Kinetics and mechanism of the chlorine oxide ClO + ClO reaction: pressure and temperature dependences of the bimolecular and termolecular channels and thermal decomposition of chlorine peroxide

Abstract

The kinetics and mechanism of the C10 + C10 reaction and the thermal decomposition of ClOOCl were studied using the flash photolysis/long path ultraviolet absorption technique. Pressure and temperature dependences were determined for the rate coefficients for the bimolecular and termolecular reaction channels and for the thermal decomposition of ClOOCl. In order to determine channel-specific rate coefficients and to minimize complications associated with secondary chemistry, the reaction was studied over wide ranges of initial reactant stoichiometry and temperature. The rate coefficient for the termolecular association channel in the lowpressure limit, C10 + C10 (+M) -ClOOCl (+M) ( l ) , with N2 as a third body was measured over the temperaturerange 195-390Kandresultedinkl,~,(T) =(1.22*0.15) X 1&33exp{(833 f 34)/TJ~m~molecule-~ s-1 ( f 2 a error bounds). The 300 K rate coefficient for reaction 1 was measured for a number of bath gases.The results are k l ,~ (X10-32 cm6 molecule-2 s-l) = 0.99 f 0.05, 1.24 f 0.09, 1.71 f 0.06, 2.00 f 0.27, 2.60 f 0.17, 3.15 f 0.14, and 6.'/ f 3.6 for He, 0 2 , Ar, N2, CF4, SF6, and C4, respectively. The effective collision efficiency for M = C12 is very large and is likely due to a chaperone mechanism. Below 250 K, the reaction was in the falloff regime between second-and third-order kinetics. From the falloff data, the rate constant in the high-pressure limit, k,3w, was estimated to be (6 f 2) X cm3 molecule-' s-'. The Arrhenius expressions for the three bimolecular channels, C10 + C10 -Cl2 + 0 2 (2), ClOO + C1(3), and OClO + C1 (4), over the temperature range 260-390 K are kz(7') = (1.01 f 0.12) X exp(-(1590 f lOO)/TJ cm3 molecule-' s-1, k3(7') = (2.98 f 0.68) X 10-11 exp{-(2450 f 330)/Tj cm3 molecule-' s-l, and k4(T) = (3.50 f 0.31) X 10-13 exp{-(1370 f 150)/Tj cm3 molecule-' s-l. These expressions lead to a value of (1.64 f 0.35) X lO-14cm3 molecule-1 s-1 for the overall bimolecular rate constant (k2 + k3 + k4) at 298 K. The rate coefficient expression for ClOOCl thermal decomposition was determined to be k-l( 7') = (9.81 f 1.32) X exp{-(7980 f 320)/Tj cm3 molecule-' s-1 over the range 260-310 K. From a Third Law analysis using equilibrium constants derived from measured values of kl and k-1, the enthalpy of formation (AHOr(298)) of ClOOCl was determined to be 30.5 f 0.7 kcal mol-'. The equilibrium constant expression from this analysis is K,(T) = (1.24 f 0.18) X 10-27 exp((8820 f 440)/Tj cm3 molecule-'. From the observed activation energy for reaction 4 and the literature activation energy for reaction -4, the OClO enthalpy of formation was calculated to be 22.6 f 0.3 kcal mol-'.

Nitric acid trihydrate nucleation and denitrification in the Arctic stratosphere

“…The detailed list of reactions included in CLaMS is given in Table A1 in the appendix. Chemical reaction rates and absorption cross sections are based on Sander et al (2011) including ClOOCl photolysis, which is based on Papanastasiou et al (2009). Exceptions are the reaction rate for ClOOCl formation, which is taken from Nickolaisen et al (1994) and the ClOOCl equilibrium constant which is taken from Plenge et al (2005).…”

“…Chemical reaction rates and absorption cross sections are based on Sander et al (2011) including ClOOCl photolysis, which is based on Papanastasiou et al (2009). Exceptions are the reaction rate for ClOOCl formation, which is taken from Nickolaisen et al (1994) and the ClOOCl equilibrium constant which is taken from Plenge et al (2005). These values are based on Geophysica measurements as suggested by Sumińska-Ebersoldt et al (2012).…”

ClOOCl photolysis at high solar zenith angles: analysis of the RECONCILE self-match flight

“…Simulations were carried out for combinations of the four J ClOOCl parameterisations discussed above and two parameterisations of k rec : JPL 2011 and Nickolaisen et al (1994). JPL 2011 k rec was chosen as the current recommendation to the scientific community, whereas k rec published by Nickolaisen et al (1994) due to its good agreement with both atmospheric observations and unimolecular theory as shown by von Hobe et al (2007).…”

Evidence for heterogeneous chlorine activation in the tropical UTLS

“…The latter is shown by the dotted lines in Fig. 9, where a combination of k f according to Nickolaisen et al (1994) and a ClO/ClOOCl equilibrium constant according to Plenge et al (2005) is used that has been shown by von Hobe et al (2007) to more realistically reproduce ClO and ClOOCl mixing ratios made in the Arctic.…”