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

Abstract

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 CHCHO* and CHCHOH* 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 CHCHO to the enol-form CHCHOH plays a significant role in the unimolecular reaction of CHCHO. The possible products of the reaction are formed via unimolecular decomposition of CHCHO and CHCHOH. The computed rate coefficients reveal that the dominant channel at low temperatures and high pressures is the formation of CHCHOH due to the low barrier height for CHCHO → CHCHOH isomerization process. However, at high temperatures, the product channel CH + CHO becomes dominant.

Multichannel Gas-Phase Unimolecular Decomposition of Acetone: Theoretical Kinetic Studies

“…A recent theoretical study on the multichannel thermal decomposition of acetaldehyde by our research group has revealed that the CH 3 CHO ⇄ CH 2 CHOH process is indeed an important process. 19 To date, all experimental and theoretical studies on the elementary unimolecular decomposition of acetone are concentrated on its dissociation to CH 3 CO + CH 3 . As mentioned above, recent experimental studies on CH 3 COCH 3 and a similar molecule, CH 3 CHO, reveal that other reaction paths could be of importance in the kinetics of the unimolecular reaction of acetone.…”

“…44,45 The computed effective reduced masses and energies along the hindered internal rotations are fitted by Fourier series, and their corresponding sums of states are included in total sum of states. 19 The above-mentioned theory is just applicable for estimating the sum of states of a tight transition state. As will be shown in the next sections, two distinct channels of bond breaking are considered in the acetone decomposition, leading to products sets of CH 3 + CH 3 CO and H + CH 3 COCH 2 .…”

“…Recently, a similar conclusion has been reached on the unimolecular thermal reaction of acetaldehyde. 19 To date, all experimental data are reported on the rate coefficient for the formation of CH 3 CO + CH 3 . The thermal rate constants for the formation of CH 3 CO + CH 3 from unimolecular decomposition of acetone computed over temperatures between 500 and 2000 K (P = 500 Torr) are demonstrated in Figure 5.…”

Mechanism and kinetics of the reaction CH3 + CH3CHO: Ab initio semiclassical transition state theory study

“…[ 1–8 ] In acetaldehyde pyrolysis and photolysis, it has been shown that the CH 3 radical production is probable, specifically in the high‐temperature regime caused by the combustion processes. [ 9–12 ] These super‐reactive radicals can play a significant role in the fate of acetaldehyde molecules as chain carriers and can react either with acetaldehyde itself or with other species produced.…”

“…Acetaldehyde, recognized as an intermediate in oxygenated fuel combustion, [ 31,32 ] and its decomposition process have received some theoretical interests within recent years. [ 11,12,33 ] Interestingly, almost none of them has reported a comprehensive theoretical work on an important secondary reaction like CH 3 CHO + CH 3 . This lack of knowledge on the reaction CH 3 CHO + CH 3 in the literature drew our attention to explore the mechanism and kinetics of this reaction.…”