Kinetic study of the gas-phase reactions of hydroxyl radicals and chlorine atoms with cis-3-hexenylformate

Recently, it has been demonstrated that grass and cut grass are important sources of C6 esters’ emissions into the atmosphere. The esters emitted as part of the wound defense mechanisms are responsible for the smell of freshly cut grass and are also highly photochemically reactive. Thus, these grass emissions should be included in models of urban photochemical pollution so that accurate smog predictions can be obtained. In this work, we have carried out a kinetic study of the gas-phase reactions of chlorine atom and hydroxyl radical with cis-3-hexenylformate (HCO₂CH₂CH₂CH=CHC₂H₅) for the first time. This was performed in a Teflon static reactor at room temperature and atmospheric pressure of nitrogen or air using gas chromatography with flame ionization detection. Kinetic rate coefficients obtained for cis-3-hexenylformate were (in units of cm³ molecule^-1 s^-1) (2.45 ± 0.30) × 10^-10 and (4.61 ± 0.71) × 10^-11 for chlorine and hydroxyl radical reactions, respectively. The reaction between the ester and chlorine atoms is very fast, approaching the collisioncontrolled regime; nevertheless, the rate constant for the reaction with hydroxyl radical is lower. In addition, the rate coefficients for the reactions of cis-3-hexenylformate with different atmospheric reactive species were estimated through a Quantitative Structure–Activity Relationship model in which High Occupied Molecular Orbital energy of the formate is used as a theoretical molecular descriptor. Tropospheric lifetimes, based on the homogeneous sinks of the cis-3-hexenylformate, are estimated from the kinetic data obtained in the present work. The relatively short atmospheric lifetimes of the ester indicate that the compound will be oxidized near its anthropogenic origin source.

Keywords
Atmospheric lifetimes; Chlorine atoms; Cis- 3-hexenylformate; Hydroxyl radicals; Rate coefficients; Smog chamber