IR Spectroscopy

Useful Concepts

Infra-Red (IR) Spectroscopy

An infra-red or IR spectrum is produced by absorption of energy (4000-400 cm^-1) due to vibrations of polar covalent bonds.
This provides an important method for identifying the functional groups within an organic molecule, the region 4000-1000 cm^-1 is most useful for this.
Below 1000 cm^-1 the spectra tend to be more complex and hard to assign. However, this fingerprint region can be used to confirm a structure by direct comparison (known vs unknown).
The most useful information you should be able to read in an IR spectrum is what functional groups are present.
Remember that different functional groups can be "viewed" as combinations of bond types, e.g. an ester is C=O and C-O.

Avoid the temptation to try to assign every peak.

Hookes' Law

To help understand IR, it is useful to compare a vibrating bond to the physical model of a vibrating spring system. The spring system can be described by Hooke's Law, as shown in the equation given on the left.

Consider a bond and the connected atoms to be a spring with two masses attached.

Using the force constant k (which reflects the stiffness of the spring) and the two masses m₁ and m₂, then the equation indicates how the frequency, ν, of the absorption should change as the properties of the system change.

As examples of this look at the positions of C-C (1000 cm^-1), C=C (1600 cm^-1) and C=C (2200 cm^-1), and the positions of C-H (3000 cm^-1) and C-C (1000 cm^-1).

The absorptions you should 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 we are interested in. Note that it is rarely useful to look for C-C since most organic molecules will have them.

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 given in the tables. You will normally be given copies of the tables in exams, but you will save yourself lots of time if you know approximately where the various bonds absorb.

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, e.g. If the formula has only one O, then you cannot have an ester (CO₂R) or an acid (CO₂H)! or you should be able to distinguish an aldehyde and a ketone by checking the proton NMR (9-10 ppm) !