Chemistry 324 - Spring 2008

The first problem on the homework assignment asks you to predict reaction products, but because I cut my lecture off before the final page of notes, I failed to give you some crucial information.

The final page of notes can be downloaded here. Please read it before you try to work the problem.

The emphasis of my treatment of frontier MO theory has been on its application, not its theoretical foundation. When it comes to FMO, I hope that you will to be able to use it and read articles in which others refer to it.

Of course, I could say a lot more about orbitals if time permitted. If you feel like you would like to spend some extra time on this material (and this applies to everything that we cover), please come see me. Physical organic chemistry is one of my favorite subjects (nerd! nerd!) and I am more than happy to talk about any aspect of it.

Today’s PowerPoint (revised W, Feb 27)

HW #5 (due next Monday) is based on two reactions that appear in paper #5 (to be discussed on Friday). So, even though we will discuss the paper before the homework is due, it probably would make sense to try the homework first.

When you generate a list of models, e.g., conformers, there are two ways to get information about them. One is tedious, but obvious. The other is very fast, but relies on obscure buttons.

Tedious & (perhaps) obvious

The obvious part: whenever you examine any model, you can always get its energy by selecting Display: Properties and you can always get the value of a dihedral angle by selecting Geometry: Measure dihedral (there is also a blue button you can use) and selecting 4 atoms.

The tedious part: you will have to repeat this for each molecule in your list. In addition, each energy and each angle will have to be written on a piece of paper and then (yawn) typed into Excel.

Very fast (uses obscure buttons)

First, select Display: Spreadsheet. To add relative energies, select the molecule that you think/know has the lowest energy, click the top of a blank column in the sheet, click Add…, and click rel. E (and kcal/mol). This adds a column of energy data to your sheet.

Second, use Geometry: Measure dihedral (or the blue button) to get the value of the dihedral angle that interests you. When this value is displayed, a little red-&-yellow “P” button will appear next to the value in the lower right-hand corner of the window. Click the “P”. This adds a column of dihedral data to your sheet.

Third, and the best part, click-drag in the sheet to select the data you want (you probably will need to drag the edges of the spreadsheet window to make it larger). Select Edit: Copy and then paste the data into Excel.

Last Monday (Feb 11) we spent some time discussing ring strain in two bicyclic molecules: 2-norbornene and its 7-oxa analog. While several groups contribute to ring strain in these molecules, any difference in ring strain might be due to angle strain at position 7.

Spartan’s model database contains high-level models (MP2/6-31G*) of both molecules. The C-C7-C and C-O7-C bond angles are 93.9° and 95.6°, respectively. In our discussion, we had noted that the ideal angles for carbon were larger than those for oxygen ( 109° in CH4 v. 104° in H2O). Comparing the bond angles in the bicyclics with the ideal angles, we might predict that 2-norbornene is more strained. And this might actually be true. Using total energies from the model data, along with HF/3-21G ZPE energies, I have estimated that the following reaction is exothermic by -5.6 kcal/mol. This is consistent with strain release at atom 7.

You might wonder if CH4 and H2O are the best models for estimating ideal bond angles. These molecules contain H-Z-H combinations while the bicyclics contain C-Z-C. As it happens, the experimentally measured bond angles in CH3-Z-CH3 are 112.4° (Z = CH2) and 111.7° (Z = O) (note: the MP2 angles are in nearly perfect agreement with experiment). These angles are quite similar, although the COC angle is still smaller.

I received an email on Sunday (sent in the afternoon, but not read until late at night) that there was a misunderstanding about the availability of HW #2. Since the original due date, M, Feb 11, in class, no longer seems practical, I am changing the due date to W, Feb 13, in class.

I will also make a greater effort to notify everyone in the class each time material gets posted to our web site.

I drew the product in problem 2a incorrectly. My original drawing shows a vinyl carbonate ester; the correct formula is an allyl carbonate ester. This functional group does not participate in the Diels-Alder addition and it does not really affect your answer. The online version of HW #1 has been corrected.