Abstract
The geometries of bond length
and angles, highest occupied molecular orbital, lowest occupied
molecular orbital, electrostatic potential, dipole moments, partial
atomic charges, and vibrational energies were calculated for
bromoflouride, ethylene, and ethylbenzene using three different levels
of molecular orbital theory for each molecule. The three theories were
3-21G, 6-21G, 6-31G, and DZV. The best level of theory was DZV then
6-31G, 6-21G, and lastly 3-21G. The reason one basis set was better than
another is that it used more basis sets in the calculations. The best
level of theories were different when finding certain characteristics.
The best level of theory for vibrational frequencies was DZV, for
geometries it was AM1, and for dipole moments it was 6-31G.
Use hyperlinks to view information of the molecules
Ethylbenzene Bromoflouride Ethylene
Introduction
The important properties of a molecules
can be predicted from its know electronic structure since it predicts
much of its reactivity. Using this knowledge predicting other useful
properties like dipole moment, polarizablity, vibrational frequencies,
probability of absorption of light, and tendency to donate electrons.
Potential energy operators becomes more important when there are more
atoms present in the molecule. The lowest energy of a molecule is in its
lowest occupied orbital. The optimized geometry is present in the
molecule with the lowest energy. The basis sets the theories used was
made by linear combinations of wavefunctions. This is all done using the
variational principle. The expectation value of the hamiltonians was
used since the wavefuntions didn't have eigenvalues, since they were not
eigenfunctions. The expectation value was then divided by the
normalization constant. The calculations are very long to do by hand so a
computer program was used to do the calculations very quickly. It was
then possible to find many important qualities of molecules which were
stated above.
Three theories of molecular geometry calculations
were used to determine important aspects of the molecules. The reason
three different theories were used was to see which ones would give the
best values. It wasn't always the largest basis sets that gave the best
values, this could be that the calculations were to large and gave large
values. The AM1 and PM3 theories were used first to optimize the
molecules geometries. The best calculations of these theories is called
Ab initio which all the integrals are calculated. The list of largest
basis set theory to smallest is as follows, DZV, 6-31G, 6-21G, and
3-21G.
A computer program known as GAMESSQ was used to
calculate the molecular orbitals of all our molecules. First the
molecules were all built on Avogadro. Opening the molecule in MacMol and
optimizing geometries was the next step. Then the optimized geometries
were further calculated in GAMESS to create a 3-21G, 6-21G, 6-31G and a
DZV basis set which all gave different geometry optimizations. Using the
best theory dipole moments, vibrational transitions, and UV-Vis data
could then be found and compared to thortical calculations.
Discussion
Each level of theory gave different
results but the 6-31G theory gave the best geometry optimizations. DZV
also gave good values for bond geometries and for some calculations was
better than the 6-31G theory. The dipole moments for the molecules found
using the theories was close to the theoretical values and were about
10% off the theoretical. For our non-polar molecule the DZV theory gave a
dipole measurement which could result from the large amount of basis
sets used in the calculations.
The vibrational frequencies were found very easily
using the calculations and give a good depiction of hoow molecules move
when they are excited. DZV gave good vibrational frequencies but the
values were not very close to the values found on NIST. Overall the
calculations done using the theories were helpful for visualizing
vibrational frequencies, and molecular orbitals of molecules. They were
not very close in values to the theoretical but they were close enough
that the calculations gave valid depictions of what should be observed.
References
1. Gutow, J. Molecular Orbital Calculations
2014, p 1-3
2. CRC Handbook of Chemistry and Physics; 68th edition; Weast, R; CRC Press, Inc.: Boca Raton 1987
3.
List of experimental data for ethylbenzene, bromofluoride, and ethylene 2015,
http://cccbdb.nist.gov/exp2.asp accessed Mar. 5, 2015