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In-Depth Information
O
HN
NH
NH
N
O
N
O
4-oxo-TEMPO
Porphyrexide
1
2
1) H
3
O
+
2) Ag
2
O
+
N
MgBr
N
O
O
N
OMgBr
(H. Wieland
et al.
5
)
OMe
OMe
MeO
MeO
PhCO
3
H
N
O
N
H
(K. Meyer
et al.
6
)
Scheme 5.1
R
2
R
2
R
3
R
3
R
1
R
1
R
4
R
4
N
O
O
N
O
N
N
O
N
N
O
O
(A)
(B)
R
5
R
5
(a)
(b)
Scheme 5.2
Principal mesomeric structures of a nitroxide (left) and a nitronyl nitroxide (right)
5.2 Nitroxide structure
5.2.1 Characteristics of the aminoxyl group
The aminoxyl group of nitroxides is characterized by a
π
N
−
O
three-electron bond resulting from the overlap
of the 2p
z
orbitals of the nitrogen and oxygen atoms, and in terms of valence bond theory the contribution
of the two main mesomeric structures, (A) and (B), to the actual electronic structure must be considered
(Scheme 5.2). As a result of this three-electron N-O
system, the N-O bond of an aminoxyl group has
a bond order of 1.5, as indicated by the bond energy of about 100 kcal/mol, and the bond length d
NO
(1.25 A
π
1.30 A), which is midway between the energy and the bond length of a N-OH single
<
d
NO
<
20 A)
11d,13
.
The spin density is mainly distributed on the nitrogen and oxygen atoms, being slightly higher on the
latter. The spin distribution depends on the degree of pyramidalization of the nitrogen atom and on the
polarity of the embedding medium.
From O-H bond dissociation energies in hydroxylamines, the gain in energy from the delocalization of
the unpaired electron between the nitrogen and oxygen atoms amounts to 30 kcal/mol.
11d,14
43 A) and a N
=
O double bond (145 kcal/mol;
bond (53 kcal/mol;
∼
1
.
∼
1
.
A lower value
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