Molecular Vibrations – a simple tutorial 3

Molecular Vibrations – a simple tutorial

Assuming the ratio of 35Cl:37Cl is exactly 3:1 (it’s not quite!), the ratio of each form will be

What do I mean by redundancy? The rogue atom in the second and third columns can lie in each of the three positions – thus the extra times three. [At Southampton University I found most students come up with ratios 27:9:3:1].

If you look at Figure II the band system near 365cm^-1 fits quite well so we assign this band to the umbrella mode V₅.

So -there we are – we can explain convincingly, the Raman spectrum of chloroform

Does our explanation also work for the infrared? Unless special equipment is available, infrared spectra are recorded from 4000 to 400cm^-1, so the lowest two bands (the ones that caused us the most trouble) don’t appear in the i.r. In Figure III, I show the infrared spectrum in transmission from a thin liquid film.

Figure 3.

As we have done before, lets start at high frequencies and work down.

The CH stretch at 3019cm^-1 appears loud and clear in the infrared as it would because the dipole in the molecule must change as the vibration occurs. The band at 1216 – very clear in the infrared. In fact, it’s relatively stronger than it is in the Raman.

Turning now to the C-Cl stretching modes – the asymmetric is MUCH more intense than it is in the Raman and vice versa for the symmetric mode exactly as one expects symmetric R, asymmetric IR. In the table below, we collect all the assignments together. As you can see – a vibration strong in the infrared is weak in the Raman and vice versa. This is a normal state of affairs. The modes below 400cm-1 – another reason for buying a Raman accessory for your FTIR!

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