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?

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 the graph?

Answer 13

Peaks

Question 14

What do peaks help us predict?

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?

Answer 16

IR absorptions on formula sheet

Question 17

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

Answer 17

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 present
Identity of unknown compound can be worked out now.

Question 18

What does each peak represent

Answer 18

A fragment of the molecule

Question 19

What does the peak on the far right represent

Answer 19

The molecular ion

Question 20

4 stages of mass spectrometry

Answer 20

Ionisation
Acceleration
Deflection
Detection

Question 21

Ionisation

Answer 21

The molecules are bombarded with electrons of which some will collide with enough force to knock an electron off the molecule – these molecules now have a +1 charge

Question 22

Acceleration

Answer 22

The ions are accelerated so they all have the same kinetic energy

Question 23

Deflection

Answer 23

The ions are passed through a magnetic field and will deflect in proportion to their mass and charge

Question 24

Detection

Answer 24

The beam of ions are detected electrically

Question 25

Number of carbon atoms forumla (using the peaks)

Answer 25