Abstract:

The catalytic effect of H₂O and six kinds of organic acids (e.g., formic acid) on the reaction of CH₃CHOO with H₂O is studied at the CCSD(T)//B3LYP/6-311+G(d,p) level. The results reveal that two possible channels exist as the double proton transfer and addition, of which the latter dominates for the non-catalytic reactions. For the additions, the OH of water is added to the α-C of CH₃CHOO, and the H atoms migrate to the end oxygen atoms. Catalysts such as H₂O and organic acid can form a hydrogen-bonded complex with CH₃CHOO, which promotes the H transfer and thus significantly reduces the elementary reaction energy barrier and apparent activation energy when compared with that of the non-catalytic reaction. The catalytic effect is proportional to the strength of the organic acids. For example, for the formation of syn-HAHP catalyzed by H₂O (pK_a=15.7), formic acid (pK_a=3.75) and oxalic acid (pK_a=1.23), the energy barrier is reduced from 69.12 to 40.78, 18.88 and 10.61 kJ·mol^-1, respectively. In addition, the non-catalytic reaction has a positive activation energy, whereas the catalytic reactions have an negative apparent activation energy.

Key words: Criegee intermediate, CH₃CHOO, Acid catalysis, Addition reaction