L6 - Infra-Red Spectroscopy

objectives

Principles of Infra-Red Spectroscopy

• 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
• The frequency of vibration can be found by detecting when the molecules absorb electro-magnetic radiation.
• Various types of vibration are possible.
• Examples include... STRETCHING and BENDING

• a beam of infra red radiation is passed through the sample
• a similar beam is passed through the reference cell
• the frequency of radiation is varied
• bonds vibrating with a similar frequency absorb the radiation
• the amount of radiation absorbed by the sample is compared with the reference
• the results are collected, stored and plotted
• A bond will absorb radiation of a frequency similar to its vibration(s)

IDENTIFICATION OF PARTICULAR BONDS IN A MOLECULE

• The presence of bonds such as O-H and C=O within a molecule can be confirmed because they have characteristic peaks in identifiable parts of the spectrum.

IDENTIFICATION OF COMPOUNDS BY DIRECT COMPARISON OF SPECTRA

• The only way to completely identify a compound using IR is to compare its spectrum with a known sample.
• The part of the spectrum known as the ‘Fingerprint Region’ is unique to each compound.

• Infra-red spectra are complex due to the many different vibrations taking place in each molecule.
• Total characterisation of a substance based only on its IR spectrum is almost impossible unless one has computerised data handling facilities for comparison of the obtained spectrum with one in memory.
• The technique is useful when used in conjunction with other methods
  • -nuclear magnetic resonance spectroscopy and mass spectroscopy.
• Peak position depends
  • on bond strength
  • masses of the atoms joined by the bond

• strong bonds and light atoms absorb at lower wavenumbers
• weak bonds and heavy atoms absorb at high wavenumbers

Interpreting Spectra

Fingerprint Region

• organic molecules have a lot of C-C and C-H bonds within their structure
• spectra obtained will have peaks in the 1400 cm^-1 to 800 cm^-1 range
• this is referred to as the “fingerprint” region
• the pattern obtained is characteristic of a particular compound the frequency
• of any absorption is also affected by adjoining atoms or groups.

IR Spectrum of a Carbonyl Group

• carbonyl compounds show a sharp, strong absorption between 1700 and 1760 cm^-1
• this is due to the presence of the C=O bond

IR Spectrum of an Alcohol Group in Alcohols

• alcohols show a broad absorption between 3200 and 3600 cm^-1
• this is due to the presence of the O-H bond in Alcohols

IR Spectrum of a Carboxylic Acid

• carboxylic acids show a broad absorption between 2500 and 3300 cm^-1
• this is due to the presence of the O-H bond in acids
• they also show a strong absorption around 1700 - 1750 cm^-1
• this is due to the presence of the C=O bond

IR Spectrum of an Ester

• esters show a strong absorption between 1750 cm^-1 and 1700 cm^-1
• this is due to the presence of the C=O bond
• There is also a sharp, strong absorption between 1300 cm^-1 and 1000 cm^-1
• this is due to the presence of the C-O bond in Esters

What is it?

• One can tell the difference between alcohols, aldehydes and carboxylic acids by comparison of their spectra.