Computational Chemistry

(Last Revision: November 17, 2008)

• ABSTRACT:

The student will use the Gaussian/Gaussview 03W computational package to calculate molecular properties. You will calculate the isomerization energy for butadiene with density functional theory, calculate an ab initio potential energy curve for the HCl molecule, and determine the structure of a molecule of your choosing.

• TEXT REFERENCE:

"Experiments in Physical Chemistry", Garland, Nibler, and Shoemaker, Eighth Ed., McGraw-Hill, 2009, pp. 70-71; 82-85.

• OTHER REFERENCES:

• GENERAL DESCRIPTION AND THEORY:
  We may use computational methods to predict molecular properties. In the first part of this “experiment”, you will calculate the potential energy curve for the HCl molecule using the unrestricted Hartree-Fock method, the Møller-Plesset second-order perturbation method, and a density functional method (B3LYP) with the 6-31G* basis set. Your report for this section will include a graphical comparison of the Morse potential and the ab initio potentials. In the second part, you will calculate the energies of the cis and trans conformers of butadiene using the B3LYP method. In the third part, you will determine the structure and vibrational spectrum of a moleucle of your choosing.
  
  
• EQUIPMENT:

The spectroscopy laboratory workstation running Gaussian/Gaussview 03W.

• CHEMICALS:

None.

• LABORATORY PROCEDURE:

Part 1: Open Gaussian 03W and open the file in your folder labeled HF HClpot.gjf. Run the file without modifications. This will This will calculate the HF energy of the HCl molecule as a function of bond length. Repeat this with the DFT HClpot.gjf file and MPpot.gjf file. The output files should be saved as HF HClpot.out , DFT HClpot.out, and MP HClpot.out in your folder. Once the job is complete, the output file will have a list of bond lengths and energies (in Hartrees). Open the file labeled HF HClvib.gjf and modify it by changing the bond length to match that of the lowest observed energy in the preceding calculation. Run this program, which will determine the optimal geometry for the molecule and generate a vibrational spectrum. Repeat this calculation using the DFT HClvib.gjf and MP HClvib.gjf files. The output files should be saved as HF HClvib.out, DFT HClvib.out and MP HClvib.out file in your folder.

Part 2: Use Gaussview to build a rough model of a butadiene backbone. Add hydrogens and optimize the geometry using the b3lyp method and the 6/31g* basis set. Calculate the vibrational spectrum to verify the optimization. Select the four carbon atoms and rotate the torsion angle by 180 degrees. Reoptimize, and calculate the vibrational spectrum for this conformer.

Part 3: Build a molecule of your choosing and optimize the geometry using the b3lyp method and the 6/31g* basis set. Calculate the vibrational spectrum to verify the optimization.

• CALCULATIONS:
  
  Plot the HCl potentials (energy in kJ/mole versus bond length in picometers) from your calculations. On the same graph, plot the Morse potential for HCl using the parameters found on page 70 of the text. Compare the equilibrium separations and vibrational frequencies with the literature values.
  
  Determine the isomerization energy for butadiene by taking the difference in enthalpy between the two conformers. Compare the enthalpy of isomerization obtained in your calculation with that found in the literature.
  
  Give a one-paragraph description of the properties found for you chosen molecule.
  
  Your report will be saved as a Word document in your folder.
  
  
• LITERATURE VALUES: