Chemistry Reference
In-Depth Information
however, revealed an extremely complex set of
J
i
that defined the exchange parameters of this heterospin
structure.
254
It was found that the above-mentioned compensation terms formed so sophisticated combi-
nations that they remained absolutely hidden without a quantum chemical study. In accordance with the
scheme on Scheme 13.28, only significant values of
J
i
are given:
J
1
=
55 and 88 cm
−
1
,
J
2
=−
290 and
82 cm
−
1
. It can be seen that some of these are com-
parable in magnitude but opposite in sign. Therefore, quantum chemical studies of exchange interactions
in complex multispin systems can be useful for understanding and reconstructing the
370 cm
−
1
,
J
3
=
40 cm
−
1
,
J
4
=
111 cm
−
1
,and
J
5
=−
−
µ
eff
(T) or
χ
T(T)
experimental dependence.
13.6.6 Contrast agents
In magnetic resonance tomography (MRT) - a tool allowing the rapid, non-invasive study of tissues of
living organisms and effective visualization of pathological processes
255
- visualization is based on the
difference between the rates of spin - lattice (T
1
)
proton relaxations in biological tissues.
The different pulse sequences used in MRT afford images, on which the contrast depends on the relaxation
times T
1
or T
2
.
256
However, it is not always possible to reach the required level of visualization even
with modern technology. To amplify the signal from a pathological area, it is possible to use special
compounds capable of penetrating in this area and enhancing contrast on T
1
-orT
2
-weighted images (T1-
VI and T2-VI).
257
These are mainly gadolinium(III) salts with polydentate ligands, such as deprotonated
diethylenetriaminepentaacetic acid (Magnevist, Omniscan)
258
, or sometimes manganese or iron compounds
(Teslascan, Abdoscan). However, despite the high contrast of images obtained when using these samples,
their use can incur certain risks. That is why the paramagnetic metal ion that accelerates spin relaxation is
generally introduced in the form of a coordination compound with a highly dentate organic ligand. These
ligands should annihilate the charge on the metal ion and, additionally, hold the ion tightly in the complex,
because the human organism has a very low tolerance to free transition metal ions.
259
The thermodynamic
stability of the complexes can be increased only due to the chelate effect of polydentate ligands because
after complexation for rare earth elements and high spin Mn
2
+
and Fe
3
+
ions, the stabilization energy
with a crystal field is absent.
260 - 266
However, the natural limitation on the number of coordination sites
restricts the theromodynamic stability of these complexes. In addition, polydentate ligands of the type
of diethylenetriaminepentaacetic acid specially synthesized for these purposes are not metabolites, but
alien objects for living organisms. This fully applies to various complexes of rare earth elements with
nitroxides
267 - 269
(Scheme 13.29).
An alternative approach to the safety of contrast agents is the use of pure organic paramagnets based on
non-toxic stable nitroxides. In this situation, a transition from metal-nitroxide systems to free nitroxides is
and spin - spin (T
2
)
X
O
N
O
N
N
N
O
O
O
Ln
N
N
N
N
N
H
3
C
N
CH
3
N
N
NH
N
X X
[Ln(L
63
)
2
X
3
],
H
3
C
CH
3
O
h
2
-NO
3
,Ln=Y,La,Gd
L
62
X=
Scheme 13.29
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