Quantum Calculations on Hydrogen Fluoride, Difluoroamine, and p-Xylene

Introduction
The simple theories of bonding that we learn in General Chemistry are powerful and useful. These theories, which include Lewis structures, VSEPR, and hybridization, are simple models that help predict chemical properties. We should verify the usefulness of our simple predictions with molecular orbital (MO) theory. If the theoretical calculations are done carefully, we can learn a lot about chemical structure by comparing our Lewis structures and hybridization with the molecular orbitals.

The calculations in this include geometry (bond lengths, angles), energy (molecular orbital energies), vibrational frequencies, UV-Vis spectra, atomic charge distribution,  electrostatic potential, and dipole moment. Molecular orbital theory is based on approximations also. These calculations are done with some of the best available calculation methods (Molecular Mechanics and Ab initio). The potential energy versus bond length for each level of theory for hydrogen fluoride was generated to show that the bigger the basis set the better the geometry. A UV-Vis spectrum for p-xylene was used as a comparison for calculations of the transitions energies of the two highest levels of theory for NHF2.

Experimental1
The software wxMacMolPlt was used to construct the following molecules: hydrogen fluoride, difluoroamine, and p-Xylene. Molecular mechanics optimization on the structure was performed in Jmol and the new coordinate results was saved as .xyz files. Then wxMacMolPlt was used to generate AM1 and PM3 geometry optimization input files (.inp) for the GAMESS computation package. If the calculation worked, the results in each molecule's were saved as .log files. The ab initio molecular calculations were carried out using GamessQ and the Gaussian type basis sets were 6-21G, 6-31G, DZV (double zeta valence).

Conclusion
 As mentioned in lecture, MO theory is more fundalmentally correct than VSEPR or hybridization theory and it can often allow for accurate chemical properties prediction.  Although computational results usually have systematic errors, but the results were used to predict the structure and bonding of molecules.  Generally, the larger the basis set the more accurate the calculation and the more computer time that is required.

Click on the images below to view results for the model and calculations of hydrogen fluoride, difluoroamine, p-Xylene.
Hydrogen Fluoride (HF)
 Difluoroamine (NHF2)
 p-Xylene
HF
 NHF2
 xylene


Referenes
1. Mihalick. J; Gutow. J. Molecular Oribital Calculations. Uw Oshkosh, 2011.