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R'
R"
R'
R"
N
NN
N
N
N
N
N
(b)
R
(a)
R
R'
R"
R'
R"
N
NN
N
N
N
N
N
(d)
(c)
R
R
Figure7.3
ResonancestructuresforKuhnverdazyls
6
.
donating or electron withdrawing aromatic substituents on the nitrogen atoms show predictable changes
in the hyperfine couplings: more electron poor groups (e.g., nitrophenyl) have relatively smaller hyperfine
couplings to the N1/N5 pair as the electron withdrawing groups compete for the nitrogen's lone pair.
12,15,20
The EPR spectra of 6-oxoverdazyls
7
and 6-thioxoverdazyls
8
differ in subtle but systematic ways
from those of the Kuhn verdazyls
6
.For
7
and
8
the N1/N5 constants are consistently smaller than
N2/N4; typical values for N1/N5 are 5.1 - 5.4 G if R
/R
4.5 G if the R
/R
aryl; the
N2/N4 constants are consistently 6.3 - 6.6 G.
42,47
The differences between these and the Kuhn radicals can
be understood in the context of the resonance structures of Figure 7.3 as applied to the 6-oxoverdazyl
framework: the electron withdrawing carbonyl group adjacent to N1/N5 competes for electron density
from these atoms, thereby decreasing the relative importance of resonance structures
c
and
d
(compared to
a
and
b
). From a practical perspective, the EPR spectra of 1,5-dialkyl-6-oxoverdazyls are rendered more
complex by additional hyperfine coupling: alkyl group
=
alkyl and
∼
=
α
-protons hfc's of between 5.2 - 5.8 G, 2.9 - 3.0 G,
and 1.2 - 1.5 G are typical ranges for R
=
R
=
methyl, CH
2
Ph, and isopropyl, respectively.
Verdazyl radicals have been subjected to a large number of computational investigations. Kuhn verdazyls
6
have been studied computationally, mainly with semi-empirical (McLcachlan,
15,67,69
INDO/PNDO
70
)
methods, although a more recent density functional theory (DFT) study has been reported.
71
In con-
trast, calculations on 6-oxoverdazyl derivatives have been principally based on DFT or hybrid (HF/DFT)
methods,
51,56,72 - 75
and occasionally multiconfigurational methods (CASSCF) have been employed.
75,76
In
general terms, the unpaired electron in all verdazyl types is part of a 7
-electron system delocalized over
the N1-N2-C3-N4-C5 portion of the heterocycle; C6 is not formally conjugated because it is either a
saturated carbon (in
6
) or part of an exocyclic double bond (
7, 8
). The singly occupied molecular orbital
(SOMO) in all cases is a
π
* orbital based on the four nitrogen atoms (Figure 7.4a shows the SOMO
for a derivative of
6
with R
π
isopropyl)
51
. One of the nodal planes of this
orbital passes through both C3 and C6, which consequently prevents direct conjugative overlap with any
π
phenyl and R
R
=
=
=
framework which may exist on the C3 substituent. However, spin polarization effects (i.e., electron
correlation) produce a small amount of spin density on C3 (as well as N1 and N5 when appropriate) sub-
stituents. The trends in calculated spin density distributions for the Kuhn and 6-oxoverdazyls corroborate
the experimental nitrogen hyperfine coupling constants (Table 7.2). Overall, the spin density is more or
less equally shared by the four nitrogen atoms in the Kuhn verdazyls, with slightly higher spin found on
N1/N5. In contrast, for the oxoverdazyls the N2/N4 pair carries substantially more spin than N1/N5. In all
cases the four ring nitrogen atoms carry the majority of the spin density.
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