Investigation of methyl-vinyl ketone, 2- and 3-butenal decomposition pathways through semi-automated variable reaction coordinate transition state theory

The reactivity of unsaturated aldehydes and ketones is of interest both in combustion, as they are formed as intermediate species in the oxidation of biofuels, as well as in atmospheric chemistry, as they appear as intermediate products in the oxidation of alkenes, such as isoprene. In this work we investigate the reactivity of the simplest unsaturated aldehydes and ketones, namely methyl vinyl ketone (MVK), 2-, and 3-butenal (2- and 3-BUT), on the C₄H₆O singlet potential energy surface (PES). An important part of this study is the accurate determination of rate constants for the seven major barrierless decomposition channels: the bond fissions of MVK to (1) CH₂CHCO + CH₃ and (2) CH₃CO + C₂H₃, the bond fissions of 2-BUT to (3) CH₃CHCH + HCO, (4) CHCHCHO + CH₃ and (5) CH₂CHCHCHO + H, and the bond fissions of 3-BUT to (6) CH₂CHCH₂ + HCO and (7) CH₂CHCHCHO + H. Rate constants were determined using variable reaction coordinate transition state theory (VRC-TST), as implemented in EStokTP, through the extensive use of automated procedures to exploit both single and multireference methodologies and to semi-automatically carry out the determination of minimum energy paths, the evaluation of geometric and high level correction potentials, and the definition of the pivot points necessary to construct the dividing surfaces. As a result of this investigation, we propose a novel mechanism for the decomposition of 2-butenal to carbon monoxide and propene, involving isomerization to 3-butenal through two keto-enol tautomerization steps. Rate constants for both the dissociation and the reverse recombination pathways, as well as for the whole PES are determined for all the investigated pathways by integrating the master equation as a function of temperature and pressure, finding a good agreement with available experimental data, and are reported in a format suitable to be used both for atmospheric and combustion kinetic simulations.