Carbazolyl Nitrenium Ion
TABLE 2. Singlet-Triplet Gaps
carbazolyl nitrenium ion. In the previous study, TD-DFT
calculations predicted absorptions at 645, 637, and 647 nm for
the diphenylnitrenium ion, 4,4′-dichlorodiphenylnitrenium ion,
and 4,4′-dibromodiphenylnitrenium ion, respectively, close to
the experimentally found absorptions of 640, 670, and 690 nm.
Also of relevance, TD-DFT predicts absorption bands for the
carbazolyl radical at 643 and 525 nm, in reasonable agreement
with the observed bands at 620 and 570 nm. However, a TD-
DFT calculation (B3LYP/6-311G(d,p)) on the closed-shell
singlet carbazolyl nitrenium ion gives predicted absorption bands
at 249 and 488 nm, in poor agreement with the observed
absorptions at 570 and 620 nm
Structure Number
∆EST (B3LYP/6-31G(d,p))
1
2
3
-2.3, -2.3a
-5.8
-7.8
a B3LYP/6-311G(2d,p)//B3LYP/6-31G(d,p).
to be ground state singlets, with the pyrrolyl nitrenium ion 1
predicted to have the smallest singlet-triplet energy splitting
of -2.3 kcal/mol (a negative value indicates a singlet ground
state). Benzannulation of the pyrrolyl nitrenium ion 1 to obtain
2 and 3, as might be expected, alters the singlet-triplet gap in
favor of the singlet, giving ∆EST values of -5.8 and -7.8 kcal/
mol, respectively (see Table 2).
Electron Configuration of the Observed Nitrenium Ion.
While all of the reactivity of the observed transient is similar
to the reactivity seen for previously characterized closed-shell
n2 nitrenium ions, three observations are suggestive of an n,p
open-shell singlet diradical configuration.
Few reasonable decay pathways can be proposed that are
consistent with these results and the identity of the transient as
the closed-shell n2 singlet nitrenium ion. One possibility is that
the nitrenium ion 3 decays to give a nitrenium ion of a different
electron configuration that has unfavorable absorption properties
or absorption bands outside the detection window of our LFP
spectrometer. For example, the singlet nitrenium ion could
undergo intersystem crossing to give the triplet carbazolyl
nitrenium ion. This possibility is unlikely given that DFT
predicts the closed-shell singlet to be the ground state with a
reasonably large energy gap (7.8 kcal/mol) to the triplet state.
(Further, when DFT errs in predicting the singlet-triplet gap,
it is almost always by overly favoring the triplet state.) Another
possibility is that the closed-shell nitrenium ion decays to give
a different singlet state configuration of 3. This possibility is
also not supported by calculations, however, as both DFT
(B3LYP) and small basis-set CASSCF(10,10)/3-21G calcula-
tions predict the closed-shell singlet state to be the lowest energy
singlet electron configuration. The closed-shell singlet DFT
wavefunction was found to be stable with respect to breaking
orbital symmetry (restricted f unrestricted stability), and
CASSCF predicts that the lowest energy singlet configuration
consists of primarily the n2 singlet state (0.82 determinant
weight). On the other hand, if the observed transient is the n,p
singlet excited state, an obvious unimolecular decay pathway
is relaxation to the n2 singlet ground state. While a lifetime of
∼0.3 µs is unusually long for an excited state, such a
configurational change would be expected to be slow for the
carbazolyl nitrenium ion because the change in orbital angular
momentum resulting from the electron switching from the p to
the n orbital is not similarly compensated by a change in spin
angular momentum. In terms of chemical reactivity, however,
the lifetime of the observed transient nitrenium ion is short,
and product formation could arise from either the excited n,p
singlet state or from the ground (n2) singlet state depending on
the concentration and reactivity of the trap.
Thus, while the product studies, trapping rate constants, and
LFP spectra in the presence of traps leave little doubt that the
observed transient is a singlet nitrenium ion, the specific
electronic configuration of the observed singlet nitrenium ion
is less certain. The absorption spectrum and the first-order decay
kinetics are most consistent with the detection of an excited
n,p singlet state that relaxes to a lower-energy n2 state. However,
the observed reaction products (which may arise from both
states) are not qualitatively distinct from those products formed
from previously characterized n2 singlet diarylnitrenium ions.
Given this consideration, along with our inability to directly
detect any intermediates following the decay of the nitrenium
ion, the current evidence for this transient being the n,p singlet
carbazolyl nitrenium ion should be considered suggestive rather
than definitive.
First, the absorption spectrum of the nitrenium ion transient
is very similar to that of the carbazolyl radical, with both
intermediates having absorption maxima at 570 and 620 nm
((5 nm) in CH3CN (the two species can be distinguished from
each other by the much longer lifetime and more defined 570
nm absorption of the radical). Assuming that the absorption
bands derive principally from excitations of pi electrons, one
would expect that the closed shell singlet carbazolyl nitrenium
ion and the carbazolyl radical would have very different
absorption spectra. On the other hand, this same assumption
leads to the prediction that the open-shell n,p diradical carbazolyl
nitrenium ion would have an absorption spectrum very similar
to the carbazolyl radical as a result of having the same electron
occupation of the pi orbitals.
Second, the decay of the transient appears to be first order
(see Figure 3). While the diphenylnitrenium ion (Ph2NH+) has
a facile unimolecular decay channel in the form of a Nazarov-
like cyclization (τ ) 1.5 µs) to ultimately form carbazole,42 no
such decay channel presents itself for the closed-shell singlet
carbazolyl nitrenium ion. Despite this lack of an obvious decay
channel, the carbazolyl nitrenium ion has a much shorter lifetime
(τ ) 0.3 µs) than Ph2N+ in solution. The lifetime of the
carbazolyl nitrenium ion transient is the same in CH3CN and
CH2Cl2 (suggesting that the decay does not involve solvent)
and is insensitive to the presence of oxygen, laser intensity (and
thus the concentration of nitrenium ions), and concentration of
the precursor 4a. The isolable photoproducts in the absence of
traps were found to be carbazole and collidine (27% yield of
carbazole and ca. 100% yield of collidine after 54% decomposi-
tion of a 0.01 M 4a solution in CH3CN). The remaining
uncharacterized product(s) is an insoluble residue that is
presumably poly(carbazole) oligomers similar to those seen for
diphenylnitrenium ion from reaction of the nitrenium ion with
accumulated photoproducts.4242
Last, while previously reported time-dependent density
functional theory (TD-DFT) calculations of diphenylnitrenium
ion and its halogen-substituted analogs give predicted absorption
band locations in good agreement with those found from
experiment,28 the predicted bands for the carbazolyl nitrenium
ion do not match well with the observed spectrum of the
(42) Kung, A. C.; McIlroy, S.; Falvey, D. E. J. Org. Chem. 2005, 70,
5283-5290.
J. Org. Chem, Vol. 72, No. 22, 2007 8191