 |
43 |
|
Let's recapitulate the whole issue step by step :
The chemical shift of a proton can be quantitatively expressed
as:
- the difference in the resonance frequency
Dn between the signal
nS of a standard, usually
tetramethylsilane (TMS) for proton NMR, and the signal
nPr of the proton in
question. In other words
| DnPr = distance
between sample signal and the signal of the TMS reference. |
Some additional comments:
Since the resonance frequency is proportional to the magnetic
field H,
|
n =
(g/2 p)
· Hz, |
|
where n is the resonance frequency,
g the gyromagnetic ratio,
p the constant pi, and
Hz the component of the magnetic field along
the z-axis |
it is common practice to describe the chemical shift not in units of
the magnetic field, but in the frequency unit Hertz (Hz). However, due
to the dependence of DnPr on the
field Hz, chemical shifts described on this scale can't
be compared between spectrometers of different field strengths.
- on the ppm scale according to the following relationship:
| d =
(DnPr ·
106) / n0
ppm |
| (1ppm = 1 · 10-6 (parts per million);
n0 is the basic frequency
of the spectrometer) |
On this scale the magnitude of the chemical shift of any given
proton is independent from the basic frequency
n0.
- in very old spectra on the t-scale
(only for proton resonances, and very uncommon today) according to the
formula:
|
Memorize these relationships, and then give it a shot with
these examples |
|
|
|
|
