Theoretical Studies on the Kinetics of Multi-Channel Gas-Phase Unimolecular Decomposition of Acetaldehyde

Theoretical kinetic studies are performed on the multichannel thermal decomposition of acetaldehyde. The geometries of the stationary points on the potential energy surface of the reaction are optimized at the MP2(full)/6-311++G(2d,2p) level of theory. More accurate energies are obtained by single point energy calculations at the CCSD(T,full)/augh-cc-pVTZ+2df, CBS-Q and G4 levels of theory. Here, by application of steady-state approximation to the thermally activated species CH₃CHO* and CH₂CHOH* and performance of statistical mechanical manipulations, expressions for the rate constants for different product channels are derived. Special attempts are made to compute accurate energy-specific rate coefficients for different channels by using semiclassical transition state theory. It is found that the isomerization of CH₃CHO to the enol-form CH₂CHOH plays a significant role in the unimolecular reaction of CH₃CHO. The possible products of the reaction are formed via unimolecular decomposition of CH₃CHO and CH₂CHOH. The computed rate coefficients reveal that the dominant channel at low temperatures and high pressures is the formation of CH₂CHOH due to the low barrier height for CH₃CHO → CH₂CHOH isomerization process. However, at high temperatures, the product channel CH₃ + CHO becomes dominant.