Chemistry Reference
In-Depth Information
1.3.5 Phenyl Nitrenium Cation
Although product studies indicate that parent singlet phenyl nitrene (
1 ns,
organic solvents) can be protonated in acidic aqueous solution, nanosecond time-
resolved laser flash photolysis studies failed to produce the transient spectrum of
phenyl nitrenium cation (PhNH
þ
).
23,79
This simplest arylnitrenium cation PhNH
þ
has been investigated theoretically. Using density functional theory (DFT), Cramer
et al. predicts that parent phenyl nitrenium cation has a singlet ground state that is
favored by 21.2 kcal/mol over the lowest-energy triplet state.
80
In addition, a number
of substituted arylnitrenium cations have been studied by LFP techniques with
nanosecond time-resolved UV-Vis,
21-24,63-66
IR,
67,68
and Raman
25,81,82
spectros-
copy. However, to our knowledge, spectroscopic features of the parent system
PhNH
þ
remain elusive. McClelland estimated the lifetime of PhNH
þ
is 125-
240 ps in aqueous solution based on an azide trapping experiment.
79
The time
resolution of a conventional ns-LFP system is not sufficient to observe such a short-
lived reactive intermediate. Thus, ultrafast time-resolved spectroscopy was required
to study phenyl nitrenium cation in solution.
t
1M H
+
H
2
O
H
HO
hv
1
N
NH
N
3
NH
-N
2
τ
= 125-240 ps
Acetonitrile-water
Ultrafast photolysis of phenyl azide (PhN
3
) in 100% formic acid with a 300 fs pulse
of 308 nm light results in the spectral changes shown in Figure 1.14, where artifacts
due to solvent two photo absorption have been subtracted.
57
A broad transient
absorption, centered at 520 nm, is detected at the earliest times observable (Fig.
1.14a). As discussed in Section 1.2.1.3, the broad transient absorption is assigned to
an excited state of phenyl azide (
1
PhN
3
).
In 100% formic acid,
1
PhN
3
decays within our instrument response function
(300 fs) and a growth in absorption below 400 nm is observed, which is assigned to
singlet phenyl nitrene (
1
PhN). This assignment is consistent with the singlet nitrene
spectrum reported previously.
56,83
Subsequently, the decay (
1.0 ps) of
1
PhN is accompanied by the growth of transient absorption centered at 500 nm
(Fig. 1.14b). Within experimental error, the time constant of the decay recorded at
360 nm is the same as that of the growth of transient absorption monitored at 500 nm.
In nonacidic solutions such as ACN, the decay of
1
PhN is not accompanied by the
formation of a transient species absorbing at 500 nm.
56,83
Following our study of the
efficient protonation of
o
-biphenylyl and 1-naphthyl nitrene to form the correspond-
ing nitrenium ions in 88% formic acid,
84
the transient absorption at 500 nm is
assigned to phenyl nitrenium ion (PhNH
þ
). Falvey's group studied a related species,
N
-methyl-
N
-tolylnitrenium ion
p
-MePhNMe
þ
, which has a strong absorption band
centered at 325 nm and a weak absorption tail in the visible region centered around
470 nm.
85
The weak band of
p
-MePhNMe
þ
has absorption similar to that observed
for PhNH
þ
. TD-DFT calculations predict that the parent system PhNH
þ
has two
t ¼
12.0
