Interpretative Spectroscopy CH507 (MSci) 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.
[4]
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).
[4]
e) Explain the trends in chemical shift and linewidth (Δω1/2) observed in the 95Mo NMR data given
in the table below:
Compound
δ (95Mo) / ppm
Δω1/2 / Hz
-1858
-1850
-1829
-1786
0.14
4.4
7.0
5.8
Mo(CO)6
Mo(CO)5(CNMe)
cis-Mo(CO)4(CNMe)2
fac-Mo(CO)3(CNMe)3
[7]
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
iii)
The effective magnetic moment of (NH4)2[CoCl4] is 4.6 BM at room temperature, and
does not vary appreciably with temperature.
[4,4,5]
Interpretative Spectroscopy CH507 (MSci) 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
Mn2+ in an octahedral ligand field
Ni2+ in a tetrahedral field
[8]
(d) Which of the above ions will also show significant temperature dependence? Give reasons for
your answer.
[6]
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]
(c) Oxidation of [CoII(dppe)2Cl]ClO4 with Cl2 gives a compound with formula [CoIII(dppe)2Cl2]ClO4.
This compound gives a 59Co NMR spectrum which is a very broad singlet (Δω1/2 = 14000 Hz) at δ =
+1660 ppm.
i)
ii)
Explain why no 59Co – 31P coupling is observed.
If the chloride were exchanged for iodide, would you expect the chemical shift to be
more positive or more negative? Give reasons for your answer.
[5]
Interpretative Spectroscopy CH507 (MSci) May 2008
1.
(a) Outline briefly the key fundamentals of Fourier transform NMR spectroscopy, including
comments on precession, relaxation, the FID, the Fourier transform, pulse sequences and the
advantages of multi-pulse operation..
[10]
(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 cover more than 10,000
ppm.
[4]
(d) Explain the reasons for the trends in chemical shift seen for the following two sets of compounds:
(i) [Fe(C5H5)(CO)2X] δ (57Fe)
(ii) [Co(CN)6]3[Co{P(OMe)3}6]3+
[Co(NH3)6]3+
[Co(acac)3]
X = Cl, +1263 ppm
X = Br, + 962 ppm
X = I, + 305 ppm
δ (59Co) = 0 ppm
δ (59Co) = -305 ppm
δ (59Co) = +8100 ppm
δ (59Co) = +12500 ppm
[2,5]
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 six line pattern being observed in the spectrum of an electron coupling to a single
nucleus with nuclear spin, I = 5/2.
[6]
(c) How does anisotropy arise in transition metal complexes and what is its effect on the observed
ESR spectra?
[7]
(d)
(i)What would be the coupling pattern observed in the ESR spectrum of [Co(dppe)2]2+ ?
[DATA: 31P, I = ½ ; 59Co, I = 7/2; dppe = Ph2P(CH2)2PPh2)
(ii) The spectrum shows a tetragonal distortion. What can you deduce about the structure of
the molecule?
[4,4]