenching of the photoreactions. On the basis of quenching studies with steady-state irradiations, the triplet lifetime of valerophenone at 20 C was estimated to be 52 ns,~7 times longer than that observed in hydrocarbon solvents. Since the triplet lifetime is controlled by intramolecular hydrogen abstraction, these results indicate that the rate constant for H abstraction is significantly lowered in aqueous media. The slower H abstraction in aqueous solution is attributed to stabilization of the excited , * state by water and vibronic mixing and slight inversion of the reactive n, r* triplet and the unreactive , * triplet states. This interpretation also is supported by changes in the UV absorption spectra of phenyl ketones in water compared to organic solvents. Red shifts, compared to the polar organic solvent acetonitrile, were observed in the - * transitions of valerophenone and acetophenone, reflecting stabilization of the excited , * state by water. Other results indicated that the quantum yields for valerophenone photoreaction are pH-independent from pH 9 to pH 2 but decrease significantly below ph 2. The decrease at low pH is attributed to quenching of triplet reactivity via protonation of the excited triplet state. The use of valerophenone as a convenient actinometer for studies in water is discussed; its half-lives during midday exposure to summer sunlight in temperature latitudes are <30 min.