Quantum Calculations on Oxygen, Ozone, and Flourobenzene
Lucas Esser and Stefanie Bell

Introduction

    The electronic structure of a molecule is useful to determine the reactivity. When the electron density is determined, it is possible to predict other properties of the molecule and gain knowledge of the reactivity. The dipole moment, polarizability, frequency of vibration, and electron donating character may be calculated from the electron density.

It is necessary to have a respectable understanding in quantum mechanics to use the calculations.  Since the calculations are very time intensive, software may be useful in approximating the results. Most of the approximations are done by the variation principle.  The principle states that the lowest energy approximations are closest to the actual physical quantities.

The approximation methods are very useful for complex molecules that are difficult to solve analytically. Molecular Mechanics is a program that may be used to solve classical mechanics. GAMESS, a software program with high accuracy approximations, is useful in approximating calculations for complex molecules. GAMESS is capable of two different approximation methods, semi-emperical and ab initio. Ab initio theory calculations have the best approximations for molecular mechanics. The Hartree Fock Self-Consistent Field model is used in Ab initio theory calculations with a manually selected basis set size. The basis sets allow the approximations to improve. Once the electronic structure calculations are done for each molecule with the highest level of theory, the other molecular properties may be calculated and compared to empirical values.




Experimental

    Software from the UW-Oshkosh quantum server determined calculations to obtain molecular mechanics approximations for the following molecules: Flourobenzene, Oxygen, and Ozone.  The program wxMacMolPlt calculated the initial geometries for the molecules and were saved as  “.cml” files. Each file was opened in Jmol and a molecular mechanic optimization was performed for each molecule. Once the optimization was complete the files were then saved as “.xyz” files. The optimized molecular mechanic structures were used to generate AM1 and PM3 optimized geometry files and then saved as “.inp” then exported to the program, GAMESS, via GamessQ. The log files for each molecule with completed calculations were verified by making sure they exited gracefully and then saved as a “.log” file. The “.log” files were then opened and the dipole moments at each level of theory were obtained.  

The results with the largest basis gave the best ab initio and the potential energy surfaces and bond lengths were calculated for each molecule. The following basis sets were computed: 6-21G, 6-31G and DZV and saved as ".log" files. The ".log" files were examined for graceful exit and then reopened for each molecule and level of theory. The vibrational frequencies and levels of theory were obtained for each molecule from the appropriate “.log” files.



Conclusion

    This experiment examined how computer computations can show the molecular geometry of molecules and helps to determine the other properties of molecules including vibrational frequencies, dipoles, and spectral absorption peaks. These calculations may be used to predict the accuracy in different levels of theory.  The molecular geometry computations allow us to create the molecular orbital diagrams for each molecule.  We examined 3 different levels of theory and observed each one has a different level of accuracy. Some calculated values did not compare to the theoretical values, such as the dipole moments. Using computer software for these calculations helps save time, however error may be missed if not carefully analyzed. One of the most useful calculations is the molecular geometry.  These calculations should be consistent with literature values. 


                                                                                                      
                                                                                              

                                                 Oxygen                            Ozone                       Flourobenzene


References:

1. Mihalick, J.; Gutow, J. Molecular Orbital (MO) Calculations. Chemistry 371 Lab Manual Spring 2011. Oshkosh, WI, 2011.

2. (MOPAC 6) M.J.S. Dewar J. Mol Struct. 100, 41(1983); M.J.S. Dewar, EG. Zoebisch, E.F. Healy, J.J. P. Stewart J. Am. Chem. Soc. 107, 3902(1985).

3. The Jmol Development Team. http://www.jmol.org, accessed March 2011.