Chapter 17 - Spectroscopy

Question 1

What does a mass spectrometer detect?

Answer 1

The mass-to-charge ratio (m/z) of the molecular ion, which gives the molecular mass of the compound

Question 2

Describe mass spectrometry process

Answer 2

Substance vaporised and then injected into sample inlet. Enters ionisation chamber where high energy electrons bombarded at atoms, ionising the atom. Passed into acceleration area, where electric field accelerates electrons to constant kinetic energy. Enters drift region where strong magnets bend route that the species can take. Then reaches detector and time taken for positive ions to reach the detector depends on the mass and charge. When positive ions hit the plate they gain an electron producing a flow of charge - greater the current the greater the abundance

Question 3

How is the molecular mass found from a mass spectrum?

Answer 3

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

Question 4

What is the small peak after the M+ peak on a mass spectrum called?

Answer 4

M+1 peak

Question 5

Why does the M+1 peak exist?

Answer 5

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

Question 6

What is fragmentation?

Answer 6

The molecular ions break down into smaller pieces called fragments

Question 7

What are the other peaks in a mass spectrum caused by?

Answer 7

Fragment ions

Question 8

Give an example of a fragment ion of molecular ion - CH3CH2CH2OH+

Answer 8

CH2OH+

Question 9

How do mass spectrums of substances with the same molecular formula differ?

Answer 9

Fragment ions are different, so peaks are different

Question 10

What happens when a bond absorbs infrared (IR) radiation?

Answer 10

bends or stretches more as it gains energy

Question 11

What does amount that bond stretches or bend depend on?

Answer 11

• mass of the atom in bond (heavier atoms vibrate more slowly)
• strength of the bond (stronger bonds vibrate faster than weaker bonds)

Question 12

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

Answer 12

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

Question 13

What scale is used for the frequency values?

Answer 13

Wavenumber

most vibrations of bonds are observed in 200cm^-1 to 4000 cm^-1 range

Question 14

What happens when the sun emits visible and IR radiation on earth?

Answer 14

Radiation passes through atmosphere to the earth’s surface, where most of it is absorbed. However, some is re-emitted from earth’s surface in the form of longer wavelength IR radiation

Question 15

Why do greenhouse gases - carbon dioxide, water vapour and methane absorb the longer wavelength IR remitted by the earth?

Answer 15

Same frequency as their natural frequency of their bonds

re-emit energy as radiation that increases temperature of the atmosphere

Question 16

Describe the process of infrared spectroscopy

Answer 16

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

Question 17

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

Answer 17

Peaks

Question 18

What is specific to each bond in a molecule on a IR spectrum?

Answer 18

Each peak is absorbed at a wavenumber that can be related to a particular bond in the molecule

Question 19

What is the area below 1500cm^-1 called on a IR spectrum?

Answer 19

fingerprint region

Question 20

What is the use of the fingerprint region?

Answer 20

Contains unique peaks which can be used to identify the particular molecule under investigation

Question 21

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

Answer 21

IR absorptions on formula sheet

Question 22

What do all organic molecules produce on a IR spectrum?

Answer 22

Peak between 2850 cm^-1 and 3100cm^-1

Question 23

Describe the IR spectrum of an alcohol?

Answer 23

• Absorbance peak within 3200 - 3600 cm^-1 caused by O-H bond
• Also peak between 1000-1300cm^-1 caused by C-O bond

Question 24

Describe the infrared spectrum of an aldehyde or ketone?

Answer 24

Peak in range 1630-1820cm^-1 caused by C=O bond. Peak typically absorbs close to 1700cm^-1

Question 25

Describe the infrared spectrum of a carboxylic acid

Answer 25

Peak in range 1630-1820cm^-1 caused by C=O bond, and a broad peak at 2500 - 3330cm-1 caused by O-H group. Peak at 100-1300 cm^-1 representing C-O bond

Question 26

Give some examples of applications of infrared spectroscopy?

Answer 26

• Identifying pollutants - detect/ measure carbon monoxide CO2 and hydrocarbons in town centres or by motorways
• Breathalysers - detect IR absorbance of the compounds in the breath

Question 27

What would the typical sequence for identification of an unknown substance be?

Answer 27

• elemental analysis - use of percentage composition data to determine the empirical formula
• mass spectrometry - use of the molecular ion peak from a mass spectrum to determine molecular mass; use of fragment ions to identify sections of molecule
• Infrared spectroscopy - use of absorption peaks from an infrared spectrum to identify bonds and functional groups present in molecule