17) Spectroscopy

Question 1

Define molecular ion M+

Answer 1

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

Question 2

The mass spectrometer detects the _ of the molecular ion

Answer 2

mass-to-charge ratio (m/z)

Question 3

Define 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, which gives the molecular mass of the compound

Question 4

Define M+1 peak

Answer 4

very small peak one unit after the M+ peak which exists because 1.1% of carbon is present as the carbon-13 isotope

Question 5

Define fragmentation

Answer 5

the process in mass spectrometry that causes a positive ion to split into smaller pieces: a positive fragment ion (appears as other peaks on the spectrum) and an uncharged radical (not detected)

Question 6

What are axis of mass spectra?

Answer 6

relative intensity (%)
m/z

Question 7

Different fragmentation patterns can help to…

Answer 7

identify an organic molecule

Question 8

What is the effect of covalent bonds absorbing infrared radiation?

Answer 8

bonds bend or stretch more

Question 9

What does the amount a bond stretches/bends depend upon?

Answer 9

the mass of the atoms in a bond (heavier atoms vibrate more slowly)
the strength of the bond (stronger bonds vibrate faster)

Question 10

Any particular bond can only absorb radiation that has…?

Answer 10

the same frequency as the natural frequency of the bond

Question 11

What is wavenumber proportional to?

Answer 11

frequency

Question 12

What happens to the sun’s visible and IR radiation after it passes through the atmosphere to the Earth’s surface?

Answer 12

most of it is absorbed however, some is re-emitted from the Earth’s surface in the form of longer-wavelength IR radiation which is absorbed by greenhouse gases

Question 13

Name 3 greenhouse gases

Answer 13

water vapour, carbon dioxide and methane

Question 14

What happens to greenhouse gases after they have absorbed longer-wavelength IR radiation?

Answer 14

eventually vibrating bonds re-emit this energy as radiation which increases the temperature of the atmosphere close to the Earth’s surface, leading to global warming

Question 15

Define infrared spectroscopy

Answer 15

an instrumentation method of analysis that identifies bonds from absorption of the infrared radiation of different wavelengths
means of identifying functional groups present

Question 16

Describe the 3 steps of infrared spectroscopy

Answer 16

1) sample placed in IR spectrometer and a beam of IR radiation (200-400cm-1) is passed through the sample
2) molecule absorbs some IR frequencies and emerging beam of radiation is analysed to identify the frequencies that have been absorbed by the sample
3) IR spectrometer connected to computer that plots a graph of transmittance against wavenumber

Question 17

Each peak is observed at a wavenumber, which can be related to…?

Answer 17

a particular bond in the molecule

Question 18

Define fingerprint region

Answer 18

an area of an infrared spectrum below 1500cm-1 that gives a characteristic pattern for different compounds

Question 19

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

Answer 19

due to the presence of C-H bonds

Question 20

How would you recognise an alcohol from IR spectra?

Answer 20

absorbance peak within the range of 3200-3600cm-1 caused by the O-H bond

Question 21

How would you recognise an aldehyde/ketone from IR spectra?

Answer 21

absorbance peak within the range of 1630-1820cm-1 caused by the C=O bond

Question 22

How would you recognise a carboxylic acid from IR spectra?

Answer 22

absorbance peak within the range of 1630-1820cm-1 caused by the C=O bond
AND broad peak at 2500-3330cm-1 caused by the O-H group

Question 23

GIve 2 applications of infrared spectroscopy

Answer 23

remote sensors to identify and monitor localised pollution

IR-based breathalysers (the more IR radiation absorbed, the higher the signal, the more ethanol in the breath)

Question 24

In combined techniques, what can we find out by elemental analysis?

Answer 24

percentage composition data -> empirical formula

Question 25

In combined techniques, what can we find out by mass spectrometry?

Answer 25

M+ peak -> molecular mass

fragment ions -> identify sections of a molecule

Question 26

In combined techniques, what can we find out by infrared spectroscopy?

Answer 26

absorption peaks -> identify bonds + functional groups present