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

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

The 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

What are the uses of IR spectroscopy?

Pollution

• 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

Breathalysers

• 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

Greenhouse gases

• Greenhouse gases such as CO₂, H₂O, and CH₄ absorb infrared radiation due to the vibrations of their polar bonds

• This absorption contributes to the greenhouse effect, as the energy is re-emitted and trapped in the Earth's atmosphere

• The effectiveness of a greenhouse gas depends on the type of bonds it contains and how strongly they absorb infrared radiation