Quantum Calculations of Chlorine Fluoride, Carbon Dioxide, and 2,2'-fluoro biphenyl
Abstract
The optimal geometries, highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), electrostatic potentials, and partial atomic charges were determined for chlorine fluoride, carbon dioxide, and 2,2’-fluoro biphenyl through the use of molecular orbital theory. Calculations were completed from lower levels of theory such as AM1 and PM3, and also at higher levels of theory including 6-21G, 6-31G, and DZV. The geometric optimizations were best produced by the higher level theory calculations overall, with chlorine fluoride having the same bond lengths for the 6-21G, 6-31G, and DZV calculations. Carbon dioxide was the same with all three higher level calculations giving the same value, and the 2,2’-fluoro biphenyl bond lengths were best produced by the 6-21G calculation.
Introduction
Different properties of a molecule, such as bond length, atomic orbitals, electrostatic potential, and partial atomic charge, can all be predicted using multiple types and levels of calculations. This was done by using a molecule’s optimal geometry and most stable configuration to give the lowest energy. While atoms with only one electron have exact, known solutions, atoms with multiple electrons are more complicated and harder to solve, therefore the variational principle was used in this lab1. This can help approximate true wavefunctions using linear combinations of trial wavefunctions. These trial wavefunctions are not eigenfunctions, however, so the expectation value of the energy had to be calculated. The multiple levels of calculations produced different levels of accuracy with the calculations using larger basis sets of wavefunctions being the more accurate calculation. Using computer software, it has become much easier and faster to produce these calculations.
Calculating the expectation value can be done using different methods. The first being the semi-empirical quantum mechanical methods called AM1 and PM3. This used experimental data of the elements in the molecule for the parameterization. The next set of calculations, called Ab initio, produced a more accurate result and used the Hartree-Fock self-consistent field approximations. The levels of theory used in this lab are 6-21G, 6-31G, and DZV. These calculations are the ones using the larger basis set, and therefore took longer to complete.
Experimental
Multiple types of software were used in this lab to obtain the results. This started with using Avogadro2 to build the molecules and find the initial guesses of chlorine fluoride, carbon dioxide, and 2,2’-fluoro biphenyl optimal geometries produced as AM1.xyz and PM3.xyz files. MacMolPlt3 was used to find better geometry optimizations of each molecules, and then run through GamessQ4 to obtain an AM1.inp and PM3.inp file which in turn were run through GamessQ again as 6-21G.inp files to 6-31G.inp files, and finally to DZV.inp files. After the calculations were complete, Jmol5 was used to produce visuals of these results.
Conclusion:
The highest level of theory that produced the best results for FCl was DZV. For the other two molecules it seemed that the 6-21G calculation produced the best results. These types of calculation can be useful when looking for the information in this lab for a molecule with more than one atom because otherwise it would be too much to calculate by hand. This is especially helpful when one is looking at large compounds, such as the 2,2'fluoro biphenyl, since the number of calculations goes up exponentially high with larger and larger molecules.
To access these calculations, use the links provided: Chlorine Fluoride Carbon Dioxide 2,2'-fluoro biphenyl
References
Gutow, J. Quantum Calculations of Molecular Properties 2017, p 17-28.