Chemistry Reference
In-Depth Information
magnetochemistry, which is again experimentally simple, and also the subject of attention
below.
7.3.1
Peak Performance - Illustrating Selected Physical Methods
There are a number of techniques that rely on applying specific protocols that lead to
excitation and a resonance condition, reported experimentally as the appearance of a peak(s)
in a spectrum by using appropriate instrumentation. It is beyond the scope of this textbook
to develop the theory behind these many methods, but it is appropriate to illustrate two of
the most common techniques - IR and NMR spectroscopy.
NMR relies on change in nuclear spin through absorption of energy in the MHz range,
where each atom of a particular element in a compound in a unique molecular environment
will yield a signal in a unique position, reported as a chemical shift (
). Moreover, atoms may
interact with close neighbours in many cases, to produce a splitting pattern superimposed on
the gross chemical shift that is characteristic of this neighbouring environment. This makes
the technique powerful as a weapon in determining structure, particularly in solution. The
majority of elements can produce an NMR spectrum, but the traditional element targeted
is hydrogen, because
1
H is of high isotopic abundance, has simple spin characteristics, and
the highest sensitivity. Another popular element used,
13
C, has a relative sensitivity of only
∼
1
H, and a significantly lower isotopic abundance (only
1%), so
that its detection is consequently more demanding. Even the metals in complexes can be
examined directly, but suffer from low sensitivities and/or inappropriate isotopic abundance,
as well as other limitations that can lead to very broad peaks and extreme chemical shifts.
Nevertheless, modern NMR instruments offer adequate to excellent determination of spectra
of a vast number of elements, although
1
H,
13
C,
19
F and
31
P remain most commonly available
in commercial instruments, and dominantly the former two are used. This means that, for
coordination complexes, it is usually the organic ligands that are being probed by this
technique rather than the metal or even the ligand heteroatoms. The MRI instrument used
for whole-body scanning in medicine is a type of NMR instrument, but focussed on variation
of the properties of water molecules in different environments as the basis of its operation.
To illustrate the NMR technique very simply, we shall draw on some simple complexes.
The NMR method is most applicable to diamagnetic metal complexes, and we shall re-
strict examples to low-spin d
6
Co(III), which has no unpaired d electrons and is therefore
diamagnetic. To remove strong signals from the solvent H atoms, spectra are routinely
measured in a deuterated solvent in which D atoms replace all H atoms; further, by using
an aprotic solvent like CD
3
CN, any H/D exchange issues met for molecules with readily
exchangeable centres like NH and OH in the common NMR solvent, D
2
O, are re-
moved. If we consider highly symmetrical [Co(NH
3
)
6
]
3
+
, all six ammonia ligands are in
equivalent environments, and so the
1
H NMR spectrum should yield a single peak with one
specific chemical shift. If we turn to [CoCl(NH
3
)
5
]
2
+
, the four ammonia ligands around
the plane of the metal are equivalent, but the one
trans
to the chloride ion is unique, so
two peaks of different chemical shifts should result, in a ratio of 4:1. For [CoCl
2
(NH
3
)
4
]
+
,
the
trans
isomer has all four ammonia ligands equivalent, and thus one peak, whereas
the
cis
isomer has two different types, two opposite chloride ions and two opposite other
ammonia molecules, so two peaks in a ratio of 1:1 will result (Figure 7.1). For
fac-
and
mer-
[CoCl
3
(NH
3
)
3
], the former will yield a single peak, the latter two peaks in a 2:1 ratio.
Thus you may see how molecular symmetry is being defined by the NMR pattern. If we
were to replace the NH
3
ligands by CH
3
NH
2
ligands, we would get two sets of peaks
1.6% compared with
∼
