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Spectroscopy 2814 1

INFRA - RED SPECTROSCOPY
Introduction Different covalent bonds have different strengths due to the masses of different atoms at
either end of the bond. As a result, the bonds vibrate at different frequencies (imagine two
balls on either end of a spring) . The frequency of vibration…

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2 2814 Spectroscopy



Axes Vertical Absorbance the stronger the absorbance the larger the peak.

Horizontal Frequency wavenumber (waves per centimetre) / cm-1
or Wavelength microns (µ); 1 micron = 1000 nanometres


A typical IR spectrum
Wavelength (microns)
2.5 3 4 5 6 7 8 9 10 12 15

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Spectroscopy 2814 3

CHARACTERISTIC ABSORPTION FREQUENCIES OF SOME FUNCTIONAL GROUPS

Bond Class of compound Range, cm-1 Intensity

C-H Alkane 2965 - 2850 strong
CH3 - 1450 medium
1380 medium
CH2 - 1465 medium
Alkene 3095 - 3010 medium
1000 - 700 strong
Alkyne 3300 (approx) strong
Aldehyde 2900 - 2820…

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