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    Elemental chlorine is composed of two chlorine atoms.  This diatomic molecule has the highest electron affinity and is considered one of the most electronegative elements.  Given its linear structure, chlorine is a non-polar molecule as shown below by the following Lewis structure:

    Although the above structure depicts the correct bonding structure of the molecule, it is not an accurate depiction according to the molecular orbital (MO) theory.  To better understand the nature of the bonding and anti-bonding molecular orbitals, three levels of theory were performed on an initial guess of the molecular structure.  These three levels (Molecular Mechanics, MOPAC3, and Ab initio) served as the predication of the quantum mechanical system for nitrobenzene.

    The optimized geometry and double zeta valence (DZV) basis set were used to obtain the highest occupied molecular orbital (HOMO) for the molecule.  The blue and red surfaces above and below each sulfur atom represent electron occupied pi-orbitals.  These differences in color represent the difference in phase for each orbital. 

    The figure represents the stage when the valence electrons are in the highest energy state.  Similar to the figure of the HOMO, the lowest unoccupied molecular orbital (LUMO) is shown using the optimized geometry and DZV basis set.  Molecular orbitals are shown surrounding all atoms in the molecule, which is consistent with the molecular orbital (MO) theory, where the LUMO is the lowest energy state of unoccupied orbitals.  According to the MO theory, delocalized electrons become excited and transition from the HOMO to the LUMO.  This transition energy can be approximated using the ab initio level of theory.

By clicking on the button below, a figure of the molecule can be viewed that shows a visual representation of the lowest unoccupied molecular orbital (LUMO).

    As shown in the figure, the shape of the electrostatic potential field is more elevated surround the chlorine atoms and more depressed around the single bond.  However, the color of the field is uniform throughout, which indicates a constant potential energy to charge distribution pattern.

    The optimized geometry of the structure is in agreement with the experimental value of bond length, 1.988 Å.  Additionally, the structure is linear as previously demonstrated by its Lewis structure.

1.     Mihalick, J. Gutow, J. Quantum Calculations 2016, p 1-12.  

2.     Chlorine. Chlorine, http://webbook.nist.gov/cgi/cbook.cgi?id=7782-50-5 (accessed Mar 3, 2016).

3.     Carbon disulfide. Carbon disulfide, http://webbook.nist.gov/cgi/cbook.cgi?id=75-15-0 (accessed Mar 2, 2016).

4.     Benzene, nitro-. Benzene, nitro-, http://webbook.nist.gov/cgi/cbook.cgi?id=c98953 (accessed Mar 3, 2016).

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