Section B 4 Flashcards
In a magnetic field, the nuclear spin axis is tilted with respect to the external field direction, and it precesses around
Bo, just like a gyroscope, or spinning top. The frequency at which this occurs is the Larmor frequency, νL.
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νL is determined by γ, the gyromagnetic ratio : this is a constant for each nucleus, but is different for each nucleus
(including different isotopes of the same nucleus !), as well as Bo, the strength of the external applied magnetic field.
The spinning nuclei can resonate with electromagnetic radiation (light), via its fluctuating magnetic field vector.
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The sample is placed as a liquid or dissolved in a solvent in
an NMR tube. We also add a standard compound (e.g.,
TMS : tetramethysilane : Si(CH3)4) to calibrate the NMR
signal (1H, 13C, 29Si) - this is to measure the chemical shift.
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The usual standard for proton (1H) NMR is TMS - tetramethylsilane - (CH3)4Si.
This is a good standard : it is (a) unreactive ; (b) liquid and mixes with common solvents; (c) has a single NMR peak
that (d) occurs at lower frequency than for most protons encountered in organic chemistry (there is a higher electron
density around the protons in TMS due to the highly electron donating Si atom so the shielding constant (σ) is greater
than for most organic -R groups).
The standard (TMS) is then assigned a value of δ = 0.
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