17 - Spectroscopy

Question 1

Functions of mass spec

Answer 1

• Can be used to identify the molecular mass of an organic compound and to gain further information about its structure.

Question 2

How does the molecular ion peak work

Answer 2

organic compound will lose an electron and form a positive ion = molecular ion (M+).

Question 3

How does mass spec work - overall

Answer 3

detects the mass to charge ratio (m/z) which gives the molecular mass of the compound.

Question 4

Mass spec always…

Answer 4

PRODUCES POSITIVE CHARGE

Question 5

How to identify molecular ion peak

Answer 5

• Clear peak at the highest m/z value (on the right-hand side) of the mass spec

Question 6

What is the m+1 peak

Answer 6

very small peak one unit after the M+ peak

Question 7

Why does the m+1 peak exist

Answer 7

as 1.1% of carbon is present as the carbon-13 isotope

Question 8

How does the number of carbon atoms effect m+1 peak

Answer 8

• The larger the number of carbon atoms present, the larger the M+1 peak.

Question 9

Formular to find number of carbon atoms

Answer 9

Question 10

How does mass spec work - full

Answer 10

1. Atom is ionized to form positive ion
2. Ions accelerated to same kinetic energy
3. Ions deflected by a magnetic field - The amount of deflection depends on their mass
4. Ions are detected.

Question 11

What would make the ion deflect more

Answer 11

More mass and more positive charge = more deflection

Question 12

Uses of mass spec

Answer 12

• Analysing molecules in space
• Detecting banned substances such as steroids in athletes
• Detecting traces of toxic chemicals in contaminated marine life
• Determine the abundance of each isotope of an element
• Identifying unknown compounds
• Gain further info about structure and chemical properties of molecules

Question 13

What is fragmentation

Answer 13

• Molecular ion breaks up into smaller pieces

Question 14

What are other smaller peaks caused by in mass spec

Answer 14

fragment ions

Question 15

What does fragmentation break molecular ion into

Answer 15

positively charged fragment ion and a radical

Question 16

Are the positive ions picked up by mass spec

Answer 16

Yes

Question 17

Are radicals picked up by mass spec

Answer 17

No

Question 18

Common Fragment ions - 15

Answer 18

Question 19

Common Fragment ions - 17

Answer 19

Question 20

Common Fragment ions - 29

Answer 20

Question 21

Common Fragment ions - 43

Answer 21

Question 22

Common Fragment ions - 57

Answer 22

Question 23

Characteristics of covalent bonds

Answer 23

possess energy and vibrate (bend or stretch) around a central point.

Question 24

Atoms in molecules are…

Answer 24

In constant motion

Question 25

When does the vibration increase in covalent bonds

Answer 25

When temp increase

Question 26

What also makes bonds vibrate more

Answer 26

absorb IR radiation

Question 27

What is resonance

Answer 27

• The frequency of vibration occurs in the infra-red region of the electromagnetic spectrum • If an organic molecule is irradiated with infra-red energy that matches the natural vibration frequency of its bonds, it absorbs some of that energy and the amplitude of vibration increases

Question 28

Define stretch

Answer 28

distance between two atomic centres increase + decrease

Question 29

Define bend

Answer 29

results in a change in bond angle

Question 30

The amount a bond vibrates depends on what

Answer 30

o The mass of the atoms – heavier atoms vibrate more slowly o The strength of the bond – stronger bonds vibrate faster

Question 31

Any particular bond can only…

Answer 31

absorb radiation that has the same frequency as the natural frequency of the bond. (Specific functional groups absorb at specific frequencies)

Question 32

Why do we use wave number instead of frequency

Answer 32

• Frequency scale is very large

Question 33

How does the greenhouse effect work

Answer 33

• Most of the suns visible and UV radiation passes through the atmosphere to the Earth’s surface, where most of it is absorbed. Some is re-emitted as IR radiation. • Greenhouse gases absorb this longer wavelength IR radiation as it has the same frequency as the natural frequency of the bonds. • Eventually the vibrating bonds re-emit this energy as radiation, which increases the temp of the atmosphere (global warming)

Question 34

Purpose of IR spec

Answer 34

identify the functional groups present in organic molecules

Question 35

IR process

Answer 35

Sample placed into IR spectrometer 2. Beam of IR radiation in the range of 200-4000cm-1 fired through sample 3. Some IR radiation is absorbed by the sample, the emerging beam is analysed to work out the frequencies that have been absorbed 4. IR spec. connected to a computer that plots a graph of transmittance against wavenumber.

Question 36

The dips on an IR spec are

Answer 36

Peaks

Question 37

Analysis of IR spec

Answer 37

• Each peak is observed at a wavenumber that corresponds to a particular bond in the molecule.

Question 38

What is the fingerprint Region

Answer 38

• Contains unique peaks which can be used to identify a particular molecule. Compare to computer database/ published spectrum booklets.

Question 39

Where is the fingerprint region

Answer 39

Below 1500

Question 40

Uses of IR spec

Answer 40

• Breathalysers – detects the characteristic bonds present in ethanol. The more IR radiation absorbed, the more ethanol in the breath. • Remote sensors that analyse vehicle emissions – detects pollutants (CO/CO₂/hydrocarbons).