Coupling in NMR Flashcards
What other interaction, bar chemical shift, determines the appearance of an NMR spectrum in solution
Spin-Spin Coupling
What is Spin-Spin coupling interaction
Nuclei (with non-zero spin) which are near neightbours in a molcule can effect each others energy
This can have a small contribution to the local magnetic field at the nucleus which depends on the spin state of the neighbour
If the neighbour has spin of i = 1/2
How many spin states are available
There are two spin states available
With orientations parallel and anti-parallel to the applied magnetic field
If i= 1/2
How is the local magnetic field affected
The two spin states can add to or subtract from the local magnetic field and so resulting in contributions at ±J/2
Where J is the size of the coupling
If i = 1/2
What affect will this have on the NMR spectrum produced
This gives a splitting into a doublet in the spectrum
So if J is the size of the coupling
How is it transmitted
It is transmitted through the bonds which connect the two coupled nuclei
If J is the size of coupling
Is J affected by the strength of the magnetic field?
J is independent of the strength of the applied magnetic field
(as it is measured in Hz not ppm)
If J is the size of the coupling
What will cause the size of J to decrease
J will decrease as the number of bonds between the coupled nuclei increases
One-bond couplings (¹J) are usually substantially larger than two (²J) and three-bond (³J) ones
¹⁹F, is going to have what type of value for i
Hence, how will the molecules align in Fluoroacetic acid
¹⁹F will have a half value fori
Hence 50% of the molecules in a sample of fluroacetic acid contain a ¹⁹F spin in the paraellel oritentation
And 50% spin in the anti-parallel orientation
The ¹⁹F molecules have spin
How will this affect the local magnetic field and hence appearance of the spectrum
- The effect of a parallel orientation is to augment the local magnetic field at the carbon atoms - increasing the resonance frequency and the shift
- The effect of an anti-parallel oritnetation is to reduce the local magnetic field decreasing the resonance frequency and shift by the same amout
- Overall producing a splitting in the spectrum to give a doublet
How is the value of J shown on an NMR spectrum for a doublet
The value of J is the frequency seperation between the two lines in the doublet
How would the two doublets produced in fluroacetic acid differ and why
¹J is larger than ²J
This is because the one bond coupling is much greater than the two bond one
Difluroacetic acid has two fluroine molecules
There are three ways for the two ¹⁹F molecules to orient in the applied magentic field
What are they
- Both spins are parallel to applied magnetic field. In this case both ¹⁹F spins augment the local magnetic field at the carbon nucleui = increased frequency
- Both spins are anti-parallel to the applied magnetic field. In this cause both ¹⁹F spins reduce the local magnetic field at the carbon nuclei, resulting in a decreased frequency
- One spins paralllel, one anti-parallel. The opposing effect of the two ¹⁹F spins cancel, and no overall change in frequnecy
- Results in a triplet with a 1:2:1 ratio
In general, if a nucleus is coupled to n neightbouring equivalent spins with i = 1/2 the resulting multiplet is split into …
n + 1
(Pascals Triangle)
When are couplings not observed
- No splittings are observed when the two coupled nuclei are in equivalent environments
- Couplings to isotopes with low natural abundance are normally neglected
- Splitting due to coupling to ¹H are often removed experimentally from other NMR spectra by ‘decoupling’
- Couplings to spins with i > 1/2. This is due to these spins rapidly interconverting between the spin states by a processes called relaxation
Explain in the case of ¹³C, why couplings to isotopes with low natural abundance are normally neglected
Only 1.1% of naturally occuring carbon is the NMR ¹³C isotope
Only 0.012% will have a ¹³C nucleus in both positions in a dicarbon compound like ethanol
These molecules will show a splitting, but there is usually such a small proportion of these molecules in the sample that the corresponding signals are of vanishingly low intensities
What is proton decoupling and how is it carried out
When ¹³C spectras which are recorded have the coupling interaction to ¹H removed
Involved irradiating the sample at the ¹H resonance frequency while recording the spectrum
It allows the spectrum to become simplified as shown in the top NMR
Couplings to spins with i > 1/2. This is due to these spins rapidly interconverting between the spin states by a processes called relaxation
Name one execption to this
Deuterium ²H, where i = 1
it has three spin states: i = -1, 0, +1
which add, subtract or have no effect on the local magnetic field
Results in a triplet of three lines of equal intensity
What is the common solvent for NMR studies
What effect will it have on the NMR spectrum
Deuterated chloroform (CDCl₃)
1% of the molecules contain a ¹³C nucleus which is coupled to the neighbouring ²H, resulting in the expected triplet
The chlorines vanish due to relaxations however
¹H chemical shifts can also be affected by which factors
Electronegativity of nearby groups
Aromaticity of nearby carbon atoms
Extent of hydrogen bonding
How many peaks would this molecules have on a 1H NMR and why
There are only 4 inequivalent types of hydrogen nuclei, so there are 4 peaks in the spectrum
This is due to a plane of symmetry and rapid rotation about single bonds meaning that all protons in methyl groups are equivalent
Why are the heights of peaks important in ¹H NMR
the area under each peak is proportional to the number of ¹H nuclei in that environment (integrals)
The taller the integral, the larger the area under the peak and the more protons within the same environment
These can be worked as a ratio for the proportion of hydrogens
Ethyl acetate would form 3 peaks
Are there any splitting within these peaks
There is no splitting in the purple methyl group protons as the adjacent atom is an oxygen
The two red methylene protons have a splitting of 4, as the adjacent carbon is attached to 3 hydrogen (n+1)
The green methyl group has a splitting of 2, as the adjacent carbon is attacked to 2 hydrogens (n+1)
How do double bonds affect splitting patterns
Use acrylonitrile as your example
Bond rotation is restricted, protons bonded to the same carbon usually equivalent to another
The purple and green proton are not equivalent (as there is no rotation around the double bond)
Therefore the red, green and purple protons will have different couplings
For the purple proton the multiplet pattern is a doublet of doublets (not n+1)
