Cl2O4 in the Stratosphere  Week 2: Ab Initio Calculations

1. Introduction
   
   
2. The Calculations
   
   
3. PCOL Contract Computational Chemistry Web Site (CompChem)
   
   
4. Questions
   

Introduction

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 possible. 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. Different groups will work on different molecules and then share their results.

 

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Assignment 2: Ab Initio Calculations

We need to calculate the energy of the molecules in the above reaction. Refer to the table below to find the molecule(s) for which you will be responsible. Use GaussView to build the molecules. If you are working with a small molecule, you can use its known bond length and bond angle.

1. Do the calculations at the MP2 level with a 6-31G(d) basis set using the computational chemistry program, Gaussian.

2. Run a geometry optimization and frequency calculation. (e.g., # opt freq rmp2/6-31G(d)).

3. Record the sum of the electronic and thermal enthalpies.

Send your results to Dr. Lever, who will forward them to the entire group.

James O2ClOClO3	Build the molecule in GaussView. Because this molecule is relatively large, add the following to the Link 0 tab under edit in order to delete scratch files and avoid filling up the hard disk. Copy the first two lines below and paste before the %chk=filename. %rwf=a,245mw,b,245mw,c,245mw,d,245mw,e,245mw,f,245mw,g,245mw,h,245mw %NoSave %chk=filename
Cindy ClOClO3	Build the molecule in GaussView. Because this molecule is relatively large, add the following to the Link 0 tab under edit in order to delete scratch files and avoid filling up the hard disk. Copy the first two lines below and paste before the %chk=filename. %rwf=a,245mw,b,245mw,c,245mw,d,245mw,e,245mw,f,245mw,g,245mw,h,245mw %NoSave %chk=filename
Zach O3	The experimental bond length and bond angle in ozone are1.278 Angstroms and 116.5o respectively.
Zach O2 (triplet)	The experimental bond length in oxygen is 1.21 Angstroms. This is the ground state of molecular oxygen. Because it is a triplet state, the calculation must be an unrestricted MP2 calculation.
Zach O2 (singlet)	The experimental bond length in oxygen is 1.21 Angstroms. This is an excited state of molecular oxygen.You can use the same bond lengths for the singlet as for the triplet ground state.

Questions

You can use the discussion above and the Computational Chemistry References web site to help with your answers.

1.The Hartree-Fock method uses the variation principle, so the coefficients of the basis set functions in the wavefunction are chosen to minimize the calculated electronic energy. What must be true about the calculated Hartree-Fock energy as compared to the exact electronic energy?

2. We are using the 6-31G(d) basis set in our calculations for this project. Suppose we choose a larger basis set, such as a 6-311++G(d,p).

a) In general, if you use a larger basis set would you expect the calculated bond lengths and bond angles to be more or less accurate than those calculated with a smaller basis set?

b) How would the larger basis set affect the amount of computer time needed for the calculation?

3. What is the advantage of using a Post-SCF method, such as MP2, for calculations over the Hartree-Fock method?

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

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 acknowledges 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