Animating the vibrations

Now we are ready to look at the output from the frequency, or vibrational analysis. We will be animating the vibrations and looking at the computed IR spectrum.

• if your frequency job is not open, open it now
• From the main menu along the top of the screen (in gaussview) choose "Results" and then choose "Vibrations":

  

• a new window will open called "Display Vibrations":

  

• arrange your windows so that you can see both the molecule window and the vibration window
• in the molecule window rotate your molecule so it is not completely in the plane of the screen
• in the vibration window highlight the top vibration, then check the "Show Displacement Vectors" box, and finally click on the "Start" button, the molecule should start vibrating!
• look at all the vibrations by highlighting them one after another down the list. Note which vibrations look like bond stretches and which look like angle deformations.
• If you have any vibrations with a negative number under the frequency heading this means your molecule is not fully optimised, go back and repeat the frequency analysis making sure that you take the log file from the optimization. If that doesn't work repeat your optimization, and then frequency analysis. If you are still unsuccessful get a demonstrator to help you determine what you have done wrong.
• you should have 6 vibrations, with frequencies similar (but not necessarily identical) to my example
  • notice that some of the vibrations have the same wavenumber and intensity, these vibrations are degenerate (remember that degenerate means "the same energy" and wavenumber is proportional to energy)
  • degenerate vibrations give rise to a single peak in the spectrum (because they are the same energy)
  • there are two sets of degenerate vibrations, one is a bond stretch and one is a bond angle deformation. From animating the vibrations we can see that the bond stretch is, in this case, higher in energy than the angle deformation.
• the number in the IR column identifies the intensity of each vibration, notice which vibrations have a very low intensity, these vibrations will not be experimentally observable.
• in the vibration window click on the button "spectrum", and enlarge the window that opens, this is the computed IR spectrum for your molecule

Update your wiki

• Link to your completed 6-31G(d,p) BH₃ frequency file on your wiki
• Include a table listing the vibrations and their intensity in your wiki. Careful with the accuracy of your numbers, check the accuracy page! Indicate in your table if a vibration is IR active or not and if it is a stretch or bend.
• Take a snap-shot of the spectrum and include it in your wiki.
• In your wiki explain why are there less than six peaks in the spectrum, when there are obviously six vibrations.
• Here is an example of my progressing wiki page.

When you are ready close the molecule window move onto the next step