Chemistry Reference
In-Depth Information
hand, the predicted triplet state spectrum does not agree well with the experimental
results. Thus, the conclusion that these species have singlet ground states appears
to be verified.
6.8 HETEROARYL NITRENIUM IONS
There are two types of heteroaryl nitrenium ions:
endo
-heteroaryl nitrenium ions,
wherein the formal nitrenium ion center is located on a nitrogen in the (nominally)
aromatic ring and
exo
-heteroaryl nitrenium ions, wherein the nitrenium center is
attached to the aromatic ring. Of the former, the pyrrolidinium, indolenium, and
carbazolenium ion were examined experimentally and computationally. These
species are interesting because the
n
2
singlet states would be formally antiaromatic,
and thus destabilized. This raises the possibility that the triplet state, open-shell
singlet, or the
n
2
singlet might be preferred.
Experiments and calculations on the carbazolenyl nitrenium ion
were, there-
fore, undertaken to determine if antiaromaticity would lead to unusual electronic
behavior.
48
Calculations at the DFT level, however, showed that the singlet state was
the ground state. Computed nucleus-independent chemical shift values (NICS) as
well as thermochemical analysis of isodesmic reactions were strongly suggestive of
significant antiaromatic character for carbazolenium ion.
The possibility of forming an open-shell (
n
,
80
) singlet state, which is implicitly
ignored in single reference DFT calculations, was also considered. To this end,
multireference CASSCF/3-21G calculations were also carried out. At this level,
the ground state is dominated by a single determinant and can be well described as an
n
2
singlet.
LFP experiments, wherein carbazoleniumwas generated from a stable precursor
p
79
, create a transient species that absorbs at 620 nm (Fig. 6.16). This species reacts
rapidly with nucleophiles, as would be expected for a singlet nitrenium ion.
However, two observations suggest that the detected species is not the ground
state,
n
2
singlet. First, its transient spectrum differs significantly with the UV-Vis
spectrum calculated by TD-DFT for the
n
2
singlet state. In contrast, other mono-
N
+
N
+
N
N
+
355 nm
300 ns
1
(
n
,
p
)
80
1
(
n
2
)
79
TDDFT: not determine
Expt: 690 nm (weak)
TDDFT: 530 nm (strong)
Expt: not detected
FIGURE 6.16.
Laser flash photolysis of the pyridinium ion provides a short-lived species
tentatively identified as an excited state (
n
,
p
) singlet state.
