Molecular Orbital Diagrams

[bubbles]

In a molecular orbital diagram that shows the bonding of two electrons (like a diatomic molecule), there are sigma and pi bonds, and the sigma and pi antibonds. Here is an example: http://www.westga.edu/~chem/courses/chem410/410_09/sld024.htm

I don't understand how to determine where to put the pi and sigma orbitals on the diagram. The order of the different molecular orbitals (the bonding pi and sigma orbitals) are different for different atoms and elements. Is there any way to know the their order other than memorizing them without making sense of them?

Thanks in advance.

 

[Gokul43201]

In a molecular orbital diagram that shows the bonding of two electrons (like a diatomic molecule), there are sigma and pi bonds, and the sigma and pi antibonds. Here is an example: http://www.westga.edu/~chem/courses/chem410/410_09/sld024.htm

I don't understand how to determine where to put the pi and sigma orbitals on the diagram. The order of the different molecular orbitals (the bonding pi and sigma orbitals) are different for different atoms and elements. Is there any way to know the their order other than memorizing them without making sense of them?

The simple answer is "No - there is no easy way to make sense of the positions. Any theoretical determination of their values is very complex". But that's not the complete answer.

First of all notice that there's an ambiguity almost only when it comes to the relative positions of $1\pi$ and $3\sigma$ (with molecules involving s- and/or p-block elements). The other MOs have pretty well-defined positions that mostly do not change qualitatively relative to each other.

So the question filters down to (if I haven't misunderstood you): how do I know if $1\pi$ should lie above or below $3\sigma$? To this end I suggest a simple rule of thumb that will work in the majority of cases. [tex]E_{1\pi} > E_{3\sigma}[/itex] is $2\pi ^*$ is unoccupied or singly occupied; else (if doubly or more occupied) it is the other way round.

Let me also add that in most cases (though not at a grad level spectroscopy course), the instructor will not terribly care about the relative positions of these particular MOs, unless they've specifically lectured about it in class.

 

[Blueshift5]

Molecular orbital diagrams are an important tool in understanding the bonding and properties of molecules. The placement of pi and sigma orbitals on these diagrams is determined by the symmetry and energy levels of the atomic orbitals involved in the bonding process. This can be a complex concept to grasp, but there are a few general rules that can help guide the placement of these orbitals on the diagram.

Firstly, sigma orbitals are typically lower in energy than pi orbitals. This means that they will be closer to the nucleus and have a greater contribution to the overall bonding in the molecule. On a molecular orbital diagram, sigma orbitals are typically shown as a single line connecting the two atomic nuclei.

Pi orbitals, on the other hand, are higher in energy and are typically shown as two lines above and below the sigma orbital. These orbitals have a more complex shape and are formed by the overlap of two atomic orbitals that are perpendicular to the bond axis.

The specific order of the pi and sigma orbitals on the molecular orbital diagram depends on the atoms involved in the bonding process. However, there are some general trends that can be observed. For example, in diatomic molecules, the sigma bonding orbital is typically the lowest in energy, followed by the pi bonding and then the pi antibonding orbitals.

To determine the specific order for a particular molecule, it is important to consider the atomic orbitals involved and their relative energies. This can be done by looking at the electronic configurations and relative positions of the atoms in the periodic table. However, it is also important to note that this order may change in more complex molecules with multiple bonds and different atomic orbitals involved.

In summary, the placement of pi and sigma orbitals on a molecular orbital diagram is determined by the symmetry and energy levels of the atomic orbitals involved in the bonding process. While there are some general trends, the specific order may vary for different molecules and must be determined by considering the electronic configurations and relative positions of the atoms involved.