SLIDE 2 4/11/2016 2
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Atoms for NMR
1H, 2H, 3H, 3He, 4He, 12C, 13C, 14C, 14N, 15N 16O, 17O, 19F, 23Na, 31P, etc.
The most frequently applied nuclei: 1H, 13C, 15N, 17O, 19F, F, 31P
Signal depends on:
- Magnitude of the magnetic moment
- Concentration of the isotope
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The split of the energy states (Zeemann)
N N S S
Spinning nucleus with it’s magnetic field alligned with the external field. Spinning nucleus with it’s magnetic field alligned against the external field. α-spin state, favourable, lower energy
N S N S
β-spin state, unfavourable, higher energy
The energy difference between the spin states does depend on the strength of the external magnetic field.
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m: magnetic moment of the individual atom
Orientation and reorientation on the microscopic level
µ µ H=0; M=0 µ µ µ H>0; M>0 M=Σµi
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Graphical representation
Remember, the energy is proportional to the frequency!
H E
at no magnetic field there is no difference between these states
α-spin state β-spin state
∆E1 ∆E2
<
H1 H2
<
N S N S S N N S www.medschool.pte.hu
An example: CH4
H E
α-spin state β-spin state
h x 300MHz
0 T 7,05 T 11,75 T
h x 500MHz We can probe the energy difference of the α - and β - state of the protons by irradiating them with EM radiation of just the right energy. In a magnet of 7.05 Tesla, it takes EM radiation of about 300 MHz (radio waves). So, if we bombard the molecule with 300 MHz radio waves, the protons will absorb that energy and we can measure that absorbance. In a magnet of 11.75 Tesla, it takes EM radiation of about 500 MHz (stronger magnet means greater energy difference between the α - and β - state of the protons).
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What is the problem with this concept?
It is difficult to compare the data from two instruments with different strenghts of magnetic fields.
Let’s use the chemical shift!
What is this? Much simpler than it may sound…
