Molecular Orbital Calculations for ClBr, NO₂, and Phenylacetylene (C₈H₆)

By: Laura Hagen and Kosy Phimmasene

Abstract

Using different software, three different molecules were created, calculated, and re-calculated to attempt to create accurate presentations of the three molecules chlorobromide, nitrogen dioxide, and phenylacetylene. After comparing different calculations for the best fit molecules, another software was used to create molecular orbitals, electrostatic potential, partial atomic charges, and different vibrations.

Introduction

Molecular orbital (MO) theory is a method for writing electronic wavefunctions of diatomic molecules where the distribution of a single electron can be defined over the entire molecule.¹ Using MO allows for different properties of molecules to be predicted with very good accuracy. The best approximations of wavefunctions occur when the variational principle is applied. The principle states that the wavefunction with the lowest energy is the best approximation for the actual wavefunction.² The caveat with using this theory is that it can be done relatively easily within the domain of a single atom with one electron; however, as more atoms and more electrons are introduced, it becomes vastly more complicated.

 

The experiment performed utilized a number of software programs performing calculations needed to obtain the MO information for three molecules. MO calculations were performed on Chlorobromide (ClBr), Nitrogen Dioxide (NO₂), and Phenylacetylene (C₈H₆). The software programs used for calculations and web design were Avogradro, MacMolPlt, GAMESS(Q), Igor, Jmol, and Seamonkey.

 

Avogadro was the software for drawing the molecules and performing simple molecular calculations like bond length and bond angles. MacMolPlt performed geometry optimization using MOPAC, which is a semi-empirical method for calculating the geometries of molecules. The semi-empirical method allowed for the use of a number of Hamiltonians to optimize the geometry, with AM1 and PM3 being used throughout the initial calculations. The best level of MO theory utilized was Ab initio where the basis sets for the wavefunction calculations involve all integral calculations.³ The Ab initio basis sets used were: 3-21G, 6-21G, 6-31G, and DZV (double zeta valence). GAMESS was the software that ran quantum mechanical calculations for all levels of theory and GAMESSQ allowed for queuing up the necessary calculations for GAMESS to perform.

Igor was the statistical software used to plot potential energy versus bond length for the diatomic molecule. Lastly, Jmol and Seamonkey were used to create and edit the webpage.

Links to Molecules

Conclusion
The molecular orbitals calculated in the experiment was done through two semi-empirical methods using MOPAC and these were, AM1 and PM3. The best results used the Ab initio theories of 6-21G, 6-31G, and DZV. One of the calculations used for ClBr was 3-21G. Overall the calculations for all three molecules were very close to the experimental values found from the NIST website and other open sources. Most of our results had the best calculated values using DZV theory except for the ClBr which used 6-31G.

References

1. Cooksy, Andrew. Quantum Chemistry and Molecular Interactions; Pearson: New Jersey, 2014; pp 214.

 

2. Gutow, Jonathon. Molecular Orbital (MO) Calculations; laboratory manual: University of Wisconsin-Oshkosh, revised Feb 2015.

 

3. Gutow, Jonathon. Molecular Orbital (MO) Calculations; laboratory manual: University of Wisconsin-Oshkosh, revised Feb 2015.