Module 4: Chapter 17 - Spectroscopy

Question 1

What is the structure of an electromagnetic wave?

Answer 1

Electric and magnetic fields both perpendicular to each other and the direction of travel of the wave. The electric and magenetic fields vibrate at the same frequency as each other

Question 2

What happens when an atom/molecule/ion absorbs UV or visible light?

Answer 2

The electrons move to higher energy levels

Question 3

What happens when an atom/molecule/ion absorbs infrared light?

Answer 3

The bonds vibrate

Question 4

What happens when an atom/molecule/ion absorbs microwaves?

Answer 4

The molecules rotate

Question 5

What happens when an atom/molecule/ion absorbs radio waves?

Answer 5

The nuclear spin changes

Question 6

What are the 3 types of bond vibrations?

Answer 6

• Symmetric stretch (distance between atoms changes)
• Assymetric stretch (distance between atoms changes)
• Bending (change in bond angle)

Question 7

How does IR spectroscopy work?

Answer 7

1. The sample under investigation is placed inside an IR spectrometer
2. A beam of IR radiation in the range 200-4000 cm⁻¹ is passed through a sample
3. The molecule absorbs some of the IR frequencies , and the emerging beam of radiation is analysed to identify the grequencies that have been absorbed by the sample
4. The IR spectrometer is connected to a computer that plots a graph of transmittance against wavenumber

Question 8

What is the measure of the frequency absorbed in IR spectroscopy?

Answer 8

Wavenumber (cm⁻¹)

Question 9

What is the equaton for wavenumber?

Answer 9

wavenumber = 1/wavelength

Question 10

What range is IR light absorbed in?

Answer 10

4000 - 200 cm⁻¹

Question 11

What range of IR light is used to identify functional groups?

Answer 11

above 1500cm⁻¹

Question 12

What range of IR light is used for fingerprinting?

Answer 12

Below 1500cm⁻¹

Question 13

What is the fingerprinting region?

Answer 13

Below 1500cm⁻¹, it contains unique peaks for every compound so can be used to identify a specific compound or to check if it is a pure substance

Question 14

How can you tel the IR spectrum for an O-H in an alcohol and an O-H in a carboxylic acid apart?

Answer 14

Alcohol is usually a smooth curve whereas acid is usually jagged

Question 15

Why is wavenumber used as the units of IR spectoscopy?

Answer 15

The frequency values would be extremely high, so wavenumber is used as it is proportional to frequency

Question 16

How do greenhouse gases lead to global warming?

Answer 16

Greenhouse gases often contain C=O, O-H, and C-H bonds. Greenhouse gases in the atmosphere can absorb longer-wavelength IR radiation from the sun as it has the same natural frequency as these bonds. Eventually, the vibrating bonds in these molecules re-emit this energy as radiation that increases the temperature of the atmosphere close to the earths surface, leading to global warming

Question 17

What does a bonds natural frequency depend on?

Answer 17

All bonds vibrate at a characteristic frequency (in the infrared region of the EM spectrum), the frequency depends on:
* the mass of the atoms in the bond
* the bond strength
* the type of vibration

Question 18

What does an IR spectrum look like?

Answer 18

transmittance % on the y axis, wavenumber on the x axis (starting from 4000 down to 0)

Question 19

Why do all organic compounds produce a characteristic peak between 2850 and 3100cm⁻¹?

Answer 19

2850 to 3100cm⁻¹ is the range of absorption for a C-H bond

Question 20

Where will an alcohol contains peaks in an IR spectrum? (dont specify wavenumber)

Answer 20

At the range for O-H, C-H, and the fingerprint region

Question 21

Where will a ketone/aldehyde contains peaks in an IR spectrum? (dont specify wavenumber)

Answer 21

At the range for C=O, C-H, and the fingerprint region

Question 22

Where will a carboxylic acid contain peaks in an IR spectrum? (dont specify wavenumber)

Answer 22

At the range for O-H, C-H, C=O, and the fingerprint region

Question 23

What are 2 applications of IR spectroscopy?

Answer 23

• The detection of pollutants such as carbon monoxide and nitrous oxide using the IR fingerprints in vehicle emissions and busy town centres
• IR-based breathaliser tests pass a beam of IR radiation through the carptured breath in the sample chamber and detect the IR absorbance of the compounds in the breath. The characteristic bonds of ethanol may be detected, the higher the absorption reading (lower the transmission) the more ethanol in the breath

Question 24

What is infrared spectroscopy?

Answer 24

Use of absorption peaks from an infrared spectrum to identify bonds and functional groups present in the molecule

Question 25

How does mass spectrometry work?

Answer 25

• Ionisation - gaseous atoms are bombarded by electrons from an electron gun and are ionised to form ions of 1+ charge
• Acceleration - ions are charged and are accelerated by the electric field
• Deflection - the ions ar deflected by a magnetic/electric field, separating them according to mass (higher mass = less deflection)
• Detection - the ions are detected and a graph of abundance against m/z values is plotted

Question 26

How does the mass spectrometry of larger molecules work?

Answer 26

Once the molecules have been ionised, they have formed a molecular ion. Some of these molecular ions will undergo fragmentation to form fragment ions of smaller masses. Both the fragment ions and the molecular ions will be detected and plotted. From this you are able to identify the molecular mass (from the molecular ions) and information about the structure of the molecule (from the fragment ions)

Question 27

If there is a fragment with a m/z value of 15, what is the likely group?

Answer 27

[CH₃]⁺

Question 28

If there is a fragment with a m/z value of 17, what is the likely group?

Answer 28

[OH]⁺

Question 29

If there is a fragment with a m/z value of 28, what is the likely group?

Answer 29

[C=O]⁺ or [C₂H₄]⁺

Question 30

If there is a fragment with a m/z value of 29, what is the likely group?

Answer 30

[C₂H₅]⁺

Question 31

If there is a fragment with a m/z value of 43, what is the likely group?

Answer 31

[COCH₃]⁺ or [C₃H₇]⁺

Question 32

If there is a fragment with a m/z value of 45, what is the likely group?

Answer 32

[COOH]⁺

Question 33

If there is a fragment with a m/z value of 57, what is the likely group?

Answer 33

[C₄H₉]⁺

Question 34

Determine the structure of compound F

EXTREMELY IMPORTANT EXAMPLE QUESTION!!!

Answer 34

Question 35

What is a molecular ion?

Answer 35

A molecular ion is the positive ion formed in mass spectrometry when a molecule loses an electron

Question 36

How do you determine the molecular mass of the molecule from a mass spectrum?

Answer 36

You use the molecular ion peak (M⁺ peak). The M⁺ peak is the clear peak at the highest m/z value on the right-hand side of the mass spectrum

Question 37

What is the M + 1 peak and how does it occur?

Answer 37

A very small peak (known as the M + 1 peak) is usually found one unit after the M⁺ peak. This exists due to different isotopes of atoms within the molecule (such as 1.1% of carbon atoms being the carbon-13 isotope).

Question 38

Why may there be many smaller peaks around the larger fragmentation peaks?

Answer 38

• Due to different isotopes of atoms that may slightly affect the m/z value
• Some molecular ions/fragment ions may be given a +2 charge causing their m/z value to half

Question 39

What happens in fragmentation?

Answer 39

Fragmentation breaks a molecular ion into 2 species, a positively charged fragment ion and a radical. Any positive ions formed will be detected by the mass spectrometer, but the uncharged radicals will not be detected

Question 40

What is the equation for the fragmentation of propan-1-ol into a fragment of size 31?

Answer 40

Question 41

What do fragmentation peaks allow you to do?

Answer 41

They allow you to identify the structure of the molecule, helping you identify where in the molecule functional groups and side chains are located