Ab initio mechanism and multichannel RRKM-TST rate constant for the reaction of Cl(P-2) with CH2CO (ketene)

The potential energy surface for the most important pathways of the reactio n between Cl(P-2) and ketene has been studied using the ab initio G2(MP2) m ethod. A variety of possible complexes and saddle paints along the minimum energy reaction paths have been characterized at the UMP2(full)/6-31G(d,p) level. The calculations reveal that the addition-elimination mechanism domi nates the Cl + CH2CO reaction and the direct hydrogen abstraction pathway i s negligible. It-is interesting to note that the addition reaction starts b y the formation of a p-pi complex (P pi C), and subsequently the chlorinate d acetyl radical CH2ClCO((2)A') and the chloroformyl methyl radical CH2CClO ((2)A ") are formed through the isomerization of P pi C. The C-C bond sciss ion of CH2ClCO((2)A') leads to the products CO and CH2Cl. The three-center HCl elimination from P pi C, occurring via a high energy barrier (TS3) and a weakly bound hydrogen bonding (HBC1), was proposed to account for the min or yield of the HCl + HCCO observed experimentally. Multichannel RRKM-TST c alculation has been carried out for the total and individual rate constants for various channels using the ab initio data. The "loose transition state " in the barrierless reaction entrance was determined by fitting the known experimental rate constant at 295 K. The kinetic calculations in this work can explain reasonably all the previous experimental results. In the temper ature range 300-1500 K and the atmospheric pressure of NZ, the total rate c onstants exhibit negative temperature dependence and can be fitted to the e xpression k(T) (1.0 +/- 0.2) x 10(-15)T(-1.58+/-0.02) exp(-565 +/- 2/T) cm( 3) molecule(-1) s(-1). Meanwhile, the rate constants show the typical fall- off behavior in the pressure range 10(3)-10(8) Torr. At lower pressures (P <10(3) Torr), the rate constants are pressure independent and the major pro ducts are CO and CH2Cl over the whole temperature range of interest. At the high-pressure limit (P > 10(8) Torr), the stabilization of P pi C dominate s the reaction. It is found that two radical products, namely CH2ClCO((2)A' ) and CH2CClO((2)A "), might be detectable in the fall-off region.