Competing Mechanistic Channels in the Oxidation of Aldehydes by Ozone

The reaction of ozone with aldehydes has been studied intermittently for over 100 years, but its mechanism remains uncertain. Experimental results support two reaction channels: radical abstraction of the acyl hydrogen and addition to form a five-membered ring tetroxolane. We have studied the aldehyde-ozone reaction by DFT and CCSD(T) calculations. CCSD(T)/6-311+G(d,p)//M05-2X)/6-311+G(d,p) calculations predict two competitive pathways for the oxidation of formaldehyde by ozone. Abstraction of the acyl hydrogen by ozone has a barrier of 16.2 kcal/mol, leading to a radical pair, which can combine to form a hydrotrioxide; this species may subsequently decompose to a carboxylic acid and singlet oxygen. In the second reaction channel, addition of ozone to the carbonyl is stepwise, with barriers of 19.1 and 23.0 kcal/mol, leading to a five-membered ring tetroxolane intermediate. This process may be reversible, consistent with earlier observations of isotopic exchange. The two channels connect by an intramolecular hydrogen abstraction. Ring opening of the tetroxolane by an alternate O-O bond cleavage, followed by spin inversion in the resulting diradical intermediate, can give a carbonyl oxide plus O-3(2). It is also possible that reaction of triplet oxygen with carbonyl oxides can produce ozone by the reverse route. These two stepwise reaction channels, hydrogen abstraction and addition to the C=O bond, explain much of what has been observed in the long history of ozone-aldehyde chemistry. Known reaction rates and the substantial barriers to both channels support an earlier conclusion that aldehyde oxidation by ozone is too slow to be of importance in atmospheric chemistry.