Molecular Orbital Diagrams

But most people will expect you to know that the s-\pi^* MO will always lie above the s-\sigma MO and that the s-\sigma MO will always lie above the s-\sigma^* MO. In summary, determining the position of pi and sigma orbitals on a molecular orbital diagram can be complex and there is no clear way to make sense of their positions. However, a simple rule of thumb is that the 1pi orbital will typically be above the 3sigma orbital, unless the 2pi* orbital is unoccupied or singly occupied. In most cases, the relative positions of these orbitals may not be important, but it is generally understood that the s-pi* orbital will always be above the s
  • #1
bubbles
97
0
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.
 
Last edited by a moderator:
Physics news on Phys.org
  • #2
bubbles said:
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 [itex]1\pi[/itex] and [itex]3\sigma[/itex] (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 [itex]1\pi[/itex] should lie above or below [itex]3\sigma[/itex]? 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 [itex]2\pi ^* [/itex] 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.
 
Last edited by a moderator:
  • #3


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.
 

FAQ: Molecular Orbital Diagrams

1. What are molecular orbital diagrams?

Molecular orbital diagrams are graphical representations of the distribution of electrons in a molecule. They show the energy levels and orbitals that the electrons occupy in a molecule.

2. How are molecular orbital diagrams different from Lewis structures?

Molecular orbital diagrams take into account the energy levels and orbitals of the constituent atoms to show the distribution of electrons in a molecule. Lewis structures only show the bonds and lone pairs of electrons in a molecule.

3. How do you interpret a molecular orbital diagram?

In a molecular orbital diagram, the lower energy levels are filled first with electrons according to the Aufbau principle. The bonding orbitals are filled with electrons, while the anti-bonding orbitals remain empty. The total number of electrons in the bonding orbitals should equal the number of electrons in the constituent atoms.

4. What is the significance of molecular orbital diagrams?

Molecular orbital diagrams provide a visual representation of the stability and reactivity of a molecule. They can also predict the magnetic and electronic properties of a molecule.

5. How are molecular orbital diagrams useful in chemical bonding?

Molecular orbital diagrams show the overlap of atomic orbitals to form bonding and anti-bonding orbitals. This helps to explain the strength of chemical bonds and the stability of molecules.

Similar threads

Back
Top