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

• An electron is lost to form a positive ion

- Electrons from an electron gun knock off electrons in the outer shell

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 does the mass to charge ratio give you ?

Answer 8

It gives you the molecular mass of the compound

Question 9

How is an electron lost ?

Answer 9

Electrons from an electron gun knock off electrons in the outer shell

Question 10

How do you locate the M+ peak ?

Answer 10

It is the big peak furthest to the right of the graph

Question 11

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

Answer 11

It bends or stretches more as its gains energy

Question 12

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

Answer 12

• 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 13

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

Answer 13

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

Question 14

What scale is used for the frequency values?

Answer 14

Wavenumber (200/cm to 4000/cm)

Question 15

How does infrared spectroscopy work?

Answer 15

• Sample placed inside an IR spectrometer
• A beam of IR radiation is passed through the sample
• Molecule absorbs some of the IR frequencies and the emerging beam is analysed to find the absorbed frequencies.

Question 16

What is the term given to the dips in the graph?

Answer 16

Peaks

Question 17

What do peaks help us predict?

Answer 17

The bond types in the molecules

Question 18

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

Answer 18

• Fingerprint region

- The peaks in this region are unique to each molecule.

Question 19

What is the typical sequence for identification of an unknown substance?

Answer 19

• Elemental analysis - determine empirical formula
• Mass spectrometry - Determine molecular mass and identify sections in molecule from fragment ions
• Infrared spectroscopy - identify bonds and functional groups presentIdentity of unknown compound can be worked out now.