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Title: Physical Chemistry - Magnetic Shielding, Chemical Shift, Nuclear Coupling, Multiplet splitting, limitations of continuous pulse NMR and Pulse NMR
Description: Physical Chemistry - Magnetic Shielding, Chemical Shift - with standard frequencies and requirements, Nuclear Coupling, Multiplet splitting with diagram, limitations of continuous pulse NMR and Pulse NMR.
Description: Physical Chemistry - Magnetic Shielding, Chemical Shift - with standard frequencies and requirements, Nuclear Coupling, Multiplet splitting with diagram, limitations of continuous pulse NMR and Pulse NMR.
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Physical Chemistry - Magnetic Shielding, Chemical Shift, Nuclear Coupling, Multiplet splitting,
limitations of continuous pulse NMR and Pulse NMR
Magnetic Shielding
- The exact resonance frequency of a nucleus is a function of the static magnetic field seen by the
nucleus - small changes make measurable change in resonance frequency
...
- Addition of magnetic field produced by electrons ς B is proportional to and opposite to applied
field: ς B = - σ B : where σ is shielding constant (usually positive, but can be negative) and B is
magnetic field at nucleus (discussed in previous notes)
...
g
...
Shifted from an agreed 'standard'
frequency o
...
However, ς =
x 106 (ppm)
- Represents a chemical shift that is independent of field, and so will apply to the same nucleus in a
molecule in any NMR machine
...
- Thus we can use NMR spectroscopy to identify protons and other nuclei in a sample in different
chemical environments or different parts of a molecule, to obtain quantitative information to help
identify
...
g
...
Nuclear Coupling
- There is an interaction between nuclei in a molecule, which affects energy levels of all other nuclei
...
- Without detailing eneergy considerations, we can generalise the fact that:
A group of P equivalent nuclei split a neighbouring group into P+1 lines, with intensities given by the
Pth line of Pascal's triangle
...
It is independent of
the field or measurement frequency and so is quoted in Hz, not ppm
...
- Thermal transitions between levels occur frequently, contributing to noise - arranging of many
scans required to partially cancel noise
...
Hence signal to noise ratio (S:N) improves by only
...
Pulse NMR
- Instead of continuous wave at constant frequency, we can 'hit' the nuclei with a pulse of
electromagnetic energy - disturbing all nuclei at once
...
However, each nucleus oscillates at a different rate depending on local magnetic field,
and these oscillations interfere with one another
...
Title: Physical Chemistry - Magnetic Shielding, Chemical Shift, Nuclear Coupling, Multiplet splitting, limitations of continuous pulse NMR and Pulse NMR
Description: Physical Chemistry - Magnetic Shielding, Chemical Shift - with standard frequencies and requirements, Nuclear Coupling, Multiplet splitting with diagram, limitations of continuous pulse NMR and Pulse NMR.
Description: Physical Chemistry - Magnetic Shielding, Chemical Shift - with standard frequencies and requirements, Nuclear Coupling, Multiplet splitting with diagram, limitations of continuous pulse NMR and Pulse NMR.