Infrared frequency-modulation probing of Cl+C3H4 (allene, propyne) reactions:: Kinetics of HCl production from 292 to 850 K

Absolute rate coefficients for the reactions of chlorine atoms with allene (propadiene, H2C=C=CH2) and propyne (HC=C-CH3) have been measured as a function of temperature (292-850 K) and pressure (4-10 Torr) with a laser photolysis/CW infrared long path absorption technique. The reactions are initiated via pulsed laser photolysis of Cl-2 and monitored through CW infrared frequency-modulation spectroscopy of the HCl product. At room temperature (292 K), the reaction of Cl with allene proceeds almost exclusively through addition, with little HCl generated through either abstraction or elimination. HCl production increases with temperature, and becomes unity at T greater than or equal to 800 K. Quantitative HCl yield measurements allow the contributions of addition and abstraction/elimination to the total rate to be determined. The rate coefficient for HCl production is described between 292 and 850 K by the Arrhenius expression, k(metathesis)(allene)(T) = (3.7 +/- 1.7) x 10(-10) exp-[-(1671 +/- 286)/T] cm(3) molecule(-1) s(-1), (all error bars +/-2 sigma precision only). In contrast with the Cl + allene reaction, both addition and metathesis appear to be important channels in the reaction of Cl + propyne near room temperature. The reactions display biexponential HCl time profiles at T less than or equal to 400 K, but at T greater than or equal to 500 K, only single-exponential evolution is observed. The HCl yield at 293 K is approximate to 70% and reaches unity at T greater than or equal to 500 K. A fit of the rate coefficient for HCl production between 400 less than or equal to T less than or equal to 800 K to standard Arrhenius form gives k(metathesis)(propyne)(T) = (3.7 +/- 1.0) x 10(-11) exp[-(685 +/- 151)/T] cm(3) molecule(-1) s(-1). The data over the whole 292-800 K temperature range exhibit curvature and are better fit by the modified Arrhenius expression (with the temperature exponent fixed at 2) k(metathesis)(propyne)(T) = (1.25 +/- 0.21) x 10(-12) (T/298)(2) exp[(500 +/- 93)/T] cm(3) molecule(-1) s(-1) (+/-2 sigma precision). Preliminary measurements are also presented for Cl + isobutene between 300 and 700 K, which were required to comet for the small butene contamination in the propyne, Measurements carried out in Ar buffer indicate that vibrationally excited HCl is formed with both allene and propyne and allow an estimate to be made for k(VET), the HCl (upsilon = 1) + C3H4 vibrational relaxation rate coefficient. In the Cl + allene reaction, approximately half (f = 0.42 +/- 0.10) of the HCl is formed in upsilon = 1, and k(VET) = (2.8 +/- 0.4) x 10(-12) cm(3) molecule(-1) s(-1); with propyne, the corresponding values are f = 0.55 +/- 0.09 and k(VET) = (8.6 +/- 3.6) x 10(-12) cm(3) molecule(-1) s(-1). The qualitatively different dynamics for Cl + aliene vs propyne can be attributed to formation of a resonantly stabilized radical with aliene (chloroallyl), which greatly enhances the addition rate. Comparisons are also made with recent ab initio calculations of energies and transition states for the Cl + C3H4 reactions.