Infra Red Spectroscopy

AS CHEMISTRY

How It Works• The electromagnetic spectrum is composed of energy that may behave both as a

particle and as a wave.• Frequency is the number of wave troughs that pass a given point in a second and

wavelength is the distance from one crest of a wave to an adjacent crest. • Frequency and wavelength are inversely related, according to the equation E = hv =

hc / λ. As frequency increases, wavelength decreases.• The wavenumber is the number of waves in one centimeter and has the units of

reciprocal centimeters (cm-1). • Since the wavenumber is inversely proportional to wavelength, it is directly

proportional to frequency and energy.

How It Works• If a range of infra-red frequencies shine one at a time through a sample of an organic

compound, some frequencies get absorbed by the compound. • A detector on the other side of the compound would show that some frequencies pass

through the compound with almost no loss, but other frequencies are strongly absorbed.• How much of a particular frequency gets through the compound is measured

as percentage transmittance.• A percentage transmittance of 100 would mean that all of that frequency passed

straight through the compound without any being absorbed. • A transmittance of only 5% would mean that nearly all of that particular frequency is

absorbed by the compound.

How It Works• Spectroscopy is the study of matter and its interaction with electromagnetic radiation. • All matter contains molecules; these molecules have bonds that are continually

vibrating and moving around. • These bonds can vibrate with stretch motions or bend motions. • Stretching can either be symmetric or asymmetric. A molecule with three or more

atoms can experience a bending vibration, a vibrational mode where the angle between atoms changes.• The energy involved in this vibration depends on things like the length of the bond

and the mass of the atoms at either end. • If the right amount of energy shined on a bond, it jump into a higher state of vibration.

The amount of energy it needs to do this will vary from bond to bond, and so each different bond will absorb a different frequency (and hence energy) of infra-red radiation.

How It WorksBond stretching• In covalent bonds, atoms aren't joined by rigid links - the two atoms are held together

because both nuclei are attracted to the same pair of electrons. • The two nuclei can vibrate backwards and forwards - towards and away from each

other - around an average position.

How It WorksBond Bending• As well as stretching, bonds can also bend. The effect of this, of course, is that the

bond angle between the bonds fluctuates slightly around its average value.

PRACTICE 1

Interpret Spectrum

• carbon-oxygen double, C=O• carbon-oxygen single, C-O• oxygen-hydrogen, O-H• carbon-hydrogen, C-H• carbon-carbon single, C-C

PRACTICE 2 1. Use Data Booklet for this practiceThis question is going to give you three isomeric compounds and three IR spectra. The three compounds you are concerned with are

• propanoic acid: CH3CH2COOH

• methyl ethanoate: CH3COOCH3

• hydroxypropanone: CH3COCH2OHa) Draw full structural formulae for the three compounds so that you can see exactly which bonds you need to think about.b) Decide which compound corresponds to each of the spectra below, explaining how you made your decisions.