11.1.4 Infrared Spectroscopy

• All covalent bonds act rather like springs, as opposed to rigid bars
• Like springs, the bonds can vibrate in a number of different ways
• The frequency of vibration occurs in the infra-red region of the electromagnetic spectrum
• If an organic molecule is irradiated with infra-red energy that matches the natural vibration frequency of its bonds, it absorbs some of that energy and the amplitude of vibration increases
• This is known as resonance

Different modes of vibration in molecules. Each mode has a characteristic frequency of vibration

Infrared (IR) spectroscopy

• Infrared (IR) spectroscopy is a technique used to identify compounds based on changes in vibrations of atoms when they absorb IR of certain frequencies
• A spectrophotometer irradiates the sample with IR radiation and then detects the intensity of IR radiation absorbed by the molecule
• IR energy is absorbed only if a molecule has a permanent dipole that changes as it vibrates
  • Symmetrical molecules such as O₂ or H₂, are therefore IR inactive

• The resonance frequency is the specific frequency at which the bonds will vibrate
• Rather than displaying frequency, an IR spectrum shows a unit called wavenumber
  • Wavenumber is the reciprocal of the wavelength and has units of cm^-1

• Characteristic absorptions can be matched to specific bonds in molecules
  • This enables chemists to determine the functional groups present

Absorption Range of Bonds

• Due to some absorption bands overlapping each other, other analytical techniques such as mass spectroscopy should be used alongside IR spectroscopy to identify an unknown compound

• The best way to understand how to interpret an IR spectrum is by looking at examples and becoming familiar with the characteristic features of an IR spectrum

Answer:

• • IR spectrum A is propanone and spectrum B is propan-2-ol.
  • In IR spectrum A the presence of a strong, sharp absorption around 1710 cm^-1 corresponds to the characteristic C=O, carbonyl, group in a ketone.
  • In spectrum B the presence of a strong, broad absorption around 3200-3500 cm^-1 suggests that there is an alcohol group present, which corresponds to the -OH group in propan-2-ol.

Fingerprint Region

• The region below about 1500 cm^-1 is called the fingerprint region and is unique to every molecule
• It has many peaks that can be difficult to assign
• These peaks represent the complex vibrational interactions that occur between different bonds within a molecule
• The value of the fingerprint region is in being able to compare the IR spectrum to a known compund from a database and coming up with an exact match
• This is particularly useful, for example, in identifying a specific member of a homologous series
  • All members of the series will show the same type of bonds present, but no two molecules will have the same fingerprint region