Theoretical investigation of the oxidation pathways of HO*-initiated reactions of acrolein, methacrolein, and trans-crotonaldehyde

Ab initio molecular-orbital calculations have been performed to investigate the reaction mechanisms of the HO*-initiated reactions of the α,β-unsaturated aldehydes: acrolein (ACR), methacrolein (MACR), and trans- crotonaldehyde (CROT). All geometries were fully optimized at the MP2(Full)/6-31G(d,p) level. The correlation energy corrections were introduced by carrying out single-point calculations using both spin-projected second-order Moller-Plesset perturbation theory (PMP2) and coupled-cluster theory (CCSD(T)) using basis sets of different sizes. All reaction pathways studied proceed via a barrierless formation of a loosely bound pre-reactive complex in the entrance channel. The transition-state structures of the HO* additions to the terminal (β) and to the central (α) carbon atoms of the C=C double bond are found to be reactant-like structures. The lowest-energy barrier pathways are found to be the aldehydic H-atom abstraction. The β-addition pathways are found to be energetically more favored than the a-addition pathways. The HO* addition and aldehydic H-atom abstraction pathways are found to be highly exoergic, with the H-atom abstraction pathway being more exoergic than the addition pathways. Although the methyl substitution at the C=C double bond of methacrolein and crotonaldehyde lowers the energies of the transition-state structures of both α- and β-addition pathways, it destabilizes the energies of the transition-state structures of the corresponding aldehydic H-atom abstraction pathways, compared with that of acrolein. Key words: ab initio, transition-state structures, acrolein, methacrolein, crotonaldehyde. On a effectue des calculs theoriques d'orbitales moleculaires ab initio afro d'etudier les mecanismes de reactions initiees par le radical HO* d'aldehydes α,β-insaturees, tel l'acroleine (ACR), la methacroleine (MACR) et le transcrotonaldehyde (CROT). Toutes les geometries ont ete completement optimisees an niveau MP2(full)/6-31G(d,p). Des corrections d'energie de correlation ont ete introduites en effectuant des calculs ponctuels a l'aide de la theorie des perturbations de Moller-Plesset du deuxieme ordre en projection de spin (PMP2) ainsi que la theorie des agregats couples [CCSD(T)], en faisant appel a des ensembles de bases de diverses tailles. Toutes les voies reactionnelles etudiees procedent, a l'entree du tunnel, par la formation sans barriere reactionnelle d'un complexe prereactif a interactions faibles. On a trouve que les structures des etats de transition resultant de l'addition du HO* aux atomes de carbone terminal (β) et central (α) de la double liaison C=C possedent des structures qui ressemblent aux reactifs. On a trouve que les voies reactionnelles impliquant la barriere d'energie la plus faible correspondent a l'enlevement de l'atome d'hydrogene aldehydique. On a trouve que les voies reactionnelles correspondant a l'addition du radical HO*et a l'enlevement de l'atome d'hydrogene sont fortement exoergiques; la voie impliquant l'enlevement de l'atome d'hydrogene est plus exoergique que les voies reactionnelles conduisant a une addition a. Meme si elle diminue les energies des structures de transition de chacune des deux voies reactionnelles des additions en a et en β, par comparaison avec celle de l'acroleine, la presence de substituants methyles sur la double liaison C=C de la methacroleine et du crotonaldehyde destabilise les energies des structures de l'etat de transition des voies reactionnelles correspondantes d'enlevement de l'atome d'hydrogene aldehydique. Mots-cles : ab initio, structure de l'etat de transition, acroleine, methacroleine, crotonaldehyde. [Traduit par la Redaction]
Introduction Aldehydes play an important role in the chemistry of the polluted troposphere. They may be emitted as primary pollutants from partial oxidation of hydrocarbon fuels, industrial uses, and from [...]