by Tony F. Yang and Emilie Levenhagen
Oct. 10, 2019
Abstract:
The molecules used in this experiment were HBr, Di-t-Butyl
Peroxide,P-xylene.
The use of different levels of molecular orbital theory could
calculate
the
geometries, highest occupied molecular orbital, lowest
unoccupied molecular orbital, electrostatic potentials, dipole
moments, partial atomic charges and vibrational energies
of all the molecules in this experiment. The multiple
theories used were AM1, 6-21G. 6-31G, and DZV. The best
theories were found to be 6-31G for geometry, DVZ for
Vibrational frequencies. and AM1 for Dipole moment.
Introduction:
Electronically known structures can be used
to determine the properties of a molecule and its reactivity.
Predictions could be made about
molecular
dipole moment, polarizability, vibrational frequencies,
probability of absorption of visible light, and tendency for
molecules to donate electrons in a reaction. Knowing that the
lowest energy of a molecule is the most stable, the optimum
geometry was determined in this lab. By dividing the
expectation value of the Hamiltonian by the normalization
constant, the value of the energy could be calculated. The
integral calculations increase exponentially as the molecule
grows, therefore computers were needed to increase the
calculation speeds.
The different ways to calculate expectation
values of a molecule is through the use of theories. The level
of theories has three main differences in which are the time
it takes to finish, the number of wave functions, and how well
the energy is predicted. AM1 and PM3 are the fastest and most
simplest of the theories as it only has two basis sets. 6-21G,
6-31G, and DZV has more basis set and is listed in increasing
order.
The program Gamess was used to calculate
molecular orbitals and then transferred to a software called
Avogradro for calculations. The calculations guessed the
geometries of the molecules. Adjusting each calculations, all
the theories were created.
Use the following hyperlinks to see the calculated values: HBr, Di-t-Butyl Peroxide,P-xylene.
Conclusion:
Out of all the theories examined, 6-31G worked the best in relations to literature values. Considering the amount of time it took to compute, it is not recommended going past 6-31G. Dipole moments for the diatomic and aromatic worked great, but the peroxide didn't. For vibrational energies, DZV was used and in comparison to IR literature values it worked well. DZV was expected to be the best calculated value due to its high basis set, but it ended up not being true for all three molecules. Noticing that DZV was not best for all value, it is better to consider doing a set of theories versus only the highest basis set.References:
1.
“All Data (Experiment and Calculated) in the
CCCBDB for One Species.” CCCBDB All Data for One Molecule, https://cccbdb.nist.gov/alldata1x.asp.
2.
“Di-Tert-Butyl Peroxide.” Di-Tert-Butyl Peroxide, National Institute of Standards and
Technology,
https://webbook.nist.gov/cgi/cbook.cgi?ID=C110054&Type=IR-SPEC&Index=1.
3.
“IR Spectrum Table & Chart.” Sigma,
https://www.sigmaaldrich.com/technical-documents/articles/biology/ir-spectrum-table.html.
4.
Oberhammer, Heinz. “Gas Phase Structures of
Peroxides: Experiments and Computational Problems.” ChemPhysChem, vol. 16, no. 2, Apr. 2014, pp.
282–290., doi:10.1002/cphc.201402700.