The grey dotted arrows show jumps which absorb light outside the region of the spectrum we are working in. This time, the important jumps are shown in black, and a less important one in grey. It isn't necessary to know about this in order to understand the rest of this page. Note: If you are interested, there is a description of how a double beam absorption spectrometer works on another page in this section. Only a limited number of the possible electron jumps absorb light in that region. Some jumps are more important than others for absorption spectrometryĪn absorption spectrometer works in a range from about 200 nm (in the near ultra-violet) to about 800 nm (in the very near infra-red). If you choose to follow this link, use the BACK button on your browser to return to this page. If you need more help with sorting out these relationships, you will find them discussed more slowly on a page in this section about electromagnetic radiation. However you do it, you must be confident about this relationship. Note: It is obviously better if you can work this out in case you forget it, but you may feel that it is a lot easier just to learn the last statement. The larger the energy jump, the lower the wavelength of the light absorbed. 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. You can see from this that the higher the frequency is, the lower the wavelength is.
That means that you need to know the relationship between wavelength and frequency. That's easy - but unfortunately UV-visible absorption spectra are always given using wavelengths of light rather than frequency. The greater the frequency, the greater the energy. You can see that if you want a high energy jump, you will have to absorb light of a higher frequency. It is easier to start with the relationship between the frequency of light absorbed and its energy: Does, for example, a bigger energy gap mean that light of a lower wavelength will be absorbed - or what? We need to work out what the relationship is between the energy gap and the wavelength absorbed. 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. Each jump takes energy from the light, and a big jump obviously needs more energy than a small one.Įach wavelength of light has a particular energy associated with it.
#Chem draw uv spectra full#
In each possible case, an electron is excited from a full orbital into an empty anti-bonding orbital. The possible electron jumps that light might cause are:
Important: If you don't know exactly what I mean by bonding, non-bonding and anti-bonding orbitals, or don't remember the diagram, go back and read the introductory page again.
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. Remember that the diagram isn't intended to be to scale - it just shows the relative placing of the different orbitals. 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:
What happens when light is absorbed by molecules? You need to read this before you go on with this page. Important: If you have come direct to this page from a search engine, you should be aware that it follows on from an introductory page explaining some essential bonding for UV-visible spectrometry. This page explains what happens when organic compounds absorb UV or visible light, and why the wavelength of light absorbed varies from compound to compound.