Gas and Solution Phase Reactivities of Triplet Aromatic Carbonyls

Abstract

Abstract. Absolute rate studies pertinent to the gas and solution phase photochemistry of triplet benzaldehyde, acetophenone and benzophenone have been carried out, and it has been confirmed that, in the gas phase, attack on a hydrocarbon by the carbonyl results in the expected hydrogen abstraction. Also absolute rates in both the gas phase and solution (acetonitrile) are quite comparable. For a given carbonyl the differences in the rates of abstraction are largely accounted for by activation energy changes. The activation energies vary from a high of 4.8 kcal mol-' for abstraction of a primary hydrogen by acetophenone to a low of 1.4 kcal mol-' for abstraction of tertiary hydrogen by benzaldehyde. At room temperature the relative reactivities of primary, secondary and tertiary C-H bonds for all the carbonyls are 1:23:190. In solution benzophenone is two times more reactive than acetophenone, while benzaldehyde is ten times more reactive. In the gas phase benzaldehyde is about 18 times more reactive than acetophenone. Molecular orbital calculations indicate that the greater reactivity of benzaldehyde as compared to acetophenone stems from steric hindrance in the latter. The quenching of acetophenone triplets by various other substrates including naphthalene, benzene, isopropanol and triethylamine is also discussed.

Substituted acetophenones. Importance of activation energies in mixed state models of photoreactivity

“…Reson., 1,451 (1969); M. St.-Jacques and C. Vazlrl, Ibid., 4, 77 (1972). (18) The coupling constants were slightly different from those of the corresponding protlo compounds, to an extent definitely outside the experimental error.…”

Quenching of n,π*-Excited States in the Gas Phase: Variations in Absolute Reactivity and Selectivity

“…Some gas-phase studies for other ketones such as acetone, biacetyl, and acetophenone have also become available. − ,, ,, Scaiano and co-workers investigated the triplet quenching of acetone and biacetyl by four secondary and tertiary aliphatic amines on the basis of competitive product studies or phosphorescence measurements . They observed a drop in reactivity by about 1 order of magnitude in the gas phase compared to solution and a different selectivity toward the different quenchers, namely a larger reactivity of secondary relative to tertiary amines in the gas phase.…”

“…The importance of solvent effects for the mechanistic understanding of the quenching of n,π*-excited states encourages the study of these basic photoreactions in the absence of solvation, that is, in the gas phase. However, the study of quenching of n,π*-excited states in the gas phase has been restricted to case studies. − Moreover, the vapor pressure of benzophenone, which is the most extensively studied ketone in solution, is too low to allow gas-phase studies at room temperature. We have presently employed DBO to gain further insight into gas-phase photoreactivity.…”

Hydrogen abstraction in the neutral molecular cluster of benzophenone and hydrogen donors formed in a supersonic free jet expansion

“…A sapphire disk, having a thickness of 1 mm and a diameter of 25 mm, was mounted onto the sample holder using an indium gasket to maintain good thermal contact. The temperature of the copper sample holder was typically maintained near 12 K monitored by a digital temperature controller (Air Products, Model 3700).…”

“…25 In a previous investigati~n,~~ we found that excited benzophenone is photoreduced to benzophenone ketyl radical (BPK) without contributions from surrounding solvent molecules by monitoring the fluorescence of BPK. The quenching rate constant measurements of triplet benzophenone show that, in addition to its role in the condensed phase, the charge-transfer interaction also plays an important role in the photoreduction process of benzophenone-amine systems in the gas phase.…”