Qualitative Application of Molecular Orbital Theory

Orbitals are occupied by electrons, beginning with the orbital of lowest energy and filling each orbital with a maximum of two electrons (the Aufbau principle). The number of electrons is determined by the number of electrons present on the interacting atoms. The orbitals in Fig. 1.9 could be applied to systems such as H2 (one electron), H2 (two electrons), He2 (three electrons), or He2 (four electrons). A reasonable conclusion would be that H2 would be the most stable of these diatomic species because it has the largest net number of electrons in the bonding orbital 

The second-row elements including carbon, oxygen and nitrogen involve p atomic orbitals as well as 2s orbitals. An example of a heteronuclear diatomic molecule involving these elements is carbon monoxide, C=0. The carbon monoxide molecule has 14 electrons, and the orbitals for each atom are Is, 2s, 2p, and 

SECTION 1.4. QUALITATIVE APPLICATION OF MOLECULAR ORBITAL THEORY 

Just as we were able to state some guiding rules for qualitative application of resonance theory, it is possible to state some conditions by which to test the correctness of an MO energy level diagram derived by qualitative considerations. 

By applying these rules and recognizing the elements of symmetry present in the molecule, it is possible to construct molecular orbital diagrams for more complex molecules. In the succeeding paragraphs, the MO diagrams of methane and ethylene are constructed from these kinds of considerations. 

The total number of MOs must equal the number of AOs from which they were constructed. 

The symmetry of the MOs must conform to the symmetry of the molecule. That is, if a molecule possesses a plane of symmetry, for example, all the MOs must be either symmetric (unchanged) or antisymmetric (unchanged except for sign) with respect to that plane. 

As with valence bond theory, the full mathematical treatment of MO theory is too elaborate to apply to all situations and it is important to develop from the fundamental ideas of molecular orbital theory and the detailed calculations on specific systems, ideas which can be applied without the need for detailed calculation. 

A slight adjustment allows the energy level diagram to be applied to heteronuc-lear diatomic species such as HHe . Rather than being a symmetrical diagram, the He Is level is lower than the H Is level due to the increased nuclear charge on 

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One of the key assumptions of the Huckel approximation is the noninteraction of the TT-orbital system with the cr-molecular framework. This, as was mentioned, is a good approximation for completely planar molecules where the a framework is in the nodal plane of the tt system. For other molecules, as for example when an sp carbon is added as a substituent group, this approximation is no longer entirely valid. Qualitative application of molecular orbital theory can be enlightening in describing interactions between the tt system and substituent groups. In valence... 

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