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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 |