Kinetics of the gas-phase reaction of atomic chlorine with selected monoterpenes

Abstract

Halogens may play an important role in the chemistry of hydrocarbons in certain regions of the troposphere. We have studied Cl-atom-initiated reactions of several monoterpenes (α-pinene, 2-carene, 3-carene, myrcene and γ-terpinene) at atmospheric pressure of ca. 760 Torr over the temperature range 283–323 K in N₂ and air. The decay of the terpenes was followed using a gas chromatograph with a flame ionisation detector (GC-FID) and the rate constants were determined using the relative rate technique with n-nonane as a reference compound. The Cl atoms were generated by UV photolysis of trichloroacetylchloride (CCl₃COCl) at λ≅250 nm. Room temperature rate constants are found to be (in units of 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, with uncertainties representing ± 2σ) 5.3 ± 0.1 for α-pinene, 5.8 ± 0.1 for 2-carene, 5.2 ± 0.1 for 3-carene, 8.5 ± 1.8 for myrcene, and 10.4 ± 0.6 for γ-terpinene. No temperature dependence was observed for the reactions within our experimental uncertainties with the exception of Cl + γ-terpinene (ln k = (−27.9 ± 0.8) − (2100 ± 200)/T). Data on the rate constants of the Cl reactions with monoalkenes and dienes, obtained in this work and taken from the literature were correlated with alkene HOMO energies within the framework of PFMO theory. The calculated semi-empirical parameters can be used to predict Cl-atom rate constants for alkenes in the troposphere. To gain a deeper insight into the reaction mechanism, several ab initio and DFT quantum chemical calculations were also carried out.