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.

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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5.     The Jmol Development Team. http://www.jmol.org, (accessed February 2016.)

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