What is Spectroscopy?
– 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
What is NMR?
- 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)
Name some nuclei that are NMR active
1H, 13C, 31P, 19F
How do you determine if the atomic nuclei of an atom is NMR active?
- 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
Principles of NMR spectroscopy
- There is an energy difference between the two alignments
- Radio waves make low energy spin ‘flip’ to high energy status
What does the exact magnetic field seen by each nucleus depend on?
- the externally applied magnetic field
- AND on the electronic environment surrounding the nucleus
What is a carbon ‘environment’?
• Within a molecule two carbon environments are identical
only when the bonding and neighbours are identical.
What determines the number of signals in an NMR spectrum?
• The number of signals in an NMR spectrum reflects the
number of carbon environments in the molecule
Define ‘chemical shift’
- The position of a signal is called its chemical shift
- Referenced to tetramethyl silane (TMS) (CH3)4Si which is set to 0
What does ‘down field’ mean in terms of position on the spectrum?
Down field is the position furtherst to the left
(think of it like on a piano, the lowest notes are furtheres to the left)
- List the numerical values for different bonds on the 13C spectrum
List the numerical values for different bonds on the 1H spectrum
What are two sources of extra information in a 1H spectrum?
- The integral
– tells you how many H’s in a given environment - The multiplicity (splitting)
– tells you how many neighbours an H has
What is an integral?
- 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.
What is multiplicity (splitting)?
- Tells you how many neighbours an H has
- Fine structure (splitting or multiplicity) arises from the hydrogen atoms attached to the neighbouring carbon atoms
How do you determine the number of splits per signal? (n+1 rule)
- 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.
Complete the following table with observations for each information point
