Infrared Spectra (IR) Interpretation (HL) (DP IB Chemistry) : Revision Note

Infrared Spectra (IR) Interpretation

• 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 bond vibration

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

• Infrared spectroscopy is used to identify pollutants in vehicle emissions in the air

  • Sensors detect and measure the amount of pollutants such as carbon monoxide, carbon dioxide and unburnt hydrocarbons

  • This commonly occurs on motorways and in busy town centres to monitor localised pollution

• Infrared spectroscopy can be used to measure alcohol levels using roadside breathalysers

  • A ray of infrared radiation is passed through the breath that is exhaled into the breathalyser chamber

  • The characteristic bonds of ethanol are detected and measured - the higher the absorbance of infrared radiation, the more ethanol in the person's breath

What are the uses of IR spectroscopy?

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 compound 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