Mechanism of the Photochemical Isomerization and Oxidation of 2-Butenedial: A Theoretical Study

Abstract

Under tropospheric conditions, 2-butenedial is photochemically removed to produce secondary organic aerosol. Upon solar irradiation in the lower troposphere, the main photochemical products are ketene-enol (a key intermediate product), furanones, and maleic anhydride. The oxidative reaction mechanism was studied using the multireference method CASSCF to explore the hypersurface of the two most accessible singlet excited states, and by DFT for the ground state. Photoisomerization of 2-butenedial in the first excited state directly produces ground state ketene-enol upon nonradiative relaxation. From this intermediate, furan-2-ol and successively 3H-furan-2-one and 5H-furan-2-one are formed. The cooperative effect of two water molecules is essential to catalyze the cyclization of ketene-enol to furan-2-ol, followed by hydrogen transfers to furanones. Two water molecules are also necessary to form maleic anhydride from furan-2-ol. For this last reaction, in which one extra oxygen must be acquired, we hypothesize a mechanism with singlet oxygen as the oxidant.

Keywords: 2-butenedial; CASSCF; CCSD(T); DFT; reaction mechanism; tropospheric oxidation.

Scheme 3

Formation of furanones form ketene-enol, in the ground state. Blue: gas phase without explicit water molecules; black: gas phase with one explicit water molecule; red: gas phase with two explicit water molecules. For the sake of clarity, the two water molecules are not shown in this scheme, but the relevant transition structures are shown in Figure 2. ΔG(298 K) in kcal mol⁻¹.

Figure 2

Cyclization of ketene-enol TS A-B, hydrogen migrations TS B-C, and TS B-D catalyzed by two water molecules. Bond distances in Ångstrom.

Scheme 4

Formation of maleic anhydride from 2-furanol B, initiated by ¹O₂. Blue: gas phase without explicit water molecules; red: gas phase with two explicit water molecules. For the sake of clarity, the two water molecules are not shown in this scheme but a picture of TS E-F is shown in Figure 3. ΔG(298 K) in kcal mol⁻¹. * This barrier does not directly lead to F: further steps are described in the Supplementary Materials (Scheme S3).

Figure 3

TS E-F: formation of maleic anhydride catalyzed by two water molecules. Bond distances in Ångstrom.

Figure 1

Zze butenedial.

Scheme 1

Photochemical formation of ketene-enol. Potential energies in kcal mol⁻¹, calculated at the SA-CASSCF(12,10)/6-31G(d) level of theory. SA-CASSCF(12,10)/cc-pVTZ//CASSCF (12,10)/6-31G(d) energies in italics. Left: adiabatic transition energies are shown in parentheses.

Scheme 2

Conceivable ketene-enol to furanone ring closure step.

Figure 4

Active orbitals.