Molecular Orbital Calculations Lab
By: Chris Aretz and Becky Kauer

    The reactivity of a molecule is determined mostly by its electron structure. Knowing the locations of the electrons and the energies they have could lead to describing most of its properties such as bond lengths, dipole moments, vibrational frequencies, uv-vis spectrum, and molecular orbital geometries.  The calculations can be done easily for a one atom system with only one electron, however it gets more complicated with multi-atom sytems with more than one electron. To do these calculations various computer programs are needed, such as MacMolPlt, GamessQ, and Jmol. Several approximations were done by these programs, such as Huckel approximation and the variational principle. The Huckel approximation is done using determinates to solve for the enegry; assuming all beta for non-neighbors are zero, all other betas are the same, that S = 0, and all alphas are equal. This gives as many energies as there are solutions for the determinates. The variational principle sets the wavefunction psi equal to a sum of constant times psi 1, constant times psi 2... This allows the calculation of the wavefunction. Five different basis sets were used, they are AM1, PM3, 3-21G, 6-31G, and DZV. The 3-21G, 6-31G, and DZV are ab initio calculations, which give closer results to the accepted values. Some of these calulations took a long time to do and were left to calculate over a few days. These calculations were done on three molecules, a diatomic, a small polyatomic, and an aromatic compound. The diatomic was flourine, F2. The small polyatomic molecule was hydrogen sulfide, H2S. The aromatic compound was styrene, C8H8.

F2 Data
H2S Data
Styrene Data

    From looking at the data for F2, Table 1, the computational results are reasonable for the dipole moments at the PM3 and DZV theory levels. In addition, the computational results are reasonable for looking at the different molecular orbitals for molecules. However, due to the approximations made when computing the computational results are close to but not equal to the accepted values at each level of theory. The computational results are also useful if wanting to know approximate values for bond lengths, dipole moments, vibrational frequencies, uv-vis spectrum, and molecular orbital geometries without having to do additional experiments to acquire more accurate results. They are not useful if wanting to know exact values of results that can be obtained through other methods, such as the uv-vis spectrum, and vibrational frequencies.

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(2) NIST Webbook; NIST Standard Refrence Database Number 69 2008 accessed Mar. 10, 2011
(3) Spectral Database for Organic Compoundsd SDBS; SDBS Compounds and Spectral Search accessed Mar. 10, 2011
(4) Nationsl Instite of Standards and Technology; CCC and Benchmark Database 2010 accessed Mar. 10, 2011
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