F324: Module 3: Infrared Spectroscopy and Mass Spectrometry

Question 1

I) Describe how IR spectroscopy works.

Answer 1

A beam of IR radiation is passed through a sample.

Different bonds absorb different frequencies of IR radiation.

An IR spectrometer produces as graph to show what frequencies of radiation the molecules are absorbing.

You can use this to identify the functional groups in a molecule.

Question 2

I) What do the troughs indicate on a IR spectrum?

Answer 2

The ‘peaks’ show which frequency is being absorbed.

Question 3

I) How would you find the RFM of a compound using a mass spec graph?

Answer 3

You would identify the molecular ion peak which is the peak with the highest (excluding the tiny peak caused by a C13 isotope) mass/charge ratio.

Question 4

I) What do the x and y axis on a mass spec graph represent?

Answer 4

x-axis = mass/charge ratio (since the molecular ions have a charge of +1, this is their RFM)

y-axis = The relative abundance of ions in the sample (often given as a percentage)

Question 5

I) Why are fragmentation patterns useful to chemists?

Answer 5

Once you have identified the fragments you can piece them together to form a molecule with the correct RFM. They allow you to identify the structure of the compound since some molecules (e.g. propanal and propanone) have the same RFM but produce a different set of fragments.