INFRARED FREQUENCY-MODULATION PROBING OF CL- KINETICS OF HCL PRODUCTION FROM 292 TO 850 K(C3H4 (ALLENE, PROPYNE) REACTIONS )

Absolute rate coefficients for the reactions of chlorine atoms with al lene (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) wit h a laser photolysis/CW infrared long path absorption technique. The r eactions are initiated via pulsed laser photolysis of Cl-2 and monitor ed through CW infrared frequency-modulation spectroscopy of the HCl pr oduct. At room temperature (292 K), the reaction of Cl with allene pro ceeds almost exclusively through addition, with little HCl generated t hrough either abstraction or elimination. HCl production increases wit h temperature, and becomes unity at T greater than or equal to 800 K. Quantitative HCl yield measurements allow the contributions of additio n 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 erro r bars +/-2 sigma precision only). In contrast with the Cl + allene re action, both addition and metathesis appear to be important channels i n the reaction of Cl + propyne near room temperature. The reactions di splay 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 evo lution is observed. The HCl yield at 293 K is approximate to 70% and r eaches unity at T greater than or equal to 500 K. A fit of the rate co efficient 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) mole cule(-1) s(-1). The data over the whole 292-800 K temperature range ex hibit curvature and are better fit by the modified Arrhenius expressio n (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) mole cule(-1) s(-1) (+/-2 sigma precision). Preliminary measurements are al so presented for Cl + isobutene between 300 and 700 K, which were requ ired to comet for the small butene contamination in the propyne, Measu rements carried out in Ar buffer indicate that vibrationally excited H Cl is formed with both allene and propyne and allow an estimate to be made for k(VET), the HCl (upsilon = 1) + C3H4 vibrational relaxation r ate 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 corresp onding values are f = 0.55 +/- 0.09 and k(VET) = (8.6 +/- 3.6) x 10(-1 2) 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 t he addition rate. Comparisons are also made with recent ab initio calc ulations of energies and transition states for the Cl + C3H4 reactions .