Case Study: Pyrolysis of Ethyl Formate

In this study, you will generate the transition state for ethyl formate's breakdown into ethylene and formic acid. All calculations in this lab will be done at the semi-empirical level with AM1 theory. Create and optimize ethyl formate as shown below. Remember to minimize the structure in the builder before trying to optimize!

Now we need to construct the product for the reaction. Since the reactant and product must be isomers (molecules which have the same number of each atom), the product must also be a single molecule. So, resave your structure of ethyl formate as another file, called formic acid and ethylene (or something similar!). Enter the builder again, and constrain the distance between the atoms shown below.

The top constraint needs to be 1.1 Angstroms (bringing the atoms pretty close together), the bottom one is 2.4 Angstroms (nearly breaking it), and the right one is 2.1 Angstroms (again, much longer).

Minimize this structure and return to the main MacSpartan screen with both molecules open. Orient them so that they are fairly similar, and make ethyl formate the active molecule.

Open Transition Search from the Build menu. Select the formic acid and ethylene complex as the product, and pair up the atoms as requested. Remember that you can still rotate and move the molecules in this mode to make it easier to see the pairs.

When done, click Generate. Save the resulting guess at a transition structure as something different.

Do a Transition Structure calculation on this molecule at the AM1 level. Up your optcycles to at least 300, if not more. This will allow MacSpartan plenty of cycles to optimize the molecule if it made a bad guess.

When this is done, do a Frequency calculation. Don't bother submitting any surface calculations, it will just slow down your results.

If you get ONE AND ONLY ONE imaginary frequency (it is labeled "imaginary"!), then you have a possible transition state. If this does not work, go back and remake your original "guess" geometry. Think about which bonds might have presented a problem and try to modify it accordingly.

If this is successful, display the imaginary frequency. Compare it to other frequencies of this molecule, and think about why it is "imaginary". Compare and discuss your results with your lab partner and/or classmates, as always!

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