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exchange coupling between unpaired electrons localized on different centers. The synthesis and handling
of stable nitroxide multiradicals are relatively well controlled and nowadays these species are used in
many applications.
5.3.1 Electron spin-spin exchange coupling
Nitroxide multiradicals, particularly dinitroxides, have been extensively used to study the dependence of
exchange coupling (J) upon structural parameters. A few representative examples of the many recently
studied species are shown in Scheme 5.12. The diazaadamantane dinitroxide
33
currently holds the transi-
tion temperature record (T
c
48 K) for a fully organic nitroxide-based ferromagnet.
50
As expected based
on symmetry considerations, the intramolecular interaction between the two orthogonal N-O groups is fer-
romagnetic. Intermolecular interactions are also ferromagnetic and a truly three-dimensional ferromagnetic
order is observed at a Curie temperature of 1.48 K.
A series of six trimethylenemethane-type dinitroxides which differ in (
tert
-butylaminoxyl)phenyl ring
torsion angles by virtue of different steric demands of the spacer groups was prepared.
51a
Electron
spin - spin exchange values were obtained from solid state magnetic susceptibility measurements.
51b
=
.
1
The
O
N
N
O
33
50
O
O
O
O
N
N
N
N
Ac
34
51
35
51
O
O
O
N
N
N
MeO
OMe
N
N
O
OMe
O
37
53
36
51
N
N
38
54
O
O
O
N
N
O
OR
RO
RO
N
O
O
N
OR
O
O
N
N
O
N
N
O
40
57
39
55
41
58
Scheme 5.12
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