Molecular Calculations of Diatomic Oxygen, Formaldehyde, and Benzaldehyde
Molecules will be in the geometry that have the lowest energy.
These geometries can be calculated using advanced computer modeling
programs. "GAMESS" is one such program. In GAMESS, there
are options to perform calculations as MOPAC or ab initio. MOPAC
calculations are based off semi empirical calculations of electron
overlap intergrals needed to calculate the Hamiltonian. Ab initio
calculations are based off intergral calculations. Ab initio
calculations are more in depth and often yield more accurate
calculations. One draw back of ab initio calculations is that
they often strain the computer in which the calculations are being
made. This slows down the computer and often takes several hours
to complete.
After the optimized
geometries of molecules have been obtained, other computer programs can
be used to model other aspects of the molecules. The HOMO,
vibrational data, and the potential of bond stretching can all be
modeled from various programs. These calculations can then be
related to scientifically accepted values. Often times, the
calculated values are similar to the accepted values. This makes
sense considering these values are often calculated using computer
programs. The reason for the discrepencies between the accepted
values and the calculated values is often due to the depth of
calculations performed. At times, there are large differences
between the calculated and accepted values.
- As described above, the molecules were subjected to five diferent levels of moleculare orbital theory broken into two groups
- Below are images of each molecule studied. Click on each image to advance to calculated data.
In
conclusion, the calculations made by this software were very useful in
determing the probable shape of the molecules, electromagnetic
configurations, and visualizing sub shells. The calculations for the
molecular geometries, bond angles and lengths, agreed reasonably well
with those that are scientifically accepted. The vibrational
frequencies calculated provided another look into the workings of
various molecules. The calculated vibrational frequencies could
then be related to actual vibrations seen on IR spectra. The
"GAMESS" software provided a good starting location. Even through
the software gave a good starting spot for calculations, more precise
calculations could be performed to better the results. The bright
spot of using this software was the very good geometries that were
obtained. Even tough these geometries were not completely
accurate, they work for most analysis. As molecules become more
complex, drawbacks of this software begin to appear. This
software does would not handle comlex molecules well. Calcuations
for large molecules would take days or even weeks to complete.
The error associated with complex molecules would be large.
This software works by guessing at the geometries of the
molecules. An error in one geometry would lead to a bigger error
in the next geometry, because the previous geometry is used as a
starting point for the next calculation. Large molecules could
also pose a problem for this software in terms of local energy minima.
Large molecules could have several geometries. If the
software became bogged down in the wrong geometry, the best geometry
produced would be for the local minima and not the geometry with the
lowest overal energy.