Oxidation of isoprene by titanium oxide cluster cations in the gas phase

Abstract

The heterogeneous oxidation of isoprene (C₅H₈) by metal-oxide particles, such as the typical mineral aerosols TiO₂, plays an important role in the isoprene atmospheric chemistry. However, the underlying mechanism of C₅H₈ oxidation remains elusive owing to the complexities of aerosol surfaces and reaction channels. Herein, we report the gas-phase reactions of Ti_xO_y⁺ (x = 1–7, y = 1–14) cations with isoprene by using mass spectrometry and density functional theory (DFT) calculations. Five types of reaction channels were observed: association, hydrogen atom transfer (HAT), C–C bond cleavage, combined oxygen atom transfer (OAT) and HAT and combined OAT and C–C bond cleavage. It is noteworthy that formaldehyde is known as the major oxidation product of isoprene/hydroxyl radicals in the atmosphere. In addition, CO has not been observed in the reactions of isoprene with gas-phase ions. Therefore, the reaction mechanisms of CH₂O and CO generation observed in Ti₂O₅⁺/C₅H₈ and Ti₄O₈⁺/C₅H₈ systems were further investigated by DFT calculations, and the calculated results are in agreement with the experimental observations. In these two reactions, both Ti and O atoms can be the adsorption sites for C₅H₈. The reaction channels and mechanistic information gained in these gas-phase model reactions may offer fundamental insights relevant to the corresponding oxidation processes over titanium oxide aerosols in the atmosphere.