NMR

Question 1

What is Spectroscopy?

Answer 1

– Involves absorption of part of the electromagnetic spectrum
– Triggers a change of energy level occupancy within the molecule
– The light absorbed is related to the process involved

Energy Absorbed =>Signal in spectrum => Structural info

Question 2

What is NMR?

Answer 2

• Process used in analysis and structure determination (NMR). Provides info on structural connectivity.
• NMR uses energy in the radio frequency region
• Stands for nuclear magnetic resonance
• Involves ‘flipping’ the spin of a nucleus in a magnetic field
• Used to image soft tissue in animals (MRI)

Question 3

Name some nuclei that are NMR active

Answer 3

1H, 13C, 31P, 19F

Question 4

How do you determine if the atomic nuclei of an atom is NMR active?

Answer 4

• The atomic nuclei behave as if they’re spinning about an axis
• Act like ‘mini-magnets’ (because nuclei are charged)
• When put into an external magnetic field, NMR active nuclei align parallel or anti-parallel to it

Question 5

Principles of NMR spectroscopy

Answer 5

• There is an energy difference between the two alignments
• Radio waves make low energy spin ‘flip’ to high energy status

Question 6

What does the exact magnetic field seen by each nucleus depend on?

Answer 6

• the externally applied magnetic field
• AND on the electronic environment surrounding the nucleus

Question 7

What is a carbon ‘environment’?

Answer 7

• Within a molecule two carbon environments are identical
only when the bonding and neighbours are identical.

Question 8

What determines the number of signals in an NMR spectrum?

Answer 8

• The number of signals in an NMR spectrum reflects the
number of carbon environments in the molecule

Question 9

Define ‘chemical shift’

Answer 9

• The position of a signal is called its chemical shift
• Referenced to tetramethyl silane (TMS) (CH3)4Si which is set to 0

Question 10

What does ‘down field’ mean in terms of position on the spectrum?

Answer 10

Down field is the position furtherst to the left

(think of it like on a piano, the lowest notes are furtheres to the left)

Question 11

• List the numerical values for different bonds on the 13C spectrum

Answer 11

Question 12

List the numerical values for different bonds on the 1H spectrum

Answer 12

Question 13

What are two sources of extra information in a 1H spectrum?

Answer 13

1. The integral
   – tells you how many H’s in a given environment
2. The multiplicity (splitting)
   – tells you how many neighbours an H has

Question 14

What is an integral?

Answer 14

• Tells you how many H’s in a given environment
• The relative intensities of the signals is proportional to the number of hydrogen atoms which give rise to the signal.

Question 15

What is multiplicity (splitting)?

Answer 15

• Tells you how many neighbours an H has
• Fine structure (splitting or multiplicity) arises from the hydrogen atoms attached to the neighbouring carbon atoms

Question 16

How do you determine the number of splits per signal? (n+1 rule)

Answer 16

• If a hydrogen atom has “n” equivalent hydrogen atoms on adjacent carbon atoms, its NMR resonance will appear as a signal which is split into “n+1” lines - this is called the n+1 rule.

Question 17

Complete the following table with observations for each information point

Answer 17