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J. Phys. Chem. 1993,97, 7947-7954
The Possible Role of C103 Isomers in Relation to Stratospheric Ozone Ani Rauk,' E. Tschuikow-Roux,' and Yonghua Chen Department of Chemistry, The University of Calgary, Calgary, Alberta, Canada T2N IN4
Mark P. McGrath and Leo Radom' Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia Received: February 2, 1993; In Final Form: May 20, 1993
After considering all possible connectivities for structural isomers of molecular formula C103 on the doublet ROHF/6-3 1G(d) potential energy surface, four minimum energy structures were identified and studied further at some or all of the ROHF,UHF, CIS, UMP2, RMP2, and QCISD(T) levels of theory using various basis sets ranging in quality from 6-3 1G(d) to 6-3 11+G(3df). Among the four Clo3 isomers (and alternative electronic states for two of them), only the ground state (,AI) C,, structure was found to be stable at all correlated levels of theory. In particular, the isomers with connectivities ClOOO and OClOO are found to be unstable at the RMP2 and QCISD(T) levels of theory (6-31G(d) basis set). The implications of these results to the chemistry of stratospheric ozone depletion are discussed. In addition, predictions of a variety of physical properties of C3, C103 (,AI) were made, including vibrational spectra and thermochemical tabulations based on G2 energy data.
Introduction Chlorine trioxide has been postulated as a reaction interas well as an unstable end product* in the chlorine photosensitized decomposition of ozone. Although the C1+ O3 reaction has been studied for some 60 years, the details of its mechanism remain obscure. In particular, the role assigned to chlorine trioxide and which of its several possible isomeric forms might be involved is not well understood, despite intensive recent investigations of the atmospheric C10, cycle in the context of Cl/ClO-catalyzeddepletion of stratosphericozone. An important, and as yet unexplained, feature (vide infra) is the fact that O2 is known to suppressthe quantum yield, -@(03), for ozone removal in the chlorine photosensitized decomposition of 03. This observation, first reported by Norrish and Neville: has been qualitatively confirmedby Wongdontri-Stuper et al.5 and DeMore et aL9 Prasad and Adams6 proposed an explanation for this suppression of O3loss by postulating the formation of a 'loosely bound" asymmetrical C10-02 association complex according to the reaction C10
-- +
+ 0,+ M
+
C1O*O2 M
(1) The C10.02 complex would remove C10 from theozone-depleting chain and hence supress ozone depletion. Prasad and Adams' scheme includes the further reaction of C10-02:
c10 + C10*O2
-
CI,
0, + 0,
(2a)
2c100 (2b) ClOO OClO (2c) Using computer simulation with an extended reaction set, Prasad and Adams6 found that the experimental data of WongdontriStuper et al.9 for -@(03) could be matched satisfactorily with a value for the equilibrium constant for reaction 1 of K1 = 3.7 X lO-19cm3(at 298 K). This assessment was made using the initial assumption that the overall rate for reaction 2 is the same as the overall rate for reaction 3 of C10 with itself with subsequent adjustments of K1 and the individual channels of reaction 2. Prasad7 further proposed that the C10.02 complex could also explain the discrepancies between measured and modeled stratospheric C10 profiles. Assuming that the 'missing" C10 in the
+
To whom correspondence should be addressed.
-
c10 + c10 c1, + 0,
(34
+ c1 OClO + c1 ClOO
(3b) (34
observed C10 mixing ratio at different altitudes (and hence different temperatures) was in the form of the C10.02 complex, Prasad7 derived the temperature dependence of the equilibrium constant, K1 = 3.3 X 10-24exp(3470/T) cm3, which translates to a value of 3.8 X 10-19 cm3 at 298 K. The fact that the values of K1 obtained independently from laboratory data and stratospheric measurements were in good agreement was interpreted as providing further support for the existenceof a C10-02complex. Recently, DeMore and Tschuikow-Roux10studied the ultraviolet absorption spectrum and chemical reactivity of the C10 dimer produced by the photolysis of Cl2/03, C12/C120, or C120 alone at 195-217 K. The experiments were carried out both in the gas phase and in the cryogenic solvents CF4, C02, and NzO, with N2 pressurization. The results showed that ClOOCl was the only dimer formed
+
+
-
+
C10 C10 M ClOOCl M (4) and that, under the conditions of the experiments, the reverse dissociation was unimportant, while reactions 3a-c were negligibly slow. In a followup study under the same experimentalconditions but with 0 2 substituted for N2, DeMorell found, by overlay of the M = 0 2 and M = N2 normalized dimer spectra, no change in the spectral shapes and cross sections throughout the spectral range of 19&350 nm, which suggested strongly that C10.02 was not formed in any appreciable amount. Also, using computer simulation, DeMore showed that within the limit of detectability of C10.02, an upper limit for K1at 197 K is 4 X 10-l8cm3.Using this value of K1 and an estimate for the entropy of CIO.02 (305 J K-'mol-'), DeMore evaluated the C10.02 bond energy (with respect.toC10 plus triplet 02) as C32 kJ mol-1 and the temperature dependence of the equilibrium constant K1 as
