Page 78 - Practical-organic-3

➢ IR spectrum region

✓ IR is most useful in providing information about the presence or absence of specific

functional groups.
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✓ The IR spectrum has two main regions: the functional group region (at ≥ 1500 cm ), and
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the fingerprint region (at < 1500 cm ).
✓ Although the entire IR spectrum can be used as a fingerprint for the purposes of

comparing molecules, the fingerprint region (as C-O, C-N & C-C bonds) is normally a

complex area showing many bands, frequently overlapping each other and difficult to

interpret. It is unique for every molecule.

 Factors affecting vibrations:

1. Type of vibration
The energy absorbed when particular bonds vibrate depends on several factors. Generally, it is

easier to bend than stretch, so bending vibrations are of lower energy than stretching vibrations for

the same bond. Therefore, absorptions due to bending tend to occur at lower wavenumbers than

stretches.

2. Strength of bonds

A strong bond will need more energy to vibrate, so stronger bonds absorb at higher wavenumbers.

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