Irradiation at 200 nm of aqueous carboxylates (formate, acetate, propionate, alanine, isoleucine, proline, lactate and oxalate) leads to nO → πCO* type excitations from the lone-pair on the carbonyl oxygen to the antibonding πCO*-valence orbital. The transition is relatively weak, but the strong absorption band of water beginning around 195 nm precludes population of higher energy excited states, and the nO → πCO* transition is thus the only excitation path for these molecules. A common feature of the above eight species is decarboxylation via a short-lived first excited state as the primary reaction channel, and the dissociation dynamics and yields of the eight photoreactions are nearly identical. The calculated excited state surfaces of the eight species are quite similar, suggesting that nO → πCO* excitation leading to decarboxylation is a general reaction channel for molecules containing a carboxylate group. Considering that the negative charge on molecules with a carboxylate group is localized on the CO2 moiety, it is counter intuitive that decarboxylation predominantly leads to the formation of CO2, rather than the CO2- radical, but experimental results indicate that the latter only constitutes a minor channel. The conundrum is resolved by the observation of electron ejection concurrent with the dissociation of CO2, with the electron being transferred to the dissociation counter product.

Primary photolysis dynamics of carboxylates in aqueous solution: decarboxylation

ACS Spring Meeting 2021

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