[17.2] infrared spectroscopy

Question 1

what is the effect of infrared radiation (IR) on covalent bonds?

Answer 1

• covalent bonds in molecules possess energy and vibrate naturally about a central point
• the bonds can absorb IR, which makes them bend or stretch more
• any particular bond can only absorb radiation that has the same frequency as the natural frequency of the bond

Question 2

what does the amount that a bond stretches or bends depend on?

Answer 2

• the mass of the atoms: heavier atoms vibrate more slowly than lighter ones
• the strength of the bond: stronger bonds vibrate faster than weaker bonds

Question 2

how do molecules change when their bonds vibrate?

Answer 2

• a bond in a molecule can stretch so that the distance between the atomic centres change
• bonds in a molecule can bend, causing the bond angle to change

Question 3

what does a ‘wavenumber’ describe?

Answer 3

• covalent bonds have characteristic bond frequencies
• the frequency values are very large, so chemists use a more convenient scale called wavenumber, which is proportional to the frequency
• the vibration of most bonds are observed in the IR wavenumber range of 200 cm⁻¹ to 4000 cm⁻¹

Question 4

describe the process of infrared spectroscopy

Answer 4

1. the sample under investigation is placed inside an IR spectrometer
2. a beam of IR radiation in the range 200-4000 cm⁻¹ is passed through the sample
3. the molecule absorbs some of the IR frequencies, and the emerging beam of radiation is analysed to identify the frequencies that have been absorbed by the sample
4. the IR spectrometer is usually connected to a computer that plots a graph of transmittance against wavenumber

Question 5

what are peaks?

Answer 5

• dips in the graph
• each peak is observed at a wavenumber that can be related to a particular bond in the molecule

Question 6

what is a wavenumber above and below 1500 cm⁻¹ used for?

Answer 6

• above 15000 cm⁻¹ is used to identify functional groups
• below 1500 cm⁻¹ is used for fingerprinting

Question 7

what is fingerprinting?

Answer 7

• the graph is complicated and contains many signals; picking out functional group signals can be difficult
• this part of the spectrum is unique for every compound, and so can be used as a “fingerprint”
• this region can also be used to check if a compound is pure

Question 8

identifying functional groups

Answer 8

• don’t need to memorise values because they’re given; use the table to read the values
• important ones for year 1 are:

> O-H stretch (alcohol)

> C=O stretch (carbonyl)

> O-H and C=O stretch (carboxylic acid)

Question 9

how can you distinguish between carboxylic acids and carbonyls?

Answer 9

• you should see another peak for the O-H bond
• O-H stretch in carboxylic acid is very broad and broader than the O-H stretch in alcohols
• carbonyls have a strong and sharp peak at ~1700

Question 10

what are some uses of infrared spectroscopy?

Answer 10

• monitoring pollution
• breathalysers

Question 11

describe a typical sequence for identification

Answer 11

1. elemental analysis - use of percentage composition data to determine the empirical formula
2. mass spectrometry - use of the molecular ion peak from mass spectrum to determine the molecular mass; use of fragment ions to identify sections of a molecule
3. infrared spectroscopy - use of absorption peaks from an infrared spectrum to identify bonds and functional groups present in the molecule