Environmental Engineering Reference
In-Depth Information
fi eld for each nucleus will vary. As a result, each
nucleus in an atom may have a slightly different reso-
nance frequency. This is called the chemical shift
phenomenon. Thus, the two kinds of carbon in an
ethanol molecule (CH
3
CH
2
OH) differ because the
CH
3
and CH
2
carbons have different chemical
environments and therefore resonate at different
frequencies. This frequency difference increases with
increasing strength of the magnetic fi eld;, conse-
quently, it is diffi cult to compare NMR spectra taken
on spectrometers operating at different fi eld strengths.
To overcome this problem, it is desirable to have a
parameter that is independent of the magnetic fi eld
to be able to use different machines. This parameter
is the chemical shift (
virtually the same place in the spectrum as that of
TMS (Skoog
et al.
1998).
The determination of chemical shift is the principal
application of NMR by which structural information
is obtained in geochemistry, because the chemical
shift is a very precise metric of the chemical environ-
ment around a nucleus. As an example, the hydrogen
chemical shift in CH
3
F is higher than that of CH
3
Cl,
because a more electronegative group (such as F)
attached to the CH system is more effective at with-
drawing electrons from the methyl protons, causing
deshielding and consequently increasing
. Shielding
decreases with increasing electronegativity of adja-
cent groups, if other infl uences are not present.
The interaction of the magnetic fi eld of a nucleus
with the magnetic fi eld of immediately adjacent
nuclei gives rise to the splitting of chemical shift
peaks. This effect is called spin-spin coupling and,
in general, is observable if the distance between these
two nuclei is less than or equal to three bond lengths.
This coupling occurs by interactions between the
nuclei and the bonding electrons. A nucleus without
coupling produces a single sharp peak characteristic
of isotropicity. If the same nucleus is in a molecule
where it experiences spin-spin coupling with a neigh-
boring nucleus, the NMR spectrum presents the
splitting of this line as two absorption lines. The
spin-spin coupling effect is a good tool for investi-
gating stereochemical relations (McGregor 1997).
δ
), defi ned as the difference
between the resonant frequency (
v
) of a nucleus in
one type of chemical environment and that of a refer-
ence nucleus (
v
ref
), divided by the spectrometer fre-
quency. This dimensionless quantity is expressed in
parts per million (ppm) because the frequency of the
spectrometer is usually in the megahertz range,
whereas the chemical shift range is in the hertz or
kilohertz range (McGregor 1997),
δ
δ
(
ppm
) =−
(
vv
) ×
10
6
v
(
Hz
)
(15)
ref
spectrometer
The chemical shift is a molecular parameter that
is dependent only on sample conditions (solvent,
concentration, temperature) and not the spectrome-
ter frequency.
In
1
H,
13
C, and
29
Si NMR spectroscopy, the refer-
ence standard is often tetramethysilane, Si(CH
3
)
4
,
abbreviated to TMS (Wilson 1987). TMS is inert,
readily soluble in most organic liquids, and its hydro-
gen atoms are more shielded than almost all other
hydrogen atoms in organic compounds, providing
agreed-upon chemical shift scales for all spectrome-
ters. In addition, TMS is easily removed from samples
by distillation (boiling point 27 °C). Moreover, TMS
is a symmetric molecule, as all the protons are identi-
cal, so it produces only one sharp, strong absorption
signal (Silverstein
et al.
2005). Therefore, the stand-
ard for protons is the resonance frequency of
1
H and
13
C in Si(CH
3
)
4
and for
31
P it is H
3
PO
4
(aq) at 85%
(Atkins 1994). For other nuclei, other standards are
adopted. However, TMS is not soluble in water, so
if using an aqueous solution, it is usually replaced by
the sodium salt of 2,2-dimethyl-2-silapentane-5-
sulfonic acid, (CH
3
)
3
SiCH
3
CH
2
CH
2
SO
3
Na, because
the methyl protons of this salt produce a peak at
3.4.3 Solid-state nuclear magnetic
resonance
Compared with liquid-state samples, solid-state
samples present additional problems for NMR spec-
troscopy. With a powder sample a broad signal is
observed, corresponding to all the possible orienta-
tions of the molecule with respect to the axis of the
applied fi eld
B
o
(and the chemical shifts related to
each of these orientations). Therefore, the solid-state
sample presents chemical shift anisotropy (CSA),
namely a non-uniform chemical shift along the
sample, owing to the directional dependence of elec-
tronic shielding in the molecule.
Another factor responsible for the broad lines
observed with a solid-state sample is the dipole-
dipole interaction, which arises from energy levels
shifted slightly by local fi elds around the nucleus,
derived from neighbouring nuclei (dipolar coupling).
