Interpretative Spectroscopy CH407 (BSc) 2006
1. a) Outline the major differences between the NMR spectra of nuclei with spin I = ½ and those
of quadrupolar nuclei.
[8]
b) Explain why the 11B NMR spectrum of BH4- is a sharp quintet, that of Ph3PBH3 is a broad
doublet of quartets and that of BHCl2(thf) is a very broad (barely observable) singlet.
[5]
c) In what circumstances do satellite spectra occur in NMR spectroscopy?
[2]
d) Deduce the form of the 17O NMR spectrum of RuO4. (99Ru has a natural abundance of 13%
and a spin of 5/2).
[5]
2. a) Describe how observed deviations from spin-only behaviour, and variation of magnetic
moments with temperature, can give an understanding of the electronic states of transition
metal complexes.
[12]
b) Explain the following observations:
i)
ii)
The effective magnetic moment of [Ni(PPh3)2Cl2] is approximately 3.4 BM.
The effective magnetic moment of [Fe(Tp)2] is 5.2 BM at room temperature
and < 1 BM below 150 K.
N
N
Tp =
B
N
N
H
N
N
Interpretative Spectroscopy CH407 (BSc) May 2007
1.
(a) Discuss briefly the main classes of bulk magnetic behaviour and rationalise in
each case how the magnetic susceptibility varies with temperature.
[6]
(b) In magnetically dilute paramagnetic materials, what factors cause deviation from
the spin-only magnetic moment at room temperature?
[5]
(c) Deduce which of the following metal ions will show deviations from spin-only
magnetic behaviour. Explain your answer.
i)
ii)
iii)
Co3+ in an octahedral ligand field (High spin)
Mn2+ in an octahedral ligand field (High Spin).
Ni2+ in a tetrahedral field
[9]
2.
(a) Describe the key principles of electron spin resonance (ESR) spectroscopy.
Include comments on the following aspects:
i)
ii)
iii)
iv)
the underlying theory;
hyperfine coupling;
the effects of anisotropy; and
measurement of ESR spectra.
[14]
(b) The compound [CoII(dppe)2Cl] ClO4 has square pyramidal coordination geometry,
with the chloride ligand in the axial site. Deduce the form of the ESR spectrum of this
compound.
DATA: 59Co, I = 7/2, 100% abundance; 31P, I = ½, 100% abundance;
dppe = Ph2P(CH2)2PPh2
[6]
Interpretative Spectroscopy CH407 May 2008
1.
(a) Discuss briefly the importance of the free induction decay (FID) in Fourier
transform NMR spectroscopy. Include comments on relaxation, the Fourier transform,
pulse sequences and the advantages of multi-pulse operation.
[8]
(b) The expression for the maximum peak height of a NMR resonance is:
Smax  | 3 Bo2 N (T2 / T T1) |
(i) Explain the meaning of each of the terms and show which can be ignored
under normal conditions.
(ii) Show how the relative receptivities of different elements can be derived
from this expression.
[2,2]
(c) Explain why 1H NMR chemical shifts typically fall within a narrow range
(approximately +30 to -30 ppm), while for some transition metal nuclei the chemical
shift range can extend over more than 10,000 ppm.
[4]
(d) Place the following RhIII complex ions in order of their 103Rh NMR chemical shift
values:
[RhCl6]3- [Rh(PMe2Ph)3Cl3] [Rh(CN)6]3- [Rh(NH3)6]3+
Give reasons for your choices.
[4]
2.
(a) Describe the interaction of an electron spin with a magnetic field and show how
the ESR phenomenon results.
[4]
(b) State the selection rules which govern transitions between electron spin states and
show how these lead to a three line pattern being observed in the spectrum of an
electron coupling to a single nucleus with nuclear spin, I = 1.
[5]
(c) How does anisotropy arise in transition metal complexes and what is its effect on
the observed ESR spectra?
[7]
(d) The [CuCl4]2- anion can exist as either square planar or tetrahedral isomers,
dependent on the counterion. What would be the difference in the ESR spectra of
these two isomers? (63Cu, I = 3/2).
[4]