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a carboxylic acid group at the
para
position, radical
7
. The general tendency of carboxylic acids to
form dimers, both in solution and in the solid state, suggested that
7
would be an ideal system to show
whether the propagation of the magnetic exchange is efficient at a long distance as well as to determine
its ferromagnetic or antiferromagnetic nature.
An extensive magnetic study, both in solution and in the solid state, with radical
7
showed that mag-
netic exchange interactions are transmitted efficiently through the hydrogen bonds.
44
Low temperature EPR
experiments performed in solution with radical
7
did not lead to any conclusive results, since supramolec-
ular aggregates are formed by weak non-covalent interactions, like Cl-Cl and
, distinct from the
initially expected hydrogen bonds. In order to overcome this drawback, solid studies with radical
7
were
performed. Radical
7
crystallized in two polymorphic phases, the
π
-
π
phases, depending on the sol-
vent used for the crystallization. As shown in Figure 2.8, in an aprotic solvent, like dichloromethane,
the
α
and
β
phase is favored, in which the radicals form nearly isolated R
2
2
(8) hydrogen bonded dimers, while
in presence of ethanol the solvent molecules are intercalated by hydrogen bonds among the two radi-
cals of dimeric entities pushing them far away. Moreover, the bulkiness of the two radicals prevents, in
both phases, short contacts between the regions with high-spin densities inside each radical subunit of
the dimers.
Solid state magnetic measurements (Figure 2.9) of the two phases of radical
7
provided evidence about
the nature and strengths of magnetic interactions inside the dimeric entities. Magnetic data of the
α
phase
clearly shows the presence of weak ferromagnetic interactions, while such interactions are opposite in the
β
α
β
phase, in which direct
hydrogen bonded radical dimers are not present, gave further evidence that the hydrogen bonded dimers
are the species uniquely responsible for this interaction. Therefore, it has been demonstrated that exchange
through hydrogen bonded bridges can occur at a long distance, even in the cases of well delocalized
radicals.
phase (Figure 2.9). The suppression of the ferromagnetic interactions in the
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
COOH
7
Figure 2.8
View of hydrogen bonded dimers present in the
phase (right) of radical
7
.
(Reprintedwithpermissionfrom[44].Copyright2002Wiley-VCHVerlagGmbH&Co.KGaA.)
α
phase (left) and
β
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