CHM 247 CH.12
Structure Determination: Mass Spectrometry and Infrared Spectroscopy
12.6 Infrared Spectroscopy
• Infrared (IR) spectroscopy: A kind of optical spectroscopy that uses infrared energy. IR
spectroscopy is particularly useful in organic chemistry for determining the kinds of
functional groups present in a molecule.
• Range of 7.8x10 m to approximately 10 m. Only a portion is used by chemists.
• Wavenumber (ṽ): The reciprocal of the wavelength in centimetres.
• Wavenumber: ṽ(cm ) = 1 / λ(cm)
• The useful IR region is 4000 – 400 cm . Corresponds to energies of 48.0 kJ/mol to 4.80
• All molecules have a certain amount of energy and are in constant motion, with their
bonds stretching and contracting constantly.
• Amount of energy a molecule contains is quantized (non-variable).Amolecule can
stretch or bend at specific frequencies corresponding to specific energy levels. When a
molecule is irradiated with electromagnetic radiation, energy is absorbed if the frequency
of the radiation matches the frequency of the vibration. This results in an increased
amplitude for the vibration.
• We can find what kind of motions a molecule has by examining an IR spectrum since
each frequency absorbed by a molecule corresponds to a specific motion. We can then
determine the kinds of bonds (functional groups) present by examining the IR spectrum.
12.7 Interpreting Infrared Spectra
• IR spectrums can be difficult to interpret; therefore they are often used for pure samples
of fairly small molecules. Complex molecules IR spectrum can used because an IR
spectrum -1 a unique fingerpr-1t for that molecule. Complex region (fingerprint region) is
1500cm to around 400cm .
• Most compounds have IR spectrums that don’t change from one compound to another.
• Helps to divide the region on an -1 spectrum into 4 parts:
- Region from 4000-2500cm which corresponds to absorptions caused by N – H, C –
H, and O – H single bonds stretching motions. N/O – H absorb around 3300 -
3600cm ; C – H stretching occurs near 3000 cm . -1
- 2500-2000 cm : T-1ple bond stretching occurs here (C≡N, C≡C).
- 2000-1500 cm : Double bonds (C=O, C=N, and C=C). Carbonyl groups around
1680-1750 cm .Alkene stretching around 1640-1