EXPERIMENTS AND THEORY ON THE THERMAL-DECOMPOSITION OF CHCL3 AND THE REACTIONS OF CCL2

Rate constants for the thermal decomposition of CHCl3 in Kr diuent hav e been measured by the laser schlieren density gradient method. The on ly decomposition process indicated is molecular elimination giving the singlet carbene, CCl2, and HCl. Rate constants are determined under d ifferent conditions of density over the temperature range 1282-1878 K, giving k(+/-15%) = 4.26 x 10(16) exp(-22 516 WT) cm(3) mol(-1) s(-1). Electronic structure calculations have provided models for both the t ransition state and molecule. With these models, both semiempirical Tr ee and Rice-Ramsperger-Kassel-Marcus unimolecular theoretical calculat ions are carried out. The experimental results agree with theory provi ded E-0 = 56.0 kcal mol(-1) and (Delta E)(down) = (820 +/- 30) cm(-1), suggesting that the barrier for back reaction is 3.8 kcal mol(-1). Cl -atom atomic resonance absorption spectrometric (ARAS) experiments, al so in Kr diluent, are then carried out, confirming that atom formation is entirely due to the thermal reactivity of CCl2. On the basis of Cl -atom yield measurements, a mechanism for Cl-atom formation is devised . Chemical simulations of the absolute Cl-atom profile data then provi de estimates of the temperature dependences for the rate constants use d in the mechanism. These results ate discussed in terms of unimolecul ar reaction rate theory suggesting that the heat of formation for CCl radicals is 100 +/- 4 kcal mol(-1) at 0 K. Our calculated results (R-C CSD(T)) extrapolated to the complete Delta(f)H(CCl20K)(0) = 53.0 and D elta fH(CCl,0K)(0) = 102.5 kcal mol(-1) and are consistent with the ex perimental results reported herein. Additionally, the results suggest that CCl2 undergoes dissociative recombination with a substantial acti vation energy.