Chemistry Reference
In-Depth Information
FIGURE 1.11.
Transient spectra produced by ultrafast photolysis of
o
-biphenylyl 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.
Ultrafast photolysis (270 nm) of
o
-biphenylyl azide (
o
-BpN
3
) in 88% formic acid
produces the spectra in Figure 1.11.
76
A transient absorption band centered at 400 nm
is formed within 1 ps, which is assigned to singlet
o
-biphenylyl nitrene (
1
o
-BpN),
consistent with our previous observation
26,27
of the same nitrene in acetonitrile. As
1
o
-BpN decays, a peak centered at 610 nm is formed, with an isosbestic point at
465 nm. By analogy with the prior example of
p
-BpN
3
, the carrier of the 610 nm band
is assigned to
o
-biphenylyl nitrenium cation (
o
-BpNH
þ
). This assignment is also
consistent with Zhu, Carra, and Bally's study of the same nitrenium ion in Ar-HCl
matrices.
76
The singlet nitrene decay in 88% formic acid is also on the timescale of
vibrational cooling of the nitrenes in acetonitrile.
26,27
Once again it is the vibra-
tionally excited, rather than the thermalized singlet nitrene, which undergoes
protonation.
Global fitting of the decay observed at 400 nm and the growth at 610 nm gives a
time constant of 7.7 ps. In acetonitrile, the lifetime of singlet
o
-biphenylyl nitrene is
16 ps, which is mainly deactivated by intramolecular cyclization to azirine
o
-BpAZ
and isocarbazole
o
-BpIC. Assuming that the intramolecular decay processes of
1
o
-BpN have the same rate constants in 88% formic acid as in acetonitrile, we deduce
that the apparent protonation rate constant is 6.7
10
10
s
1
in 88% formic acid.
Based on this assumption, we can also conclude that 52% of
1
o
-BpN is protonated in
this acidic solvent. The carrier of the 610 nm band shows only very little decay in a
3 ns time window. Its lifetime of 27 ns in 88% formic acid was determined by
nanosecond time-resolved LFP techniques.
1.3.3 1-Naphthyl Nitrenium Cation
Ultrafast photolysis (
270 nm) of 1-naphthyl azide (1-NpN
3
) in 88% formic acid
produces the spectra shown in Figure 1.12.
76
A peak centered at 380 nm is formed
l
ex
¼
