KINETICS AND MECHANISM OF THE CLO- PRESSURE AND TEMPERATURE DEPENDENCES OF THE BIMOLECULAR AND TERMOLECULAR CHANNELS AND THERMAL-DECOMPOSITION OF CHLORINE PEROXIDE(CLO REACTION )

The kinetics and mechanism of the CIO + CIO reaction and the thermal d ecomposition of ClOOCl were studied using the flash photolysis/long pa th ultraviolet absorption technique. Pressure and temperature dependen ces were determined for the rate coefficients for the bimolecular and termolecular reaction channels and for the thermal decomposition of Cl OOCl. In order to determine channel-specific rate coefficients and to minimize complications associated with secondary chemistry, the reacti on was studied over wide ranges of initial reactant stoichiometry and temperature. The rate coefficient for the termolecular association cha nnel in the low-pressure limit, CIO + CIO (+M) --> ClOOCl (+M) (1), wi th N2 as a third body was measured over the temperature range 195-390 K and resulted in k1,N2(T) = (1.22 +/- 0.15) X 10(-33) exp{(833 +/- 34 )/T} cm6 molecule-2 s-1 (+/- 2sigma error bounds). The 300 K rate coef ficient for reaction 1 was measured for a number of bath gases. The re sults are k1,M (X 10(-32) CM6 molecule-2 S-1) = 0.99 +/- 0.05, 1.24 +/ - 0.09, 1.71 +/- 0.06, 2.00 +/- 0.27, 2.60 +/- 0.17, 3.15 +/- 0.14, an d 6.7 +/- 3.6 for He, O2, Ar, N2, CF4, SF6, and C12, respectively. The effective collision efficiency for M = C12 is very large and is likel y due to a chaperone mechanism. Below 250 K, the reaction was in the f alloff regime between second- and third-order kinetics. From the fallo ff data, the rate constant in the high-pressure limit, k(infinity)300, was estimated to be (6 +/- 2) X 10(-12) cm3 Molecule-I s-1. The Arrhe nius expressions for the three bimolecular channels, ClO + ClO --> C12 + O2 (2), ClOO + Cl (3), and OClO + Cl (4), over the temperature rang e 260-390 K are k2(T) = (1.01 +/- 0.12) X 10(-12) exp{-(1590 +/- 100)/ T} cm3 molecule-1 s-1, k3(T) = (2.98 +/- 0.68) X 10(-11) exp{-(2450 +/ - 330)/71 CM3 molecule-1 s-1, and k4(T) = (3.50 +/- 0.31) X 10(-13) ex p{-(1370 +/- 150)/71 CM3 molecule-1 s-1. These expressions lead to a v alue of (1.64 +/- 0.35) X 10(-14) cm3 molecule-1 s-I for the overall b imolecular rate constant (k2 + k3 + k4) at 298 K. The rate coefficient expression for ClOOCl thermal decomposition was determined to be k-1( T) = (9.81 +/- 1.32) X 10(-7) exp{-(7980 +/- 320)/T} cm3 molecule-1 s- 1 over the range 260-310 K. From a Third Law analysis using equilibriu m constants derived from measured values of k1 and k-1, the enthalpy o f formation (DELTAH-degrees(f)(298)) of ClOOCl was determined to be 30 .5 +/- 0.7 kcal mol-1. The equilibrium constant expression from this a nalysis is K(eq)(T) = (1.24 +/- 0.18) X 10(-27) exp{(8820 +/- 440)/T} cm3 molecule-1. From the observed activation energy for reaction 4 and the literature activation energy for reaction -4, the OClO enthalpy o f formation was calculated to be 22.6 +/- 0.3 kcal mol-1.