Chapter 17 - Spectroscopy

Question 1

What does the mass spectrometer detect?

Answer 1

The mass to charge ratio of the molecular ion, which gives the molecular mass of the compound.

Question 2

How are molecular ions formed in mass spectrometry?

Answer 2

Electron is lost to form a positive ion

Question 3

How is molecular mass found from a mass spectrum?

Answer 3

The molecular ion peak (M+ peak) has to be located.

Question 4

What is the cause of the M + 1 peak?

Answer 4

Some carbon is present as the carbon-13 isotope.

Question 5

What is fragmentation?

Answer 5

The molecular ions break down into smaller pieces called fragments.

Question 6

What causes the other peaks in a mass spectrum?

Answer 6

Fragment ions

Question 7

How can a mass spectrum differentiate between structural isomers?

Answer 7

They will break apart differently and thus will have different mass spectra.

Question 8

What is the effect on a bond when it absorbs IR?

Answer 8

Bend or stretch more as they gain energy.

Question 9

What does the amount a bond streches or bends depend on?

Answer 9

Mass of the atoms in the bond - heavier atoms vibrate more slowly than lighter atoms
Bond strength - Stronger bonds vibrate faster than weaker bonds

Question 10

What is unique about each bond with regards to IR absorption?

Answer 10

Can only absorb radiation that has the same frequency as the natural frequency of the bond.

Question 11

What scale is used for the frequency values in IR absorption?

Answer 11

Wavenumber 200/cm to 4000/cm

Question 12

How does infrared spectroscopy work?

Answer 12

Sample placed inside an IR spectrometer,
A beam of IR radiation in range 200/cm to 4000/cm passed through the sample
Molecule absorbs some of the IR frequencies and the emerging beam is analysed to find the absorbed frequencies.

Question 13

What is the term given to the dips in an IR spectrum?

Answer 13

Peaks

Question 14

What do peaks help us predict in an IR spectrum?

Answer 14

The bond types in the molecules.

Question 15

What is below 1500/cm known as on the IR spectrum and why?

Answer 15

Fingerprint region

The peaks in this region are unique to each molecule.

Question 16

In an exam question what can be used to determine the molecule in question from an IR spectrum?

Answer 16

IR absorptions on formula sheet

Question 17

How to approach a combined techniques question to identify an unknown compound?

Answer 17

1. Read the question (may include hints such as whether it is saturated or unsaturated).
2. Use elemental analysis to determine the empirical formula.
3. Use the M+ peak from the mass spectrum to determine the molecular mass and therefore the molecular formula.
4. Use Infrared spectroscopy peaks to identify bonds and functional groups present.
5. Attempt to draw a structure.
6. Check answer using fragment ion peaks from the mass spectrum.