Quantum Calculations
                                                                                                           By
Jake Baus & Alexsia Richards
   
 
Over the coarse of this lab exercise GAMESS ab initio computer program was employed to determine the electronic structure of O2, HF and meta-dibromo benzene.  Specific calculations were conducted using AM1, PM3, 3-21G, 6-31G and double-zeta basis sets.  Calculations were used to display electron density, electrostatic potential, dipole moments, and bond length.  In addition three dimension diagrams were created showing the HOMO and LUMO  of each molecule.  Further studies allowed for determination of the vibrational frequencies, potential energy, and UV-Vis transitions for each molecule.


The data collected for each molecule is sumarized in the following web pages.

HF DiagramO2 DiagramMeta dibromobenzene diagram
        HF Data & Diagrams                                O2 Data & Diagrams                              Meta-dibromobenzene Data & Diagrams


Conclusion:

    For the potential energy graphs of the diatomic molecules the energy of the orbitals decreases as the internuclear separation decreases  because electron density accumulates in the internuclear region as the constructive interference between the atomic orbitals increases. However, at very small separations there is too little space between the nuclei for signifigant accumulation of electron density there.  In addition, the repulsion between the nuclei becomes large.  As a result the energy of the molecule rises as short distances, thus there is a minimum in the potential energy curve.  In comparing the IR spectrum of meta-dibromobenzene computed using the 3-21G basis set with the version obtain from NIST website the values for the peaks associated with the C-C stretching around the aromatic ring appear to be shifter to lower frequencies. Thus is can be said that although quantum mechanical software can provide you with helpful visual tools and approximate data, there is no substitution for actual experimental results.

References:

1)  CRC Handbook of Chemistry and Physics; 68th Edition, CRC Press (1987)

2)  CRC Handbook of Chemistry and Physics; 73rd Edition, CRC Press (1987)

3) Koudelka, Juraj, Exner, Otto; CCCAK; Collect Czech Chem. Commun,;
    EN50;1:1; 188-184. Retrieved from Beilstein Database, 20 February 2007.

4) NIST Mass Spec Data Center, S.E. Stein, director; Coblentz Society, Inc.
    http://webbook.nist.gov/cgi/cbook.cgi?ID=C108361&Units=SI&Mask=80#IR-Spec

5) Atkins, De Paula; Physical Chemistry 8th Edition; 2006