CH407/CH507 INTERPRETATIVE SPECTROSCOPY
Dr M.D. SPICER
2.4 Examples
2.4.1 Nuclei of Low Abundance: Satellite Spectra
To date you will have mostly dealt with NMR nuclei with natural abundances either close to
100% (1H, 19F, 31P) or with very low natural abundances (e.g. 13C, <1%). Many of the less
commonly used nuclei have natural abundances somewhat less than 100%. This can lead to
satellite spectra.
Consider the NMR spectra of WF6. The 183W spectrum consists of a septet (1:6:15:20:15:6:1)
arising from coupling to the six 19F nuclei (I = ½), as you would expect. However, the 19F
spectrum is different. Tungsten has a number of isotopes, of which only 183W is NMR active
(I = ½). 183W is only 14% abundant, while the remaining 86% of tungsten nuclei are NMR
inactive (I = 0). Consequently, for the 14% of molecules which have 183W nuclei present we
observe a doublet, while for the remaining 86% of molecules the 19F resonance is a singlet.
Thus we see a “triplet” (it’s really a doublet superimposed on a singlet) in a roughly 1:12:1
(ie 7:86:7) ratio. The splitting between the outer lines is the 1J(183W – 19F) coupling constant.
Consider the example below (Figure 2.7), which shows the 1H NMR spectrum of 1,1,2,2tetrabromoethane, Br2HCCHBr2. The central line is the resonance for the molecules which
contain 12C only. Both protons are equivalent giving a singlet. The outer pair of doublets are
the 13C satellites. These arise
from molecules with one 12C
13
and
one
C,
ie
Br2H12C13CHBr2. The 12C has
I = 0, but 13C has I = ½ and so
couples to the protons. The
1 13
J{ C – 1H} couplingis 182
Hz, given by the separation of
the centres of the satellite
doublets. The satellites are
split into doublets by 3J{1H –
1
H} coupling, since the two
protons are now inequivalent.
Figure 2.7 1H NMR of Br2HCCHBr2
Figure 2.8 below shows the 1H NMR spectrum of Si2H6, the silicon analogue of ethane. The
natural abundance of 29Si (I = ½) is 4.7 %. The remaining 95.3 % of silicon is 28Si (I = 0). So
90.82 % of molecules will have two 28Si
atoms ((0.953)2), 8.96 % will have one 28Si
and one 29Si (2 × (0.953 × 0.047)) and the
remaining 0.22% will have two 29Si atoms
((0.047)2). The resonances due to molecules
with two 29Si atoms are too weak to be
observed. The spectrum shows the usual
intense central line arising from 28Si2H6, and
a more complex pattern of satellites. The
large coupling is 1J{29Si – 1H}, giving a
doublet. The doublet is then split into a
quartet by the three protons on the 28Si.
There is also a second set of resonances,
which are mostly obscured by the central
Figure 2.8 1H NMR of Si2H6.
singlet. This arises from coupling of 29Si to
CH407/CH507 INTERPRETATIVE SPECTROSCOPY
Dr M.D. SPICER
the protons on the 28Si (2J{29Si – 1H}). Because the coupling is longer range the J-value is
much smaller (as indicated on the diagram). This doublet is split into quartets by the three
protons on the second silicon. The outermost lines of these resonances can be seen on the
central line. Note that the spectrum arising from 29Si2H6 would be a pair of doublets, with a
large 1J{29Si – 1H} coupling and a small 2J{29Si – 1H} coupling. Note that there is no 1H - 1H
coupling in this case, as all the protons are now equivalent since they are all attached to 29Si.
The next example (Figure 2.3, left) is
a partial proton decoupled 13C NMR
spectrum of Sn(CH2Ph)3Br, showing
only the resonance arising from the
CH2 group. Note that the satellites are
a pair of doublets. In this case this is
not from coupling to protons as
observed in the cases above, but
because there are two NMR active
nuclei of tin which have similar
natural abundances. 117Sn (I = ½) has
an abundance of 7.61 % and 119Sn
(also I = ½) has a natural abundance
of 8.58 %. The inner pair of doublets
is from 117Sn which has the smaller
13
Figure 2.9 C NMR of Sn(CH2Ph)3Br.
magnetogyric ratio ( = -9.578 × 107
rad T-1 s-1) and the outer pair are from 119Sn which has the larger magnetogyric ratio ( = 10.021 × 107 rad T-1 s-1). The ratio of the coupling constants 1J{119Sn-13C}/1J{117Sn-13C}
should be equivalent to the ratio of the magnetogyric ratios of the two isotopes
((119Sn)/(117Sn).
The final example of a satellite
spectrum (Figure 2.10, left) is the 1H
spectrum of GeH4. 73Ge has I = 9/2
and a natural abundance of 7.76 %.
Consequently the central singlet
(from the nuclei with I = 0) is
surrounded by ten equally spaced
lines arising from coupling to the
73
Ge nucleus (No. of lines = 2nI + 1
= (2×1×9/2)+ 1 = 10).
Figure 2.10 1H NMR of GeH4.
2.4.2 Quadrupolar Nuclei
Example 1: 71Ga NMR This series of studies was published by Barber and Taylor
(Polyhedron, 1994, 13(2), 251-260). The 71Ga nucleus is quadrupolar, with I = 5/2 and a
linewidth factor of (See Table 2.2).
The first set of spectra shows the effect of addition of KSCN to an aqueous solution in which
gallium is present as [Ga(OH2)6]3+. The spectra (Figure 2.11) can be explained by an
equilibrium between the long-lived [Ga(OH2)6]3+ and labile thiocyanate complexes:
CH407/CH507 INTERPRETATIVE SPECTROSCOPY
[Ga(OH2)6]3+
NCSH2O
Dr M.D. SPICER
[Ga(NCS)x(OH2)6-x](3-x)+
The sharp peak at δ = 0 ppm is due to
the hexaaquo species, which is
symmetrical (regular octahedral) and
exchanges water slowly, thus having a
narrow linewidth. The peak due to the
thiocyanate species gains in intensity,
becomes broader and shifts in position
as more thiocyanate is added, but it
does not reach a limiting value which
might be attributed to the species
[Ga(NCS)6]3-. At the 1:12 ratio of
Ga:NCS there is less than 1% of Ga
present as [Ga(OH2)6]3+. The gain in
intensity reflects the increasing
concentration (the area under a peak is
proportional to the number of nuclei
present). The shift in peak position
arises from the decreased shielding due
to the anionic thiocycantate ligands.
The breadth of the peaks is due to both
the lower symmetry environment of the
mixed cations, the presence of a
mixture of species and their interconversion which takes place on the
nmr timescale.
Figure 2.11 71Ga NMR spectrum of aqueous Ga3+ with SCN-.
While these aqueous thiocycanate systems are quite difficult to study, reaction of gallium
halides with thiocyanate in organic solvents give much better defined chemistry. Figure 2.12
shows the 71Ga NMR spectrum obtained from a solution prepared by mixing H[GaCl4] and a
MIBK solution of Ga3+ and NH4SCN. It is immediately clear that discreet gallium containing
species are observed (A – D) and that they are not interchanging at a significant rate on the
NMR timescale (linewidths are relatively narrow). The species arise from the following
equilibria:
[GaCl4]-
[GaCl3(NCS)]-
[GaCl2(NCS)2]-
They are identified as shown in the figure caption, the assignment of the mono and bisthiocyanate complexes being confirmed by the coupling to 14N (I = 1, abundance 99%). A
single thiocyanate coupled to 71Ga gives a 1:1:1 triplet with 1J(71Ga – 14N) = 97 Hz, while two
thiocyanates give a 1:2:3:2:1 quintet with 1J(71Ga – 14N) = 130 Hz. The number of lines is
given by the expression 2nI+1, where I is the spin and n is the number of the coupling nuclei.
Coupling constants are normally observed to decrease as the bond between the two coupling
species weakens, which implies that the Ga-N bond is weaker in the mono-substituted
CH407/CH507 INTERPRETATIVE SPECTROSCOPY
Dr M.D. SPICER
species. The area under each peak (known as the integral) is proportional to the concentration
and thus the relative abundance of the various species can be determined, giving an indication
of the relative equilibria.
Figure 2.12: 71Ga NMR spectrum showing gallium chloride/thiocyanate complexes in methyl isobutyl ketone
solvent. A = [GaCl4] -; B = [GaCl3(NCS)] -; C = [GaCl2(NCS)2] -; D = [GaCl(NCS)3] -.
The 14N NMR spectra (Figure 2.13, left) show
the addition of NH4NCS to [GaBr4]-. The
quintet at the right (e) is due to the
symmetrical NH4+ cation and the singlet at ca
δ = 265 ppm (d) is free SCN-. The first gallium
containing species to appear is [GaBr3(NCS)](a), followed by [GaBr2(NCS)2]- (b) and lastly
[GaBr(NCS)3]- (c). In this case, with bromide
as counterion there is no evidence for coupling
between 71Ga and 14N, whereas with the
analogous chloride evidence for coupling is
observed (see inset). Presumably the bromide
complexes, by virtue of the larger anion, are a
little less symmetrical than the chlorides and
the resulting increased quadrupolar relaxation
rate causes the coupling to collapse.
Figure 2.13 (Left) 14N NMR spectra of gallium
halide thiocyantate complexes. a = [GaBr3(NCS)] -;
b = [GaBr2(NCS)2] -; c = [GaBr(NCS)3] -; d = NCS-;
e= NH4+. Inset: [GaCln(NCS)4-n] - complexes
showing extra splitting due to Ga – N coupling.
CH407/CH507 INTERPRETATIVE SPECTROSCOPY
Dr M.D. SPICER
The final spectrum from this study (Figure 2.14, below) again shows the possibility of
identifying multiple species in solution and of gaining information about equilibria. The
spectrum arises from the following set of reactions:
[GaI3(OEt2)] + [GaX4]-
[GaI2X(OEt2)] + [GaX3I]- etc
This results in a series of tetrahalide anions, GaI4- (C), GaBr4- (B) and GaCl4- (A) and all the
mixed ions as well (intermediate peaks). These are the sharp peaks in the spectrum. The
broad peaks correspond to the etherate complexes. The quadrupolar broadening arises from
the asymmetric X3O coordination sphere around the gallium. Since all the peaks are seen for
the individual species, the exchange which takes place is slow on the NMR timescale.
Figure 2.14
71
Ga NMR spectrum of GaX4- anions and etherate complexes[ GaX3(OEt2)].
Example 2: 63Cu NMR
This example concerns the 63Cu (I = 3/2 , abundance = 65%. NB 65Cu also has I = 3/2,
natural abundance 35%) and 31P (I = ½, 100%) NMR spectra of a series of copper(I)
tetraphosphine complexes. These are prepared from the reaction below:
[Cu(NCMe)4]BF4 + 4 PR3 → [Cu(PR3)4]BF4
As well as using 4 monodentate phosphine ligands, two didentate ligands, such as
Me2P(CH2)2PMe2 or o-C6H4(PR2)2 can also be used. All of these complexes have copper in
the +1 oxidation state (3d10 outer electron configuration) and adopt tetrahedral (or pseudotetrahedral) structures. Because of the electron configuration, the complexes are extremely
labile and thus ligand exchange is often rapid:
[Cu(PR3)4]+
↔
[Cu(PR3)3]+ + PR3
Consider the three sets of 31P and 63Cu spectra below (Figure 2.15) taken at temperatures
varying from room temperature (300K) down to 175 K.
CH407/CH507 INTERPRETATIVE SPECTROSCOPY
Figure 2.15
Dr M.D. SPICER
31
P and 63Cu NMR spectra of [Cu(PhPH2)4] + (left), [Cu{Me2P(CH2)2PMe2}2] + (centre) and
[Cu{o-C6H4(PMe2)2}2] + (right).
In the first set of spectra all that is seen at room temperature is a broad resonance in both the
31
P and 63Cu spectra. This is indicative of fast phosphine exchange on the nmr timescale. No
coupling is observed, and the broad peaks arise from the average of different species present
in solution. As the sample is cooled the spectrum sharpens and coupling between the nuclei
becomes apparent. The 31P spectrum is a 1:1:1:1 quartet, arising from coupling to the one
63
Cu atom (I = 3/2), while the 63Cu spectrum is a 1:4:6:4:1 quintet due to coupling to four 31P
nuclei. The “optimum” spectrum is observed at about 250 K. Below this temperature the
spectrum begins to broaden once more. This is due to the effects of quadrupolar broadening.
As the temperature decreases the viscosity increases and the rate of molecular tumbling
slows. The result is an increase in correlation time and a concomitant increase in linewidth.
Similar spectra are also observed for the complexes with didentate ligands. The
Me2P(CH2)2PMe2 complex shows an ideal spectrum at room temperature. Presumably the
effect of chelation is to slow the rate of phosphine exchange significantly to the extent that
exchange broadening is not observed. However, the spectrum does broaden as the
temperature decreases, again due to quadrupolar effects. The final spectrum with oC6H4(PMe2)2 as the ligand again shows coupling at room temperature, but is already
beginning to exhibit quadrupolar broadening. In this case it is likely that the more rigid
backbone causes some distortion from regular tetrahedral geometry, increasing the electric
field gradient and thus broadening the spectrum.
The three 63Cu spectra on the left are
from [CuL4]+, where L = PPh2H (a),
AsPh3 (b) and SbPh3 (c). Note that no
spectrum can be observed from the PPh3
complex – presumably the exchange
equilibrium lie to the right, so that little
or no [Cu(PPh3)4]+ exists in solution. The
linewidths or the resonances correspond
to the rate of exchange – greatest in the
phosphine to least in the stibine (SbPh3).
CH407/CH507 INTERPRETATIVE SPECTROSCOPY
Dr M.D. SPICER
This in turn arises from the steric properties of the ligands. The cone angles of the ligands
decrease in the order PPh3 > AsPh3 > SbPh3. This is because the metal to donor distance
increases and so the angle subtended at the metal centre decreases. The result is that the steric
effect of the ligand decreases. The smaller the steric effect, the more the exchange
equilibrium lies to the left hand side and the slower the exchange. This results in
progressively narrower lines in the spectrum.
The last thing to note is that
the chemical shifts in 63Cu
NMR
are
somewhat
dependent on the ligand
types. Data are fairly limited,
but that which is know is
summarised in the chart on
the left. The bunching of
ligand types presumably
corresponds to the electron
donor ability of the ligands and their influence on the shielding of the nucleus. It is notable
that the σ-donor, π-donor ligands, X-, are most shielded and the best π-acceptors (isonitriles)
are the most deshielded.
Example 3: 59Co NMR
The 59Co nucleus has spin I = 7/2 and a relatively large linewidth factor. Consequently
linewidths can be very large (as much as 10000 Hz!). However, to compensate for this it has
a very large chemical shift range (ca -4000 to + 17000 ppm). Furthermore, there is a strong
correlation between the donor atoms around the metal centre and the chemical shift. This
arises from the large paramagnetic contribution to the shielding. Most 59Co NMR is from
cobalt(III) complexes which have a low spin 3d6 electron configuration in the ground state.
The paramagnetic contribution arises from the presence of low-lying paramagnetic excited
states.
Consider the simplistic diagram on the left. An
electron is readily promoted to the higher
h
energy set of orbitals, and the proportion of the
complexes in the excited state will increase as
the splitting energy decreases, in turn
increasing the chemical shift (ie σp  1/E).
diamagnetic
paramagnetic Since it is the ligands which determine the size
ground state
excited state of the orbital splitting (remember the
spectrochemical series) it follows that the
chemical shifts will depend very strongly on the donor type of the ligand. Consequently,
strong field ligands like CN- and phosphines, which cause large orbital splitting, come at
relatively low chemical shifts, while weak field ligands such as water and amines cause much
smaller orbital splittings and thus have much higher chemical shift values. The chart on the
following page (left) shows the observed chemical shift ranges for a variety of different donor
sets (listed down the left hand side).
It is also possible to plot NMR chemical shift (δ 59Co) against the wavelength of the lowest
energy transition in the UV-visible spectrum (which corresponds to E given as λ (m)
where m = nm) as shown in the figure below (right). The points for particular donor sets
cluster together because ligands with the same donor atoms will give similar splitting of the
CH407/CH507 INTERPRETATIVE SPECTROSCOPY
Dr M.D. SPICER
d-orbitals. The line for the first period donor atoms has a different slope to that for the second
period donor atoms. This arises from the <r--3> term in the expression for σp (see theory
section of notes). This term is the average inverse cube of the distance of the valence pelectron from the central nucleus. This is in turn affected by the metal to donor atom distance
(which is greater for P, S and Se donors). It should be noted that the effect of the
paramagnetic shielding term is not only restricted to quadrupolar nuclei. Any species with
low energy paramagnetic excited states can exhibit this effect. Thus 103Rh (I = ½ ) also has a
very wide chemical shift range for the same reason.
Finally, consider the table of data below. This illustrates once more the inverse relationship
between chemical shift values and the wavelength of the lowest energy electronic transition
and also emphasizes the sensitivity of many quadrupolar nuclei to small changes in
symmetry. Thus, while all of the complexes ostensibly have regular octahedral geometry, it
can be seen that the
linewidths vary considerably.
The two wholly symmetrical
ligands give very narrow
linewidths (for 59Co NMR!)
and exhibit couping to the six
31
P nuclei. However, small
deviations from regular cubic
symmetry, even from the
asymmetry of the ligand, are
enough to increase the
electric field gradient at the
nucleus and therefore result in
faster quadrupolar relaxation
with the consequent increase
in linewidth and loss of
coupling.