J. Phys. Chem. 1996, 100, 2985-2992
2985
Theoretical Study of the Reaction of HCl with ClONO2 Catalyzed by NO3-. “Attachment-Detachment” Mechanism for the Anion-Catalyzed Neutral Reactions Alexander M. Mebel and Keiji Morokuma* Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322 ReceiVed: September 22, 1995; In Final Form: NoVember 6, 1995X
The potential energy surface for the reaction of HCl with ClONO2 in the presence of NO3- has been studied using ab initio (MP2) and density functional (B3LYP) methods. The HCl + ClONO2 f Cl2 + HONO2 reaction is shown to have a high barrier of 45-64 kcal/mol. The NO3- catalyzes the reaction, and the entire reaction involves several associative and dissociative steps. The energetically most favorable pathway for the reaction of the complex HCl‚NO3- with ClONO2 is to follow the associative steps. Here HCl‚NO3-, which is actually Cl-‚HNO3, can attach to ClONO2, giving the NO3ClClHNO3- complex, which, after decomposing to Cl2‚NO3- + HNO3, forms the Cl2NO3HNO3- complex and then the observed products Cl2 + NO3HNO3-. All the associative steps have no barrier, and the entire reaction occurs with a negative activation energy and with the overall exothermicity of 21.6 and 17.7 kcal/mol at the B3LYP and MP2 levels, respectively. If one follows dissociative steps, the reaction complex HCl‚NO3- at first dissociates to give Cl- and HNO3, and in the second step, Cl- attacks ClONO2 to produce Cl2‚NO3-, which again decomposes to Cl2 and NO3-. These dissociative steps occur without barrier, and energetically the highest point is Cl+ HNO3. The reaction of the reactant complex ClONO2‚NO3- with HCl cannot produce Cl2 + NO3HNO3directly. Instead, the reaction proceeds by the formation of the NO3ClNO3HCl- complex and its decomposition to ClONO2 + Cl-HNO3 without barrier and then follows the same associative pathway to that starting with the reactant complex HCl‚NO3-. Free energy calculations predict that, at low temperatures, both HCl‚NO3+ ClONO2 and HCl + NO3-‚ClNO3 reactions occur by the associative steps via trimolecular complexes and exhibit a negative temperature dependence of the rate coefficient.
Introduction The energetics and mechanisms of the chemical reactions of inorganic chlorine-containing species and nitrogen oxide species (NOx) in the stratosphere are critical to our understanding of stratospheric ozone chemistry.1-3 During the austral spring over Antarctica, in the presence of polar stratospheric clouds, inert long-lived reservoir species, such as ClONO2, HCl, and HOCl, are transformed by heterogeneous reactions to photoactive species responsible for rapid and catalytic destruction of ozone. Recent field measurements of HCl and ClO concentrations in the stratosphere, performed by Webster et al.,4 showed that the reaction
HCl + ClONO2 f Cl2 + HNO3
(1)
is critical in determining the winter and spring chlorine budget within the polar vortex. Reaction 1 itself is slow in the gas phase, with the small rate coefficient, 1000 K), the II and III species will be further destabilized, and the dissociative steps which involve Cl- + HNO3 + ClONO2 and Cl2 + HNO3 + NO3- instead of III will play an increasingly important role in the reaction. 5. Dipole Moment and Polarizability Calculations for ClONO2. Okumura et al.7 calculated the collision rate constant for the Cl- + ClONO2 reaction based on the average dipole orientation (ADO) theory.20 They used for this computation polarizability of ClONO2 calculated at the MP2/6-31G(d) level, for which they reported the value of 8.61 Å3. However, it is well-known that calculated electrical properties such as polarizability are very sensitive to the basis set used and cannot accurately reproduce experiment with a poor 6-31G(d) basis set.21 Hence, we recalculated the polarizability of ClONO2 employing various larger basis sets, 6-31+G(d), 6-311+G(2df), and 6-311(2+)G(3df). For 6-311(2+)G(3df), an additional set of diffuse sp functions was added to the standard 6-311+G set22 for each atom, following the even-tempered procedure of Lee and Schaefer.23 The calculated dipole moments and polarizabilities are collected in Table 4. Polarizability is a tensor and the values presented in Table 4 are one-third of the traces for the polarizability tensors, computed with different basis sets. One can see that these values change from 4.54 Å3 at the MP2/631G(d) level to 6.28 Å at the MP2/6-311(2+)(3df) level when we use the MP2/6-31+G(d) optimized geometries of ClONO2. As was mentioned above, the structures of ClONO2 optimized at the MP2 level with 6-31G(d), 6-31+G(d), and 6-311(2+)(d) basis sets vary only slightly, and the polarizabilities obtained with these geometries at the MP2/6-31G(d) level are similar, in the 4.49-4.54 Å3 interval. We could not reproduce the value of 8.61 Å3 reported by Okumura et al.,7 although we used the same theory level and program. Addition to the basis set of diffuse functions appears to be most essential for the polarizability calculations. The dipole moment is sensitive to the use of electronic correlation. At the Hartree-Fock level even with large basis sets, the calculated values are far from experiment. If the MP2 density matrix is employed for the dipole moment calculation, we obtained 0.68-0.85 D, close to experimental value of 0.77 D.24 Meanwhile, Lee showed12,17 that there is a substantial difference between the MP2 and CCSD(T) levels of theory for the dipole moment of ClONO2; i.e., there is a substantial correlation contribution beyond MP2. At the CCSD(T)/TZ2P level, the dipole moment is 1.08 D, about 0.3 D higher than in
2992 J. Phys. Chem., Vol. 100, No. 8, 1996 experiment. Therefore, it was concluded12 that a more accurate experimental determination is needed. Concluding Remarks The potential energy surface of the reaction of HCl with ClONO2 catalyzed by NO3- has been studied using various theoretical methods. The calculations show that involvement of the anion into the reaction lowers the activation energy from 50-60 kcal/mol to a negative value. The HCl + ClONO2 + NO3- reaction can proceed by various “attachment/detachment” pathways occurring through barrierless attachment of anions to neutral molecules followed by detachment of other anions from the complexes formed. For instance, bimolecular steps involve formation of two-body A‚B- complexes, and the attaching/ detaching anions are NO3- and Cl-. In associative steps, threebody A‚B‚C- complexes are formed and, besides NO3 and Cl-, the attaching/detaching anions are Cl-‚HNO3, Cl2‚NO3-, NO3HNO3-, and NO3ClNO3-. At low and moderately high temperatures (
