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which are monomeric in the solid state.
117,118
The variety of structure types, and lack of correlation with
the nature of the substituent R (aside from steric barriers to adopting the
cis
-cofacial structure), have been
ascribed to the small differences in dimerization enthalpies between the various isomers,
88
coupled with
the fact that the dimers are weakly bound to start with (see below). It is generally understood that inter-
molecular packing effects play a strong role in determining what (if any) dimeric structure is obtained for
a particular derivative. In fact, some derivatives (R
3-cyanopenyl,
92
Cl,
106,107,110
H
104,105
) adopt more
than one kind of structure (i.e., polymorphism), and one compound - a diradical (see below) - has two
different dimerization modes within the same single crystalline phase.
119
The
solution
association thermodynamics of two dithiadiazolyl radicals has been studied quantitatively
using variable temperature EPR. The radical dimerization enthalpies for R
=
phenyl
33
t
-butyl
120
=
and R
=
are quite similar (
7.4 kcal/mol, respectively) despite the fact that they likely adopt
different dimeric structures: the probable
cis
-cofacial geometry known for the phenyl derivative in the
solid state
98
cannot be adopted by the
t
-butyl compound because of the steric bulk of the butyl group.
Several 1,2,3,5-dithiadiazolyl-based di- and triradicals have also been prepared. The very low solubility
of these materials, coupled with their tendency to dimerize in the solid state and solution, has prevented
examination of the intramolecular spin alignment in these radicals. The electronic state energetics of
diradical
39
,
121
in which the two CN
2
S
2
rings are directly attached to one another at the carbon atoms,
have been investigated using computational methods.
122
The aforementioned nodal plane present in the
dithiadiazolyl SOMO (
38
) prevents direct (conjugative) coupling of the spins. As such,
39
has two half-
filled molecular orbitals
40a
and
40b
, which are simply the linear combinations of the SOMOs on each
ring. Calculations on this diradical indicate that it has an open shell singlet ground state, with a triplet state
only 0.5 - 1.0 kcal/mol higher in energy, that is, the unpaired electrons are very weakly interacting. These
computational studies, coupled with the experimental carbon-carbon bond length of 1.49 A, indicate that
putative resonance structure
41
(a possible representation of a closed shell formulation) is not an accurate
representation of the structure of this molecule. All of the other dithiadiazolyl diradicals have a 'spacer'
group between the two rings (i.e.,
42
), and intramolecular spin interactions are expected to be even weaker
than for
39
.
−
8.4 kcal/mol and
−
N
N
N
N
N
N
S
S
S
S
S
S
S
S
S
40a
X
S
S
S
N
N
N
N
N
N
39
41
42
40b
Solid state structures have been reported for nearly a dozen 1,2,3,5-dithiadiazolyl-based di/triradicals.
Most adopt
cis
-cofacial dimeric arrangements at each ring, but the presence of two (or three) radicals on
each molecule has consequences for the solid state assemblies. Discrete dimers are often obtained, as in, for
example, the structure of the 1,4-phenylene-bridged diradical
123
(Figure 9.7a) among others.
102,121,124,125
However, for a few derivatives (the 1,3-phenylene-
126
and 5-cyano-1,3-phenylene-
127
bridged diradicals
and the 1,3,5-phenylene-
128
bridged triradical) each of the radicals within a molecule associate with
dif-
ferent
neighboring molecules, creating polymeric chains (Figure 9.7b). Two diradical derivatives remain
unassociated in the solid state. The 5-
t
-butyl-1,3-phenylene-bridged bis(dithiadiazolyl) cannot adopt a cofa-
cial structure because of the
t
-butyl substituent; instead an antiparallel, but more regularly spaced, stacked
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