Application of Time-Resolved Step-Scan Fourier Transform Infrared Spectroscopy to Photochemical Mechanistic Investigations of Alkyl Phenylglyoxylates

The photochemical reactivity of alkyl phenylglyoxylates has been investigated using time-resolved step-scan Fourier transform infrared spectroscopy. The time-resolved FTIR spectra of hexane solutions of methyl (MPG), ethyl (EPG), and isopropyl (iPPG) phenylglyoxylates and benzene solutions of MPG display two major transient absorption peaks in the carbonyl fundamental region around 1661 and 2105 cm-1. On the basis of the spectroscopic and kinetic evidence, the 1661 cm-1 transitions are assigned to the carbonyl fundamental of the triplet state of the corresponding alkyl phenylglyoxylate. The 2105 cm-1 band is attributed to the carbonyl stretch of α-hydroxyphenyl ketene, which is the postulated intermediate of the intramolecular hydrogen abstraction in alkyl phenylglyoxylates. These assignments are supported by the results of DFT calculations. From kinetic time profiles for the two observed transient infrared bands, the rate constants for the intra- and intermolecular γ-hydrogen abstraction were obtained and compared for methyl, ethyl, and isopropyl phenylglyoxylates. The reasons for the difference between the rate constants for glyoxylates and aromatic ketones are discussed.