Topic 7: Analytical techniques 1

Question 1

What is I.R. spectroscopy used for?

Answer 1

Used to identify bonds / functional groups
Can only identify the exact molecule by comparison with library spectra

Question 2

How does I.R. spectroscopy work?

Answer 2

· Infrared radiation is passed through the sample and a reference cell.
· The reference ensures that peaks due to water or carbon dioxide in the air can be cancelled out.
· The frequencies of I.R. radiation absorbed are determined by passing through a rotating prism to focus one frequency at a time onto the detector.
The spectrum shows the wavelength on the x and absorbance on the y-axis.

Question 3

What is the theory behind I.R. spec?

Answer 3

· IR radiation corresponds to the energy required to make chemical bonds vibrate more / move to a higher vibrational energy level.
· Therefore, energy of certain wavelengths is absorbed by molecules.
The actual energy depends on the mass of the atoms and the strength of the bond, so different bonds will absorb at different frequencies.

Question 4

A high Infrared absorption number indicates what?

Answer 4

A stronger bond. This is because stronger bonds need more energy to make them vibrate, so absorb a higher frequency of I.R. radiation (higher wavenumber)