Ch13 - IR Spectroscopy

Chapter 13: Spectroscopy

Infra-Red (IR) Spectroscopy

Important absorptions:

The more important absorptions that you should probably learn to recognise, in order of importance are:

	Bond	Base Value	Strength / Shape	Comments
1	C=O	1715	s, "finger"	Exact position depends on type of carbonyl
2	O-H	3600	s, brd	Broad due to H bonding
3	N-H	3500	m	Can tell primary from secondary
4	C-O	1100	s	Also check for OH and C=O
5	C=C	1650	w alkene m-s aromatic	Alkene w due to low polarity Aromatic usually in pairs
6	C≡C	2150	w, sharp	Most obvious in terminal alkynes
7	C-H	3000	s	As hybridisation of C changes sp³-sp²-sp, the frequency increases
8	C≡N	2250	m, sharp	Characteristic since little else around it

If you know these, then you can identify most of the functional groups of interest. Note that it is rarely useful to look for C-C since the large majority organic molecules will have them.

You should also be aware that the exact substitution pattern of a particular bond causes shifts in the position of the absorption and, therefore, ranges of values are typically given in most tables.

It is possible to rationalise the shifts of absorbances based on electronic effects due to proximal groups, conjugation and / or ring strain.

In general, when you are trying to work out what a molecule is, you will not just have the IR spectrum, but you will have other information as well, such as the formula or most likely the NMR. Always cross check between these sets of information. For example, if the molecular formula indicates only one O, then you can not have an ester (CO₂R) or a carboxylic acid (CO₂H) !