Molecular orbital (MO) theory is a method for writing
electronic wavefunctions of diatomic molecules where the distribution of a
single electron can be defined over the entire molecule.1 Using MO
allows for different properties of molecules to be predicted with very good
accuracy. The best approximations of wavefunctions occur when the variational
principle is applied. The principle states that the wavefunction with the
lowest energy is the best approximation for the actual wavefunction.2
The caveat with using this theory is that it can be done relatively easily
within the domain of a single atom with one electron; however, as more atoms
and more electrons are introduced, it becomes vastly more complicated.
Avogadro was the software for drawing the molecules and
performing simple molecular calculations like bond length and bond angles. MacMolPlt
performed geometry optimization using MOPAC, which is a semi-empirical method
for calculating the geometries of molecules. The semi-empirical method allowed
for the use of a number of Hamiltonians to optimize the geometry, with AM1 and
PM3 being used throughout the initial calculations. The best level of MO theory
utilized was Ab initio where the
basis sets for the wavefunction calculations involve all integral calculations.3
The Ab initio basis sets used were:
3-21G, 6-21G, 6-31G, and DZV (double zeta valence). GAMESS was the software that ran
quantum mechanical calculations for all levels of theory and GAMESSQ allowed
for queuing up the necessary calculations for GAMESS to perform.
Igor was the statistical software used to plot potential
energy versus bond length for the diatomic molecule. Lastly, Jmol and Seamonkey
were used to create and edit the webpage.
Links to Molecules
Conclusion
The molecular orbitals calculated in the experiment was done through
two semi-empirical methods using MOPAC and these were, AM1 and PM3. The
best results used the Ab initio theories of 6-21G, 6-31G, and DZV. One
of the calculations used for ClBr was 3-21G. Overall the calculations
for all three molecules were very close to the experimental values found
from the NIST website and other open sources. Most of our results had
the best calculated values using DZV theory except for the ClBr which
used 6-31G.
References
1. Cooksy,
Andrew. Quantum Chemistry and Molecular
Interactions; Pearson: New Jersey, 2014; pp 214.
2. Gutow,
Jonathon. Molecular Orbital (MO)
Calculations; laboratory manual: University of Wisconsin-Oshkosh, revised
Feb 2015.
3. Gutow, Jonathon.
Molecular Orbital (MO) Calculations;
laboratory manual: University of Wisconsin-Oshkosh, revised Feb 2015.