chemsample-infraredspectroscopy-introduction

Introduction to **Infrared Radiation** (IR) ~

On the electromagnetic spectrum, IR is between the visible range and microwave region ~ specifically between 4000 and 400 cm-1.


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Organic chemists use IR spectra to determine the structure of unknown compounds by comparing them to reference compounds. Here is an example of these spectrum ---> [|mj071203_pg2_fig5.jpg.] This graph is a plot of the percentage of IR radiation that passes through a sample versus the frequency of the lightwave. The wavenumber of the radiation is measured in wavelengths per centimeter and is equal to the frequency. The radiation that does pass through the molecule is measured as a percent transmittance. Because molecules have very complex IR spectra due to functional groups, comparing the peaks created by the radiation passing through the molecule can be very effective in determining an unknown. The absorption bands for different functional groups are listed on tables, such as [|these].

Using this radiation really has a lot to do with vibrations. At certain energies, the different types of bonds in a molecule have distinct, natural vibrations. When the frequency of the radiation is exactly that of the bonds energy then molecule absorbs that packet of energy. This produces the peaks shown in the spectra. The intensity of these peaks is directly related to the strength of the dipole moments between two molecules. Atoms that have a greater separation of charge will have bands that show up more intensely on the spectra.

The IR spectrum is generally divided up into two parts. Bands that appear between 1500 and 600 cm-1 are in what is called the fingerprint region. This is a much more complex area than further down the spectrum. It is called a fingerprint because it is unique for each molecule. There is usually a lot of activity in this region because it reports what is happening in the molecule as a whole. Above 1500 cm-1 is used primarily for identifying functional groups. One very distinct band is that which appears for -OH groups. This will be a very broad band that spans between 3500 and 3100 cm-1.

Currently there are two different methods for measuring IR spectra. One is a dispersive double-beam spectrophotometer. This method splits light into small, individual beams using prisms, mirrors and a monochromator. Fourier transformed spectrophotometer does not use a monochromator and only splits the light into two beams. A more in depth look at this processes can be explored [|here].

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