Introduction

    Many important properties of molecules can now be determined through several computer calculations, instead of by hand, allowing one to use large basis sets.  However, these calculations are not always correct and one must be skeptical of computer output.  The electonic structure of a molecule determines much of its reactivity using the probable locations of the electrons and their energies.  The wave functions for electrons in molecules are known as molecular orbitals.  Real wave functions always have lower energy than any approximation.  The best geometry is found by trying different arrangements of atoms until the energy of the system is minimized.  The larger the basis set, and the more accurate the molecular geometry, the more accurate the energy prediction will be.  
     The three molecules looked at during this experiment were hydrogen floride (HF), carbon dioxide (CO₂), and Styrene.  The geometries (bond lengths and angles), primary molecular orbitals (HOMO), dipole moments, and vibrational frequencies/modess were all evaluated using the program MacMolPlt.  Three different level of ab initio theory (3-21G, 6-31G, and 6-311G) were compared for consistency of predictions.  The best  ab initio results were used to calculate the vibrational frequencies of each molecule.  Potential energy plots were generated for our diatomic molecules using the best ab initio results as a starting point, and UVvis calculations were performed for our aromatic molecule.  The results obtained were compaired to the experimental ones found using the NIST website (National Institute of Standards and Technology).

Data/Results

    Follow the links below to view the data and results for each molecule.

    HF calculations
    CO₂ calculations
    Styrene calculations

Conclusion

    In conclusion, it has been determined that the computer calculated values and properties for HF, CO₂, and Styrene are both precise and accutate compared to the experimental values found on the NIST website.  It should be true that the best ab initio results (for the highest level of theory) be the closest in value to the experimental values because it should have a more accurate molecular geometry and therefore, a more accurate energy prediction.  However, this not always the case because there are approximations made in the programs used, and they are not always 100% accurate.  Some of the calculations do not always get ran completely and some even fail.  It was also seen that the three different level of theory were very consistent with each other, indicating that a good prediction was made.  The computational results found are more useful when you are analyzing a smaller molecule because less approximation are needed when performing the calculations, and they take less time to run.  

References

Lab Handout. Molecular Orbital/Quantum Calculations 2. J. Gutow 2005; revised 2009.

Atkins and De Paula. Physical Chemistry.8^th Edition. W.H. Freeman and Company. New York, 2006. Ch. 11 & 18.

Computational Chemistry Comparison and Benchmark Data Base.  NIST.  <http://cccbdb.nist.gov/>.

NIST Chemistry WebBook.  NIST.  U.S. Secretary of Commerce. 2008.  <http://webbook.nist.gov/chemistry/>.