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Figure3.3
MolecularstructuresandspindensitydistributionsofTEMPO,phenyl-substituted
-nitronylnitroxide,
and phenalenyl (
PLY
) UBLYP/6-31G**//UBLYP/6-31G** level of theory. Black and gray regions denote positive
andnegativespindensities,respectively.
α
features of TEMPO and
-nitronylnitroxide are characterized by the heteroatomic spin center of N-O
moieties, where most of the spin densities are localized (“spin localized nature”), with adjacent tertiary
carbon substituents. In sharp contrast,
PLY
possesses a planar, rigid structure, where the spin spreads over
the whole molecular skeleton (“spin delocalized nature”). As can be interpreted by resonance structures,
most of the spin density of
PLY
exists on its
α
positions (Figure 3.4a). This feature is well corrobo-
rated by the distribution of the non-bonding molecular orbital (NBMO) obtained from the H uckel - MO
method. Furthermore,
PLY
exhibits a high amphoteric redox ability deduced by the NBMO, and pro-
vides corresponding anion, neutral radical, and cation species with thermodynamically high stabilities
(Figure 3.4c). The sum of the oxidation potential (
E
ox
α
and reduction potential (
E
red
)
)
(obtained from
cyclic voltammetry (CV) experiments in solution),
E
sum
E
ox
E
red
, represents the amphotericity
of a given molecule. This value is generally used as a convenient experimental measure for evalua-
tion of a HOMO - LUMO (highest occupied molecular orbital - lowest unoccupied molecular orbital) gap
for closed shell molecules, but provides an estimate of coulombic repulsion between two electrons on
the same orbital for open shell molecules.
PLY
has an
E
sum
=
+
(
−
)
value of 1.6 V, which is relatively small
among hydrocarbons.
All of these features of the phenalenyl system have been attracting special attention in the fields of
physical chemistry. Notably, Haddon claimed, in 1975, the potential use of
PLY
as a component for
molecule-based conductors.
9
However, since the kinetic instability of
PLY
causes an immediate dimeriza-
tion by intermolecular
-bond formation as well as reactivity towards oxygen, all experimental effort to
isolate the pristine
PLY
as solid state has been in vain (Figure 3.4d).
10
Thus, phenalenyl chemistry for the
past a half-century has been limited mainly to solution studies under degassed and sealed conditions.
σ
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