Question

11. What characteristics of a molecule make it likely to absorb UV light in an analytically useful region of the UV spectrum (>185 nm)?

12. Why is it not typical to analyze molecules with absorbance spectrophotometry if they do only absorb light <185 nm.

Answer 1

Hi,

11.

When we were talking about the various sorts of orbitals present in organic compounds on the introductory page (see above), you will have come across this diagram showing their relative energies:

Remember that the diagram isn't intended to be to scale - it just shows the relative placing of the different orbitals. When light passes through the compound, energy from the light is used to promote an electron from a bonding or non-bonding orbital into one of the empty anti-bonding orbitals. The possible electron jumps that light might cause are:

In each possible case, an electron is excited from a full orbital into an empty anti-bonding orbital. Each jump takes energy from the light, and a big jump obviously needs more energy than a small one.

Each wavelength of light has a particular energy associated with it. If that particular amount of energy is just right for making one of these energy jumps, then that wavelength will be absorbed - its energy will have been used in promoting an electron.

We need to work out what the relationship is between the energy gap and the wavelength absorbed. Does, for example, a bigger energy gap mean that light of a lower wavelength will be absorbed - or what? It is easier to start with the relationship between the frequency of light absorbed and its energy:

You can see that if you want a high energy jump, you will have to absorb light of a higher frequency. The greater the frequency, the greater the energy. That's easy - but unfortunately UV-visible absorption spectra are always given using wavelengths of light rather than frequency. That means that you need to know the relationship between wavelength and frequency.

You can see from this that the higher the frequency is, the lower the wavelength is. So . . . If you have a bigger energy jump, you will absorb light with a higher frequency - which is the same as saying that you will absorb light with a lower wavelength.

Important summary: The larger the energy jump, the lower the wavelength of the light absorbed.

12. Experiments requiring measurements below 200 nm are generally performed in an instrument purged with nitrogen; measurements below 185 nm generally require instruments in which the sample compartment and optical path are located in a vacuum chamber.

Also, the backgroumd absorption by solvent used in UV will be high , hence measurement below 185 nm is not feasible