Infrared (IR) spectroscopy is an analytical technique that shows which bonds, and so which functional groups, are present in a molecule.A beam of infrared radiation covering a range of frequencies is passed through a sample. Covalent bonds absorb IR radiation at particular frequencies. The instrument measures how much radiation passes through the sample at each frequency.The result is an IR spectrum:
The x-axis is wavenumber in cm−1, usually running from 4000 on the left down to about 500 on the right. Wavenumber is proportional to frequency.
The y-axis is transmittance in %. 100% means no radiation is absorbed.
Absorptions appear as downward dips in the line (often called peaks or troughs).
Each type of bond absorbs within a characteristic wavenumber range, given on the data sheet. For example:
O-H (alcohols): 3230 to 3550 cm−1
O-H (acids): 2500 to 3000 cm−1
C=O: 1680 to 1750 cm−1
C-H: 2850 to 3300 cm−1The region below 1500 cm−1 is the fingerprint region, which is unique to each compound.Worked example: a spectrum has a strong, sharp dip at 1720 cm−1 and no broad dip above 3000 cm−1. The dip lies in the range 1680 to 1750 cm−1, so a C=O bond is present, and there is no alcohol O-H. The compound could be an aldehyde or a ketone.
The x-axis is wavenumber in cm−1, usually running from 4000 on the left down to about 500 on the right. Wavenumber is proportional to frequency.
The y-axis is transmittance in %. 100% means no radiation is absorbed.
Absorptions appear as downward dips in the line (often called peaks or troughs).
Each type of bond absorbs within a characteristic wavenumber range, given on the data sheet. For example:O-H (alcohols): 3230 to 3550 cm−1
O-H (acids): 2500 to 3000 cm−1
C=O: 1680 to 1750 cm−1
C-H: 2850 to 3300 cm−1The region below 1500 cm−1 is the fingerprint region, which is unique to each compound.Worked example: a spectrum has a strong, sharp dip at 1720 cm−1 and no broad dip above 3000 cm−1. The dip lies in the range 1680 to 1750 cm−1, so a C=O bond is present, and there is no alcohol O-H. The compound could be an aldehyde or a ketone.