Cl2O4 in the Stratosphere 


1. Introduction
2. Cl2O4 and Stratospheric Ozone
3. Structure of Cl2O4 
4. Ab Initio Calculations
5. Follow-Up Discussion



In this project you will play the role of a graduate student working in a research group under the direction of a physical chemist. Your major advisor, whose research interests are centered around the applications of computational chemistry, is a member of a consortium that has received a National Science Foundation grant to study gas phase processes involving halogenated molecules in the stratosphere. The other research groups that are part of the consortium are widely scattered around the country. Different groups will be working on various aspects of the problem so that they can pool their resources.

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Cl2O4 and Stratospheric Ozone

Anderson, et al 1 studied the destruction of ozone within the Antarctic polar vortex using ER-2 aircraft observations of concentrations of ClO, BrO and O3. They concluded that two mechanisms accounted for around 61% of the observed ozone destruction within the chemically perturbed region of the polar vortex. Two other mechanisms involving ClO/HO2 and ClO/O accounted for only 5% of the ozone loss. Even allowing for their estimate of an experimental uncertainty of 30%, it appears that other ozone-depleting mechanisms may be involved.

Sanders, et al 11 have reported very high levels of OClO above Antarctica during the Antarctic spring. These reports have sparked interest in the role that OClO may play in the destruction of ozone in the stratosphere.

Because Cl2O4 is a major photolysis product of OClO 2 , it is possible that Cl2O4 may play a role in polar stratospheric ozone destruction. Your advisor is planning on using computations to predict the properties of Cl2O4 with the eventual goal of understanding its reaction chemistry. In particular, it may be possible to predict whether Cl2O4 is involved in catalytic cycles that significantly deplete ozone in the stratosphere.

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Structure of Cl2O4

The first step will be to identify probable Lewis structures for the Cl2O4 molecule and to assign point groups to each possible structure. The symmetry labels of the normal modes of vibration for a given structure will allow you to predict the number of strong peaks you would expect to observe in the IR and Raman spectra of the compound. Comparing the number of predicted peaks with the experimental spectra may help determine the actual structure of the molecule.

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Ab Initio Calculations

It has been suggested 4  that Cl2O4 may play an important role in the polar stratosphere because of the ozone-depleting cycle shown at the right. It would be of interest to predict whether this cycle is likely to occur.

The second reaction in this mechanism is crucial, because it is the reaction in which the ozone is destroyed. As a first step in predicting if the mechanism is likely, we need to estimate whether this reaction is spontaneous. Assuming that the entropy change is not large for this reaction, the enthalpy change may help us decide if it is likely to occur. If the second step is not spontaneous, it is unlikely that this mechanism will be significant.

In this phase of the project we will calculate the enthalpy change for the Cl2O4 gas-phase reaction shown below.

Eventually your advisor's research group will use very sophisticated computational methods to model this reaction. For this project you and your colleagues will do a preliminary study of the reaction using ab initio theoretical methods. We will use the 6-31G(d) basis set for all molecules.

  1. Hartree-Fock theory. An ab initio method that often is used to model molecules is the Hartee-Fock model, which "provides a reasonable model for a wide range of problems and molecular systems." The Hartree-Fock model neglects the energy contributions due to electrons interacting instantaneously, though, and thus is insufficient for accurate modeling of energies involved in chemical reactions 5. Although Hartree-Fock calculations might be useful if we only wanted to model the Cl2O4 molecule, we will need to use a more accurate method to model the energetics of a chemical reaction.

  2. Post-SCF methods. There are several different ways of compensating for Hartree-Fock theory's neglect of electron correlation (the energy contributions that arise from electrons instantaneously interacting with each other). One of the least expensive ways to improve on Hartree-Fock results is the MP2 method, which has been a successful model for a wide variety of systems. We will make calculations at the MP2/6-31G(d) level to model the reaction. Because MP2 calculations can take several hours, sometimes more than a day, for molecules the size of Cl2O4 we will distribute the computational load. Different groups will work on different molecules and then share their results.

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Follow-Up Discussion

Once you and your colleagues have calculated the energies of the molecules involved in the the Cl2O4 gas-phase reaction, you will need to combine them to estimate the enthalpy change of the Cl2O4 gas-phase reaction. You also will need to think about the entire mechanism. There may be other important factors involved along with the spontaneity of the second step. These possibilities will be dealt with in a follow-up discussion that is a part of the last two assignments of the project.

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Partial support for this work was provided by the National Science Foundation's Division of Undergraduate Education through grant DUE #9751605 and by CAChe Scientific through a Higher Education Program grant.

The PCOL community that partial support for this work was provided by the National Science Foundation's Division of Undergraduate Education through grant DUE #9950809. Additional support was provided by the Camille and Henry Dreyfus Foundation. PCOL faculty also acknowledge the National Science Teachers Association which awarded the PCOL Faculty Consortium the 1998 Gustav Ohaus Award for Innovation in College Science Teaching.

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