Cl2O4 in the Stratosphere  Week 1: Point Groups and Structure of Cl2O4

1. Introduction

2. Point Groups  

3. Vibrational Spectroscopy

4. Comparison with Experiment

Introduction

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. In order to make theoretical calculations on Cl₂O_4, you need to have an idea about its probable structure.

Several structures are possible for the Cl₂O₄ molecule. The goal for this week is to see which one of these structures is most probable, including bond types (single, double, triple) and estimated bond angles. We will limit our discussion to the five structures shown below, all of which are possible.

Assignment 1: Structure of Cl₂O₄

A. Point Groups

Several structures are possible for the Cl2O4 molecule, a few of which are shown at the right. In this part of the project we will use group theory and the experimental vibrational spectrum of Cl2O4 to determine its structure. When chemists predict the structure of a new molecule, they always ask if the structures make any sense. Do they agree with the bond angles and bond lengths commonly observed in other similar molecules? Do they have a structure that resembles an existing molecule? We should ask these questions about the structures at the right. We also need to determine the point group of different configurations of the molecule so we can predict the allowed IR and Raman transitions.

1. What bond angles would VSEPR theory predict for structures 4 and 5? What bond angles would the structures actually have? Would you expect structures 4 and 5 to be very stable? Why or why not?

2. What is the structure of perchloric acid, HClO₄ and perchlorate, ClO₄^-? Which structure in the list above looks like a perchlorate?

3. Seven possible configurations of Cl₂O₄ are shown below. Also included are the symmetry labels of the normal modes of vibrations of each structure.

4. Each student is assigned configurations to investigate. What is the point group of your structure?

Send your answer to Dr. Lever, who will post it for the entire group.

	Molecular Model of Possible Cl2O4 Structure	Normal Modes of Vibration
Configuration A Zach		4 Ag, 2 Bg, 3 Au, 3 Bu
Configuration B Zach		4 A1, 3 A2, 3 B1, 2 B2
Configuration C Cindy		8 A', 4 A"
Configuration D Cindy		5 Ag, Bg, 2 Au, 4Bu
Configuration E James		5 A1, 2 A2, 4 B1, B2
Configuration F James		5 A1, 2 A2, 4 B1, B2
Configuration G All		3 Ag, 2 B1g, B3g, Au, B1u, 2 B2u, 2 B3u

B. Vibrational Spectroscopy

The fundamental vibrational wavenumbers for Cl₂O₄ have been experimentally observed ⁶ in the infrared spectrum of the gas and Raman spectrum of the liquid. At this point we will use group theory to predict the fundamental vibrations that should be infrared active and/or Raman active for each structure on the list. Because only active transitions will show up in the IR and Raman spectra (as strong peaks, anyway), we can compare the number of experimental transitions with the predictions from group theory. This should narrow the list of possible structures down.

We also want to use group theory to predict the number of Raman lines that should be polarized. Comparing this prediction with experiment should narrow the list down even more.

Using group theory and character tables to predict IR and Raman active transitions may be new material for you, so our project Web site has resource material available. You can consult the information on Group Theory and Vibrational Spectroscopy, which will tell you how to use character tables to predict IR and Raman active transitions. Information also is there on polarized Raman lines. Character tables are available on this Web site. If you need help you can talk with your professor or e-mail Dr. Lever. (llever@uscupstate.edu).

For your assigned structure you should predict the following.

1. The number of normal modes for which the fundamental transitions would be IR active.
2. The number of normal modes for which the fundamental transitions would be Raman active.
3. The number of Raman lines that would be polarized.

Send your answers to Dr. Lever, who will post them for the entire group.

C. Comparison With Experiment

A 1971 paper in the journal Inorganic Chemistry ⁶ reported the infrared spectrum of gaseous Cl₂O₄ and the Raman spectrum of liquid Cl₂O₄. We would expect Cl₂O₄ to have twelve normal modes and thus at most twelve fundamental transitions in the IR and Raman. The IR and Raman lines corresponding to the fundamental transitions are shown below. The polarized Raman bands are in red.

Two of the fundamentals around 1280 cm^-1 happen to have roughly the same frequency - this is purely coincidental. Gas phase IR measurements were not made below 400 cm^-1, so we cannot tell from this experiment if the transitions at 92, 198, 353, and 382 cm^-1 are IR active or not.

Compare the number of experimentally observed IR and Raman active transitions with your predictions. Does your assigned isomer appear to be the structure of Cl₂O₄? If not, which is the structure of Cl₂O₄?

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.

This site created by David Whisnant (whisnantdm@wofford.edu).
This page was last updated on March 29, 2011
llever@uscupstate.edu