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What Does an IR Spectrum Look Like?

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    A spectrum is a graph in which the amount of light absorbed is plotted on the y-axis and frequency is plotted on the x-axis. An example is shown below. You can run your finger along the graph and see whether any light of a particular frequency is absorbed; if so, you will see a "peak" at that frequency. If not, you will see "the baseline" at that frequency.


    Figure IR1. IR spectrum of benzene. The x-axis labels are, from right to left, 500, 1000, 1500, 2000, 3000 and 4000 cm-1.

    Source: SDBSWeb : (National Institute of Advanced Industrial Science and Technology of Japan, 14 July 2008)

    In IR spectra (spectra = plural of spectrum):

    • As you run your finger from left to right across an IR spectrum, you can see whether or not light is absorbed at particular frequencies. When the curve dips down, less light is transmitted. That means light is absorbed. The dip in the graph is called a peak. Different bonds absorb different frequencies of light, so the peaks tell you what kinds of bonds are present.

      Contributors and Attributions

    This page titled What Does an IR Spectrum Look Like? is shared under a CC BY-NC 3.0 license and was authored, remixed, and/or curated by Chris Schaller.

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