2.8 Instrumental Analysis

Question 1

What analytical processes can be used to identify compounds and calculate their molecular formula?

Answer 1

• Mass spectroscopy
• Infrared spectroscopy
• Proton NMR spectroscopy
• C13 NMR spectroscopy

Question 2

Briefly describe what happens during mass spectrometry

Answer 2

A vaporised sample of atoms of molecules is ionised into positive ions. The ions are accelerated, deflected by a magnetic field and then detected. This produces a graph with mass to charge ratio (m/z) on the x axis and relative abundance on the y axis.

Question 3

Answer 3

Question 4

How can you use a mass spectrum to deduce the relative molecular mass of a sample of a compound?

Answer 4

The peak with the highest m/z value (the molecular ion peak, M+) is caused by the whole molecule, therefore that m/z value = molecular mass.

Question 5

‘Ions in a mass spectrometer can have a 2+ charge’
True or False?

Answer 5

TRUE
Ions in a mass spectrometer can have a 2+ charge, but the 1+ charge is more common.

Question 6

Chlorine has two possible isotopes, Cl35 with a 75% abundance and Cl37 with a 25% abundance. Predict what the mass spectrum would look like for the diatomic molecule.

Answer 6

Question 7

What happens when infrared radiation is directed at a compound?

Answer 7

Bonds within the molecule absorb specific frequencies of radiation.

Question 8

What happens when specific frequencies of radiation are absorbed by bonds in a molecule?

Answer 8

The bonds vibrate. This causes the bond to stretch or bend.

Question 9

What can be identified using peaks on an infrared spectrum? How?

Answer 9

The functional groups within a molecule can be identified by comparing the peaks to known values in a data book.

Question 10

What is the fingerprint region on an infrared spectrum?

Answer 10

The region typically between 1500 cm-1 and 500 cm-1 that contains a complicated pattern of absorptions. Every compound has a unique fingerprint region.

Question 11

Why can 13C nuclei be used in NMR spectroscopy?

Answer 11

Carbon 13 nuclei can align with or against a magnetic field. The nuclei are less stable when opposing the magnetic field meaning this alignment has higher energy. Energy can be supplied to the nucleus in the form of radio waves and cause it to flip from the more stable alignment to the less stable alignment. This energy change depends on the carbon environment.

Question 12

What is a carbon environment?

Answer 12

The atoms, or groups of atoms, that a carbon atom is bonded to.

Question 13

What can be deduced from a carbon-13 NMR spectrum? How?

Answer 13

Number of different carbon environments = number of peaks on the spectrum.
Types of carbon environments - compare the chemical shift values of the peaks to the data book to identify the carbon environments.

Question 14

Answer 14

Question 15

Predict the number of peaks in the 13C NMR spectrum for 3-methylbutanal

Answer 15

Question 16

Why can protons be used in NMR spectroscopy?

Answer 16

Hydrogen nuclei can align with or against a magnetic field. The direction of this alignment can be flipped using a specific frequency of radio waves (known as resonance condition). This is because there is a difference in the energy of the two alignments. The frequency required for this change depends on the proton environment.

Question 17

How can a proton environment be identified using a proton NMR spectrum?

Answer 17

Compare the chemical shift values of the peaks to values in the data book.

Question 18

On a proton NMR spectrum, what does the ratio of the areas under the peaks indicate?

Answer 18

The relative number of protons in each environment.

Question 19

What are equivalent protons?

Answer 19

Protons in the same environment. These may be on the same or different carbons.

Question 20

What does the splitting pattern of peaks on a proton NMR spectrum show?

Answer 20

The number of protons on the adjacent carbon atoms.

Question 21

Name the first 4 splitting patterns on a proton NMR spectrum

Answer 21

Singlet (1 peak) Doublet (2 peaks) Triplet (3 peaks) Quartet (4 peaks)

Question 22

What is the n+1 rule?

Answer 22

The number of peaks in the splitting pattern is equal to the number of adjacent protons (n) plus one.

Question 23

When doesn’t the n+1 rule work?

Answer 23

● Alcohols - the chemical shift for the hydrogen atom in -OH is variable and this peak is always a singlet. -OH doesn’t cause splitting in adjacent hydrogens.
● Equivalent hydrogens - protons bonded to the same carbon (protons in the same environment) have no effect on each other in terms of splitting.
● Benzene - the splitting pattern is very complicated (called a multiplet). The peaks for a benzene ring will be found in the range 6.0-9.0 ppm.

Question 24

What does the peak at 0 ppm on a proton NMR represent?

Answer 24

TMS (tetramethylsilane)

Question 25

What is TMS used as?

Answer 25

The standard for chemical shift measurements during NMR spectroscopy.

Question 26

Why is TMS used as the standard for chemical shift measurements in NMR spectroscopy?

Answer 26

● It has 12 hydrogens in the same environment meaning a single strong peak will be produced in proton NMR.
● It contains carbon and hydrogen atoms meaning it can be used in both carbon and proton NMR.
● It contains 1 carbon environment meaning it produced a single peak in 13C NMR.
● It’s non-toxic.
● It’s inert so won’t react with the compounds under analysis.