CH405/CH505 Interpretative Spectroscopy
3.4 Examples
3.4.1 An Organic Radical, C6H6-˙.
The benzene radical anion, C6H6-˙, is generated by reacting
benzene with an alkali metal. In the ESR spectrum it has a
symmetrical seven line pattern, as shown in the figure (right).
This implies coupling to all six hydrogen nuclei (2nI+1 lines, n=6,
I=½), which in turn implies that the unpaired electron is
delocalised over all six carbon atoms of the molecule. Given that
the LUMO of benzene is a π-orbital, this is not unreasonable.
3.4.2 A Main group radical, (MeO)3PBH2˙ .
The ESR spectrum of (MeO)3PBH2˙ is shown in
the figure (left), together with a coupling tree,
which helps to explain the various couplings
which occur. The largest coupling is to
phosphorus (31P, I= ½), which gives a doublet.
This doublet is further split by the two
borohydride hydrogen nuclei (I = ½) to give
triplets of intensity 1:2:1. Finally, we have to
consider the splitting by the boron nuclei. Boron
has two isotopes, 10B (20%, I = 3) and 11B
(80%, I = 3/2). The major influence on the
spectrum is the 11B, which splits each line into a
1:1:1:1 quartet. Note however, that the
intensities of the splitting by H (1:2:1) are
carried through, so the quartets deriving from
the central line of each triplet have twice the
intensity of those deriving from the outer lines
of the triplets. It can be seen that the
experimental spectrum matches up very well to
the theoretical splitting diagram. Also note that
the very weak bands in the baseline arise from
species with 10B present.
3.4.3 Some Transition Metal Examples
The main use of ESR spectroscopy is in the study of paramagnetic transition metal
complexes. Of course, in these compounds the use of NMR spectroscopy is not possible,
and so any technique which will give information on the compound is useful. In this
respect, ESR fits the bill.
CH405/CH505 Interpretative Spectroscopy
Example 1
The first example is the ESR spectrum of [V(O)(OH2)5]2- . 51V is 100%
abundant and has a spin, I = 7/2. Vanadium(IV) has a 3d1 electron
configuration and hence one unpaired electron, making it ideal for ESR
spectroscopy (NB systems with more than one unpaired electron
generally give rise to complex spectra which are very hard to interpret).
The unpaired electron couples only to 51V (16O has I =0), which would be expected to give
an eight line pattern. However, what is observed is two overlapping eight line patterns,
which arise from the fact that one of the axes in the complex is different to the other two,
resulting in a tetragonal spectrum, with a parallel and perpendicular component. These are
labelled in the figure, though not all of the lines are resolved due to overlaps in the central
part of the spectrum. The parallel component has a larger hyperfine coupling constant than
the perpendicular component, and its g-value is at a slightly higher field.
The next example (below) is the spectrum of [V(O){S 2P(Et)(OMe)}2]. Again we see an
eight line pattern due to coupling to vanadium. In this case, however, each line is s plit into
CH405/CH505 Interpretative Spectroscopy
a triplet by coupling to the two phosphorus nuclei (100%, I = ½). Although the phosphorus
atom is not bonded to the vanadium, the unpaired electron is able to couple (more weakly)
to it, probably by some delocalisation around the VSPS chelate ring. Because the electron
is located primarily on the vanadium centre, the hyperfine coupling constant to phosphorus
is smaller than for the coupling to V.
The next example is the ESR spectrum of a solution of [Cu(acac)2]. While in the solid state
this has an aggregated structure, in solution it is probably a simple square planar complex.
This assignment is supported by the tetragonal spectrum observed, which implies one
unique axis, and two axes the same. In this spectrum we only see coupling to copper,
since the most common isotope of oxygen (16O) has a nuclear spin of zero. Copper has
two isotopes, 63Cu and 65Cu, both of which have spin I = 3/2. The group of more intense
peaks at the right of the spectrum are the g component (the peak labelled X is a spinforbidden band). The less intense peaks at the left are the g component. The hyperfine
coupling constant is much larger for g than for g and consequently on the outer line (far
left) the coupling due to the two isotopes of copper is resolved – the splitting “tree” shows
one isotope has a smaller splitting than the other (since the magnetogyric ratio γ is
different). The big difference in g-value and coupling constant for the two components
arise from the fact that the unpaired electron is probably in the d z2 orbital which lies along
(parallel to) the unique axis.
CH405/CH505 Interpretative Spectroscopy
The final example concerns the ESR spectrum of a single crystal of a copper(II) complex
of a Schiff’s base ligand, shown on the right. The first spectrum is of the crystal orientation
which gives the best peak separation and hence clearest spectrum.
What we see is coupling of the unpaired electron to the copper nucleus (I = 3/2) to give a
four line pattern with a large hyperfine coupling constant (A Cu on coupling tree on the
diagram above). Each line is then further split into a quintet by interaction with the two
nitrogen nuclei (14N: I = 1, abundance >99%). There are 2nI + 1 lines (n = 2, I = 1). The
overall spectrum may be described as a quartet of quintets.
Again the splitting due to the metal is large because the electron is located in an orbital
which is primarily of metal character. The splitting due to nitrogen is much smaller because
although the orbitals on nitrogen mix to an extent with those on the metal, the electron
spends most of its time near the metal nucleus.
The final spectrum shows the effect of rotating the crystal through an angle of 100º. The
hyperfine coupling to copper decreases very significantly. This is because the orientation
of the orbital containing the unpaired electron changes with respect to the magnetic field
direction hence altering the magnitude of the interaction. The result is overlap of the lines
and a spectrum which is qualitatively more difficult to interpret. However, data of this type,
when combined with
quantum
chemical
calculations is a very
powerful technique for
understanding
the
electron distribution in
molecules.