Chemistry Reference
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(a)
H
H
H
N
N
N
+
+
D
+
D
D
n
2
singlet
n
,
π
* triplet
π
,
π
* triplet
FIGURE 6.11.
(a) Electronic configurations for mata-donor substituted phenylnitrenium ions
(b) Kohn-Sham SOMOs calculated for
meta
-substituted phenyl nitrenium ions.
E
st
values and some selected geometric parameters for several
meta
-substituted nitrenium ions. While the C
Table 6.6 lists the
D
H bond angles vary little among
the various singlet phenyl nitrenium ions, with the triplets they fall into two groups.
Those with strong meta donors (
N
) have more acute bond angles in the range of
110-112
, implying a lone pair on the nitrogen and thus a
49-53
pp
nitrenium ion.
) also has a bond angle of 111
. Those with
Indeed, the neutral anilino radical (PhNH
no
meta
substituents (
34
),
meta
acceptors (
45
), or weak
meta
donors (
46-48
) have
bond angles of 130
-132
, implying a semioccupied
sp
2
orbital.
Further evidence for this picture can be seen in the SOMO densities for the 3,5-
diaminophenyl nitrenium ion, a ground state triplet. In this case, the both of the
SOMOs are antisymmetric with respect to the molecular plane, consistent with the
view that this nitrenium ion is a (
pp
) triplet. In contrast, the 3-fluorophenyl
nitrenium ion (
), a ground state singlet, has one SOMO that is antisymmetric with
respect to the phenyl ring's plane, and the other is symmetric, consistent with a (
n
,
46
p
)
triplet. The orbitals are illustrated in Figure 6.11.
Figure 6.12 shows a qualitative state ordering for phenyl nitrenium ions and how
they change with increasing strength of the
meta
p
donor, going from the left to the
34
right. The parent phenyl nitrenium ion (
), and those substituted with acceptors and
weak donors (F, Me
3
Si, alkyl) have singlet ground states and the lowest energy triplet
is has (
n
,
p
) configuration, as would be typical for most of the aryl series. Those
with moderately donating substituents (vinyl, sulfhydryl, and methoxy) represent
