Chemistry Reference
In-Depth Information
magnets. To overcome this, all chemical shift values are measured relative to
an internal standard (usually TMS) and expressed as 'chemical shift', repre-
sented by the dimensionless term d, the Greek letter delta. Mathematically,
this can be written as
shift observed
———————---
Chemical shift, d
10
6
oscillator frequency
The factor of 10
6
is introduced to give convenient, easily remembered values
and this gives rise to the term 'ppm' as the unit for chemical shifts. This use
of ppm should not be confused with its established use as a unit of concen-
tration (lgml
-1
or lgg
-1
) and arises because most resonances occur within
ten millionths of the available field, or within 10 units from TMS.
When an atom is placed in a magnetic field, electrons start to circulate
around the nucleus and this circulation induces a second, local magnetic
field which opposes the applied external field. This process is called
'shielding'; the electron density around the nucleus increases and the
nucleus will appear to resonate at high field position ('upfield' or at the
right-hand side of a conventional spectrum). If a nucleus is attached to an
electronegative atom such as oxygen or nitrogen, the inductive effect of the
electronegative atom reduces the electron density around the nucleus and
the atom is said to be 'deshielded' and will resonate to the left or 'down-
field'. A table of
common proton chemical shift values is shown in
Table 7.1.
Integration
In proton NMR spectroscopy, after the spectrum has been run, it is usual to
change to 'integration' mode and run the spectrum a second time. This
gives rise to a second trace (often a different colour is used) where the trace
jumps as it goes over a peak by an amount proportional to the number of
hydrogen atoms causing the signal. Strictly speaking, the integration gives
the
ratio
of hydrogen atoms responsible for the signal (i.e. the integration
for two signals consisting of 3 and 2 protons would be the same as for 6 and
4), although this is not usually a problem as the total number of hydrogen
atoms in the molecule may be known from another technique (such as mass
spectrometry).
Multiplicity and spin-spin coupling
In NMR, the signal due to the resonance of a particular nucleus is often
split into a number of peaks. This is termed the 'multiplicity' of the signal
