Investigation of the reactivity on the C 4 H 6 O PES: methyl-vinyl ketone, 2- and 3-butenal decomposition Andrea Della Libera and Carlo Cavallotti Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Italy The theoretical investigation of the reactivity on singlet potential energy surfaces (PES) is a challenging task, since it requires characterization of all the isomers that can be reached from each reacting well under investigation, the saddle points by which they are connected, and the exit channels. A further obstacle to the exploration of singlet PESs is the possible diradical nature of saddle points, which is badly described by black box single reference ab initio methods routinely adopted to investigate PESs, such as density functional theory. Finally, the fastest exit channels are usually barrierless, which require sophisticated methods to determine their reaction rates, such as Variable Reaction Coordinate Transition State Theory (VRC-TST) together with a multidimensional sampling of the PES with multireference ab initio techniques. In this work, we used the EStokTP software [1] to investigate the reactivity on the C 4 H 6 O singlet PES using automated or partially automated approaches to overcome the afore-mentioned hurdles. The interest in this system is determined by the fact that the reactivity of the main C 4 H 6 O isomers, methyl-vinyl ketone (MVK), 2-butenal (2-BUT), and 3-butenal (3-BUT), which are important intermediates in combustion and atmospheric chemistry, is only partially understood. The approach we followed consisted in the study of isomerization and decomposition pathways of MVK, 2-BUT, and 3-BUT. Saddle points connecting the isomers were identified. Geometries and frequencies were determined at the UωB97XD/ aug-cc-pVTZ level, while energies at the CCSD(T) level with extrapolation to the complete basis set and accounting for the correlation of core electrons. Diradical singlet transition states’ energies were obtained with the multireference method CASPT2. Energy barriers leading to bimolecular exit channels, namely CO and C 3 H 6 , ethene and ketene, and H 2 + CH 2 CHCHCO, were computed. The lowest energy barrier, 43.2 kcal/mol, was identified for the decomposition of 3-BUT to CO and C 3 H 6 . Rate constants of seven barrierless reactions (and corresponding recombinations) were determined using our recent partially automated VRC- TST protocol, which relies on EstokTP [1] for the preparation of the input files and VaReCoF [2] for VRC-TST calculations. All multireference calculations were performed with the Molpro suite of packages [3] at the CASPT2 level of theory. Master equation simulations were run with MESS [4] to determine the rates of all the reactive channels. The results of the simulations were compared with available experimental data, giving new insights into the decomposition pathways of these molecules. At low pressure, the main products are CO and C 3 H 6 . When pressure and temperature increase, the barrierless channel from 3-BUT to HCO + C 3 H 5 becomes dominant. In conclusion, we determined the temperature dependence of seven recombination reaction rates and analyzed in depth the reactivity of the principal isomers of the C 4 H 6 O PES, which is an important step forward in the comprehension of this system. References 1. C. Cavallotti, M. Pelucchi, Y. Georgievskii, S. J. Klippenstein, J. Chem. Theory Comput., 2019, 15 , 1122. 2. L. B. Harding, Y. Georgievskii, S. J. Klippenstein, J. Phys. Chem. A, 2005, 109 , 4646-4656. 3. H.-J. Werner, P. J. Knowles, F. R. Manby, et al. J. Chem. Phys., 2020, 152 , 144107. 4. Y. Georgievskii, J. A. Miller, M. P. Burke, S. J. Klippenstein, J. Phys. Chem. A, 2013, 117 , 12146-12154.
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