Assignment CompChem1: WebMO and Energy Calculations

Use WebMO and Gaussian to complete the following exercises. You are encouraged to comment on the additional results provided by WebMO in your answers, if appropriate.

1. Energies and dipole moments of dicholrodifluoroethane (exercise 2.2).

Use the WebMO Editor to build the RR, SS, and meso stereoisomers of HFClCCHFCl. It might be useful to choose File: Preferences...: Display Symbol to assist in visualizing the atoms. Perform a single point Hartree-Fock 3-21G calculation on each molecule. After one molecule is built and the job has successfully run, build the additional molecules from Job Manager: View: New Job Using This Geometry. To invert a chiral center, select a carbon in adjust mode and use Adjust: Invert Chiral Center.

Make a table with columns for molecule, energy, and dipole. Discuss why your answers differ from ECESM.

Include one figure of a molecule with a nonzero dipole moment vector. You can use the Paint program to capture a WebMO screen image and insert it into a Word document. First start Programs: Accessories: Paint. Then activate the WebMO window containing the image to be captured. Press Alt-Print Screen to copy that window to the clipboard. Activate Paint. Use Edit: Paste to paste the image into Paint. Use the rectangle tool to define the part of the window you wish to insert. Use Edit: Copy to copy the rectangle to the clipboard for pasting into another document (or choose Edit: Copy to... to save the rectangle to disk). Activate Word. Use Edit: Paste to paste the image into your document.

2. Ethene and formaldehyde molecular orbitals (exercise 2.4).

Build ethene and formaldehyde using WebMO. Perform a single point Hartree-Fock 3-21G calculation on each. Use “pop=reg” as additional keywords to perform a molecular orbital analysis. After the job has completed, use Job Manager: View: Raw Output to look at the Gaussian output.

What atomic orbitals are used for row 2 atoms (O and C) at the 3-21G level of theory? For H atoms? Sketch by hand the HOMO and LUMO for both molecules. Compare and contrast the properties of both molecules.

3. C13 NMR shifts of butane, butene and butyne (exercise 2.5).

Build butane, butene and butyne. Adjust the dihedral angles to result in planar, staggered conformations. Perform an NMR Hartree-Fock 3-21G calculation.

Gaussian reports absolute chemical shifts. WebMO converts these chemical shifts to shifts relative to TMS. Make a table with columns for compound, C atom (outer or inner), absolute shift, relative shift, and literature value. Justify these chemical shifts based on your chemical knowledge.

If any of the jobs failed, view the Raw Output from Job Manager and determine why the job failed. Report the line(s) in the output file that indicates the reason for failure, and explain why this happened. Build the molecule again and re-run the job, but choose Cartesian Coordinates in the Configure Gaussian Job Options window.

4. Please offer any constructive criticism you may have on how WebMO could be improved to assist you with your calculations.