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 Møller–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 α-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.