Qk. Timerghazin et Pa. Ariya, Kinetics of the gas-phase reaction of atomic chlorine with selected monoterpenes, PHYS CHEM P, 3(18), 2001, pp. 3981-3986
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.