Module 4: Chapter 17 (Spectroscopy)

Question 1

What can mass spectrometry do?

Answer 1

1. identify the molecular mass of an organic compound

2. give further information about the structure

Question 2

How is the molecular mass determined in mass spectrometry?

Answer 2

• in the spectrometer the molecule loses an electron to form a positive ion (ion is called the molecular ion)
• m/z ratio detected of the ion and it shows up as the highest mass peak on the spectrum (the molecular ion peak)
• assuming z = 1 its value will be equivalent to the molecular mass of the compound

Question 3

What is fragmentation?

Answer 3

• some molecular ions break down into smaller fragments
• two species are formed; a positively charged fragment ion and an uncharged radical
• peak heights reflect the likely hood of the particular fragmentation (the positively charged fragment, no the lost species)

Question 4

What are some applications of mass spectrometry?

Answer 4

• all molecule fragment in slightly different ways according to their structure, therefore a compound’s mass spectrum is unique and can be used to identify it (unknown compounds)
• mass spectrometry is amongst the techniques used in sport to test urine samples for banned substances

Question 5

What happens to covalent bonded molecules with IR?

Answer 5

• covalent bonds in molecule posses energy and have a natural vibration about a central point
• they can absorb infrared radiation, this makes them vibrate more
• vibrates can be stretches or bends (bends change bond angle)
• only absorbs IR that has the same frequency as the natural frequency of the bond

Question 6

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

Answer 6

1. the mass of atoms (heavier = slower vibration)

2. the strength of bonds (stronger = faster vibration)

Question 7

wavenumber = …

Answer 7

1 / wavelength in cm^3

• wavenumber is directly proportional to frequency

Question 8

What are some applications of IR spectrometry?

Answer 8

• used to monitor levels of localised pollution from nitrogen monoxide and hydrocarbons
• IR-based breathalysers measure the ethanol levels in captured breath