Covalent Bonds: Covalent bonds can absorb infrared radiation, which makes the bonds vibratebonds bend or stretch more. Stretch and bend are two types of vibration. Bonds have energy
normally and vibrate.
• Wavenumber: Wavenumber is proportional to frequency and used as frequency values are large.
Any bond can absorb radiation that has same frequency of bond.
• How Greenhouse Gas Contribute to Global Warming:
– Radiation from sun reaches Earth’s surface, where absorbed. Some re- emitted as infrared
radiation.
– Greenhouse gases absorb infrared radiation and (long residence time).
– Bonds vibrate.
– Molecules re- emit this radiation, increasing atmospheric temperature- global warming.
Therefore need to reduce carbon dioxide emissions.
• IR Process: Beam of infrared radiation in range of 200- 4000 cm-1 passed through sample.
Emerging beam of radiation analysed to identify the frequencies absorbed by sample. Computer
plots graph of transmittance (%) on x axis against wavenumber (cm-1
) on y axis.
• Fingerprint Region: An area of an infrared spectrum below 1500 cm-1
that gives a characteristic
pattern for different compounds. Better to ignore the functional groups here.
• Data Table: Must know O-H group in alcohols. C=O group in aldehydes, ketones and carboxylic
axis. COOH group in carboxylic acid.
• All organic compounds produce peak at 2850- 3100 cm-1
from presence of C-H bond, which can
be confused with O-H peak in alcohols.
• The range for peak starts right when slopes down to when becomes flat again.
• Make sure to include what functional group the bond belongs to e.g. O-H bond in an alcohol.
• Spectrum of an alcohol- a little dip where O-H usually is does not count, must have the first curve
bit.
Spectrum of an aldehyde or ketone- C=O absorbs right close to 1700 cm-1
. Also has C-O bond but
not always reliable as in fingerprint region.
• Spectrum of carboxylic acid- Carboxylic always has O-H and C=O. The O-H and C-H have combined
here.
• X is C-H. Y is C=O. Z is C-O.
• X is O-H in alcohol. Y is C-H.
• X is C-H. Y is C=O.
• Uses of Infrared Spectroscopy:
– Monitoring Air Pollution- pollutants can be identified by identifying their functional groups
and can match each pollutant to spectrum. Can measure concentration of pollutant.
– Breathalysers- breathalysers pass a beam of infrared radiation through breath. Detects
infrared radiation absorbance of bonds found in ethanol. The higher the infrared radiation
absorbed, more ethanol in breath.
• Absorption around —- so contains C-H bonds. No absorption of other functional groups as
hydrocarbon contains C and H only.