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

Halogens may play an important role in the chemistry of hydrocarbons in cer tain regions of the troposphere. We have studied Cl-atom-initiated reaction s of several monoterpenes (alpha -pinene, 2-carene, 3-carene, myrcene and g amma -terpinene) at atmospheric pressure of ca. 760 Torr over the temperatu re range 283-323 K in N-2 and air. The decay of the terpenes was followed u sing a gas chromatograph with a flame ionisation detector (GC-FID) and the rate constants were determined using the relative rate technique with n-non ane as a reference compound. The Cl atoms were generated by UV photolysis o f trichloroacetylchloride (CCl3COCl) at lambda congruent to 250 nm. Room te mperature rate constants are found to be (in units of 10(-10) cm(3) molecul e(-1) s(-1), with uncertainties representing +/-2 sigma) 5.3 +/- 0.1 for al pha -pinene, 5.8 +/- 0.1 for 2-carene, 5.2 +/- 0.1 3-carene, 8.5 +/- 1.8 fo r myrcene, and 10.4 +/- 0.6 for gamma -terpinene. No temperature dependence was observed for the reactions within our experimental uncertainties with the exception of Cl + gamma -terpinene (ln k = (-27.9 +/- 0.8) - (2100 +/- 200)/T). Data on the rate constants of the Cl reactions with monoalkenes an d dienes, obtained in this work and taken from the literature were correlat ed with alkene HOMO energies within the framework of PFMO theory. The calcu lated semi-empirical parameters can be used to predict Cl-atom rate constan ts for alkenes in the troposphere. To gain a deeper insight into the reacti on mechanism, several ab initio and DFT quantum chemical calculations were also carried out.