The Effect of β-Hydrogens on the Tropospheric Photochemistry of Aldehydes: Norrish Type 1, Triple Fragmentation, and Methylketene Formation from Propanal.

Abstract

Wavelength and pressure dependent quantum yields (ϕ, QYs) of propanal photolysis have been measured for photolysis wavelengths, λ = 300-330 nm, and buffer gases of 3-10 Torr propanal and 0-757 Torr N2. Following laser photolysis, three photochemical pathways were established, using Fourier transform infrared spectroscopy of the stable end-products. Photolysis is dominated by the Norrish Type 1 reaction, which has been reported previously, but with inconsistent quantum yields. The propanal α-hydrogen leads to a 4-center elimination of H2, as observed in CH3CHO, here leading to methylketene. The presence of hydrogen attached to the β-carbon allows a new photochemical pathway: concerted triple fragmentation into CO + H2 + C2H4 via a 5-center transition state. Neither of these channels has been reported previously. No evidence for the previously reported C2H6 + CO, C2H4 + H2CO or CH3 + CH2CHO channels, nor for phototautomerization to 1-propenol (CH3CH═CHOH) was found. Modeling of the wavelength, pressure and collision partner dependence of the QYs allows us to reconcile the previous NT1a results and make recommendations for the quantum yields of all three channels under tropospheric conditions. The general impact of β-hydrogen atoms in the photochemistry of aldehydes is to open up new pathways from cyclic transition states and to reduce the importance of other photolysis or isomerization channels.