How IR Spectroscopy works
- 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