Photochemical Formation of Enols from Carbonyls

Abstract

The gas-phase photochemistry of small carbonyls has been studied in detail for several decades, as these ubiquitous molecules are of great importance in the atmosphere. The photochemical conversion of carbonyls to their relatively unstable enol isomers has received relatively little attention. We have studied several routes of formation of the three smallest enols, using a combined experimental and computational approach. Ethenol is formed from both n-butanal, via the well known Norrish Type II (NTII) reaction on the electronically excited S1 and T1 states, and from acetaldehyde, via a recently discovered phototautomerisation reaction occurring on the electronic ground state, S0. The wavelength and pressure dependent quantum yields for both of these reactions were determined using laser photolysis and IR spectroscopy. The measured yields are discussed in the context of the potential energy surfaces of the parent aldehydes and the competing reaction channels. Stationary points on these potential energy surfaces were calculated with a range of electronic structure methods, and these methods evaluated for their accuracy and efficiency. Calculated reaction barriers were also used to examine the reactivity of butanal in more detail using master equation modelling. With comparison to experimental data, these results revealed that all three of the S1, T1 and S0 electronic states are likely to be contributing to the observed photochemical products within the UVB range. The NTII reaction of two larger carbonyls, 2-methylbutanal and 2-pentanone, was also used to obtain and characterise the previously unreported infrared spectra and absorption cross sections of 1- and 2-propenol. As these two propenols are the enol isomers of propanal and acetone, we also briefly examined the photolysis products of these two carbonyls to determine if phototautomerisation was also occurring, analogous to the formation of ethenol from acetaldehyde.