Quantum Calculations for Diatomic, Polyatomic and Aromatic Ring Molecules Amanda Leichtfuss,
Jakob Frederick |
Abstract
Quantum calculations are a theoretical way to create
molecules. By using different programs, optimized geometries, bond lengths,
dipole moments, etc. can be calculated and a model of the molecule can be
displayed. In this experiment, quantum calculations were done on 3 molecule
types: the diatomic carbon monoxide (CO), the small polyatomic ethylene (C2H4),
and the aromatic compound benzaldehyde. By going through a series of steps
using Avogadro, MacMolPlt, Jmol, and GamessQ, a model of the molecules,
orbitals, bond lengths, vibrations, UV-Vis spectra, and dipole moments were
calculated.
Introduction1
The placement of electrons in a molecule determines the
reactivity of it based on the energy in each orbital. This placement of
electrons and energies could be able to predict many properties such as
vibrational frequencies, dipole moments, reactivity, polarizability, etc.
Wavefunctions are a calculation method to describe the placement of electrons.
It is not possible to calculate the eigenfunctions and eigenvalues for systems
larger than one electron but it is possible to approximate quantum
calculations. However, it is nearly impossible to do these calculations by
hand. By using the power of computers, these calculations can be done
efficiently and effectively.
Different levels of theory can be used in quantum
calculations. Each level represents different information used in the
calculations. The variational principle allows for approximate wavefunction
calculations by adjusting and varying different functions. There are two types
of levels used: semi-empirical and ab
initio. The semi-empirical method takes experimental data to simplify the
calculations. For the ab initio
calculations, no experimental data is used and the results are based on the
Schrodinger equation. Within the each theory, the basis set can be increased to
improve the calculations. For the semi-empirical theory, AM1 and PM3 basis sets
were used and for the ab initio
theory, 621-G, 631-G, and DZV basis sets increasing in size.
In this experiment, quantum calculations were done on carbon
monoxide, ethylene and benzaldehyde. Multiple software packages were used to
obtain the end results in the links below. Avogadro2 was used to
build a rough outline of the molecules and MacMolPlt3 was used to
create AM1 and PM3 input files for the GamessQ4 software to generate
optimized geometries the other various properties. Jmol5 was used to
view the models, generate the files for the website. Jmol was able to provide
3-D animated images of the molecules and the properties described above.
Carbon Monoxide |
Benzaldehyde |
Ethylene |
Conclusions
Quantum calculations are a great way to theoretically
calculate various properties of a molecule. With the processing power of
computers, the calculations are easier for scientists. However, the
calculations can’t be exact. Many of the calculated results vary from the
experimental results depending on the basis set. So even though the
calculations are a good starting place for determining the geometries and other
properties of the molecule, they are not exact.
Another point that is worth noting is the ease of working
through the calculations for a lower level scientist. Previously, theoretical
calculations were only done for upper level scientists but again, because of
computers and easy to use software, the process is more accessible to others.
References
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