Infrared spectroscopy

Resonance

• all covalent bonds are able to 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

IR spectroscopy

• a technique used to identify compounds based on changes in vibrations of atoms when they absorb IR of certain frequencies

• The machine irradiates the sample with the 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 - so symmetrical molecules are 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 a wave number → this is the reciprocal of the wavelength And characteristic absorptions can be matched to specific bonds in molecules so functional group can be determined

Fingerprint region

• the region below 1500cm-1 is the fingerprint region and is unique to every molecule

• The peaks in this region represent the complex vibrational interactions that occur between different bonds within a molecule

• The value of this region is being able to compar IR spectrum to a known compound from a database and coming up with an exact match

• All members of the same series will show the same type of bonds present but no two molecules will have the same fingerprint region

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