Chemistry Reference
In-Depth Information
FIGURE 1.12.
Transient spectra produced by ultrafast photolysis of 1-naphthyl azide in 88%
formic acid. The spectra were generated by ultrafast LFP (270 nm) with a time window of
2-20 ps.
Source
: Reprinted with permission from Ref. 84.
within 1 ps of the laser pulse. The carrier of this absorption band is assigned to singlet
1-naphthyl nitrene (
1
1-NpN), consistent with its previous observation
26
in acetoni-
trile. As
1
1-NpN decays, a new species is formed, with an absorption centered at
495 nm and an isosbestic point at 430 nm. The carrier of the 495 nm absorption band
is assigned to 1-naphthyl nitrenium cation (1-NpNH
þ
). This is consistent with Kung
and Falvey's report of the spectrum of the closely related species
N
-methyl-
N
-1-
naphthyl nitrenium cation.
66
This assignment is also consistent with CASPT2
calculations and low temperature studies of the same nitrenium ion in Ar-HCl
matrices.
76
Thus, we have little doubt that this assignment is correct.
Global fitting of the decay at 380 nm and the growth at 495 nm give a time constant
of 8.4 ps. In acetonitrile, the lifetime of singlet 1-naphthyl nitrene (12 ps) is mainly
limited by intramolecular cyclization to form azirine 1-NpAZ. If we make the same
assumption as in the case of
o
-biphenylyl azide, that the formation of the azirine has
the same rate constant in 88% formic acid as in acetonitrile, then the apparent
protonation rate constant is 3.6
10
10
s
1
in 88% formic acid. Therefore, we can
also conclude that 30% of the singlet 1-naphthyl nitrene produced under these
conditions is protonated in this solvent. As before, it is the vibrationally excited,
rather than the thermalized singlet nitrene, which undergoes protonation. The
lifetime of 1-naphthyl nitrenium cation is only 860 ps, which explains why this
species cannot be observed by nanosecond time-resolved LFP methods.
1.3.4 2-Naphthyl Nitrenium Cation
Singlet 2-naphthyl nitrene
1
2-NpN is the shortest-lived nitrene discovered to date
(
1
2-NpN is
t ¼
1.8 ps). The apparent rate constant of intramolecular reactions for
10
11
s
1
. Based on our analysis in Sections 1.3.2 and 1.3.3, the apparent
protonation rate constants for
5.6
1
1-NpN are 6.7
10
10
s
1
1
o
-BpN and
and
