Chapter 17 - Spectroscopy

Question 1

What is the molecular ion, M+?

Answer 1

The positive ion formed when a molecule loses an electron

Question 2

What is the m/z value?

Answer 2

The mass to charge ratio, which gives the molecular mass of the compound

Question 3

What is the molecular ion peak (M+ peak)?

Answer 3

The clear peak at the highest m/z value on the right hand side of the mass spectrum. It gives the molecular mass of the compound

Question 4

Why is there sometimes a very small peak one unit after the M+ peak?

Answer 4

This is the M + 1 peak, which exists because 1.1% of the carbon is present as the carbon-13 isotope

Question 5

What is fragmentation?

Answer 5

The process by which some molecular ions break down into smaller pieces known as fragments in the mass spectrometer.
The simplest fragmentation breaks down a molecular ion into two species: a positively charged fragment ion and a radical. Positive ions are detected, radicals are not

Question 6

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

Answer 6

Fragment ions

Question 7

What are some of m/z values for common fragment ions?

Answer 7

CH3 + = 15
OH+ = 17
C2H5+ = 29
C3H7 + = 43
C4H9 + = 57

Question 8

What happens when atoms in molecules absorb IR radiation?

Answer 8

The bonds vibrate more

Question 9

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

Answer 9

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

Question 10

How does infrared spectroscopy work?

Answer 10

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

Question 11

Where is the fingerprint region and what do you do with it when reading an IR spectrum?

Answer 11

The area <1500 cm -1

We ignore it

Question 12

What are the applications of IR spectroscopy?

Answer 12

• pollutants can be identified by their IR spectral fingerprints
• breathalysers pass a beam of IR radiation through captured breath in samply chamber to detect the IR absorbance of compounds in the breath

Question 13

How do greenhouse gases affect IR radiation to cause global warming?

Answer 13

Greenhouse gases like water vapour, carbon dioxide and methane absorb longer wavelength IR radiation that is re-emitted from the Earth’s surface after travelling from the Sun.
The vibrating bonds in these molecules re-emit this energy as radiation that increases the temp of the atmosphere close to the Earth’s surface

Question 14

What is a typical sequence for the identification of an organic compound?

Answer 14

1. Elemental analysis - use of percentage composition to determine empirical formula
2. Mass spectrometry - use of M+ ion peak from mass spectrum to determine molecular mass
3. IR spectroscopy - use of absorption peaks from IR spectrum to identify bonds and functional groups present