8303157-054-Infrared-Spectro - Chem Factsheet April 2003...

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Note the axis labels: wavenumber (cm -1 ) (x-axis) transmittance (a percentage) (y-axis) The wavenumber is related to the wavelength of the IR absorbed: Wavenumber = 1 Wavelength The transmittance shows the percentage of the IR that passes through the sample cell - so the lower it is, the more is absorbed. Here is a data table showing some different IR absorption ranges of different bonds: Bond Wavenumber Range (cm -1 ) C-O 1000 - 1300 C=O 1650 - 1750 C-H (alkanes) 2850 - 3000 C-H (alkenes) 3000 - 3100 O-H 2500 - 3500 N-H 3300 - 3500 Such data was compiled by comparing the IR spectra of related compounds. Note that bonds (such as C-H) will absorb slightly different wavelengths depending on the surrounding atoms. This means that it is impossible to assign a precise wavenumber to a bond. Nevertheless, we can identify bands or ranges corresponding to specific bonds, as shown in the data table. Before working through this Factsheet you should: Have a good knowledge of the organic chemistry covered at AS and A2 level. Understand covalent bonding. Be able to recognise functional groups within organic structures. After working through this Factsheet you will: Be able to interpret simple Infrared spectra. Have a basic understanding of how an infrared spectrometer works. The A2 syllabus requires candidates to be able to interpret simple infrared spectra. At first glance this can appear to be a daunting task - the spectra themselves look complex, and background reading about how the spectra are produced can sometimes be confusing. Do not fear infrared spectra - they are a tool to help chemists determine the structure of organic chemicals. With a little experience and use of any data supplied in an exam paper, scoring marks on questions involving IR spectra is not as difficult as it may seem.
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This note was uploaded on 03/08/2011 for the course CHEM 101 taught by Professor Hard during the Spring '11 term at UT Arlington.

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8303157-054-Infrared-Spectro - Chem Factsheet April 2003...

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