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IR spectrum

An IR spectrum represents the intensity of IR radiation as a function of wave number.

Curve of the IR spectrum. Ordinate = the wave number in cm-1, the values go from 4000 to 600. In abscissa = the transmittance T or absorbance A, the values go from 12,4 to 64,7. The curve is globally increasing until around 2600 cm-1, then decreasing beyond.

On the Y-axis: transmittance T or absorbance A

T = I/I0 in % with
    I0 = light intensity measured without sample (background)
      = contribution of the apparatus + environment
    I = light intensity measured with a sample
      = contribution of the apparatus + environment + sample
Thus I/I0 = sample contribution
A = log I0/I without unit

On the X-axis: the wavenumber, in cm-1

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Most organic molecules are infrared active. However, two conditions are required to induce vibrations.

2 conditions

A molecule absorbs IR light if it has a vibration during which its dipole moment varies with distance. It can be translated mathematically by the following relationship:

The energy of the incident radiation must be equal to the energy difference between two vibrational levels of the molecule.


h = 6,63. 10-34 J.S
c = 3 10.10 cm.s-1
ω = wavenumber in cm-1
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