Help me visualize electronic vibrations

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Let's consider the simple functional group C=O (carbon double bonded to oxygen). There is only one type of vibration for this group and that's the stretching mode, where C<=>O oscillates back and forth.

What exactly is happening during the stretching? Are electrons, and therefore electron density, shifting from side to side, going from the carbon atom to the oxygen atom?

Also, I don't think I'm grasping the idea of an electronic vibrational state. Could someone help me understand this in terms of what I stated above?

Thanks.
 

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  • #2
chemisttree
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Let's consider the simple functional group C=O (carbon double bonded to oxygen). There is only one type of vibration for this group and that's the stretching mode, where C<=>O oscillates back and forth.

What exactly is happening during the stretching? Are electrons, and therefore electron density, shifting from side to side, going from the carbon atom to the oxygen atom?

Also, I don't think I'm grasping the idea of an electronic vibrational state. Could someone help me understand this in terms of what I stated above?

Thanks.
During a stretching vibration, the electrons will find themselves closer to the more electronegative atom. The bond will become polarized during this stretch/vibration. The polarizability is a requirement for infrared absorption (vibration). The electronic vibrational state is simply the quantized vibrational energy of the molecule. Symmetric stretch, assymetric stretch, wagging, rocking, breathing (for ring systems) are all adjectives that are used to describe these states. They all have unique energies which can be displayed as a spectrum. If you were looking at the gross energy state of a molecule and drew a diagram representing a particular state, E(nergy) along the Y axis with horizontal lines representing some molecular electronic state, the vibrational states could be represented as a fine structure superimposed on a particular state. Even smaller fine structure can be diagrammed as a nearly continuous spin or rotational energy (microwave) state on top of each of the energy states possible.
 
  • #3
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The rule for IR absorption is that the vibration result in a change of dipole moment.
You might want to do some reading about "normal modes of vibration" for further info. But, briefly, for molecules with more than 2 atoms, there are 3N-6 internal degrees of freedom. (The six missing are translation and rotation of or about the center of mass.) which the normal modes describe.
-Jim
 
  • #4
Stretching is due to the change in the dipole moment, which can be expressed using calc, but is easier to understand if drawn. Basically, there is a dipole center of the undisturbed molecule:
C=|=O
Place in an IR machine, and the location of the center will change due to the atoms <--- -> trying to go in different directions, with different magnitudes. So you get:
C===|=O
 
  • #5
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Stretching is due to the change in the dipole moment, ...
This reverses cause and effect. The change in dipole moment is caused by an IR allowed vibration. If a different atomic charge is associated with two atoms, an increase in the distance between them results in an increase of dipole moment.
Incidentally, the thread title is a bit garbled. Unless you are doing something outside of standard usage, molecular vibrations are taken to mean movement along the 3N-6 internal degrees of freedom of the nuclei and associated electrons - the atoms. "Electronic vibration" implies something about the movement of the electrons.
-Jim
 

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