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(a)
(b)
Figure 7.4
(a) SOMO and (b) spin density plot for 1,5-diisopropyl-3-phenyl-6-oxoverdazyl (blue
=
positive
negativespin density).
51
(Reprintedwith permission from [51]. Copyright2007 American
ChemicalSociety.)Afull-colourversionofthisfigureappearsintheColourPlatesectionofthis topic.
spin density,green
=
Table7.2
SummaryofrangesofEPRnitrogenhyperfinecouplingconstantsandDFTcalculated
spindensitiesforverdazyls
Verdazyl general
structure
N1/N5 hyperfine coupling (G)
N 1/N 5 calculated spin density
N2/N4 hyperfine coupling (G)
N 2/N 4 calculated spin density
Ar
Ar
N
NN
N
5.7-6.2
0.24
5.4-6.1
0.16
R
O
R'
R'
5.1-5.4
0.18-0.21
6.4-6.6
0.34-0.40
N
NN
N
R
O
Ar
Ar
4.4-4.5G
—
6.3-6.5G
—
N
NN
N
R
To summarize, verdazyls are delocalized radicals in terms of the unpaired electron distribution
within
the verdazyl ring, but delocalization onto the ring
substituents
is relatively ineffective. This feature of
their electronic structure is manifested in a number of different ways. For example, verdazyl-substituted
cyclophanes
29
and
30
were designed to probe the extent of through-space spin perturbation. Detailed EPR
and NMR analyses indicate no through space (transannular) spin transfer in
29
to the “remote” aromatic
group, a consequence of the small amount of spin density on the N-aromatic substituent comprising the
other half of the cylophane.
77
Analogous cyclophane-bridged diradicals
30
have correspondingly very weak
intramolecular spin - spin interactions,
78
which contrasts the relatively strong through-space interactions in
cyclophane-based triplet dicarbenes, such as
31
.
79
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