![]() 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). Important summary: The larger the energy jump, the lower the wavelength of the light absorbed. 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. 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. 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: 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 wavelength of light has a particular energy associated with it. Each jump takes energy from the light, and a big jump obviously needs more energy than a small one. 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: 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?
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