Chemistry Reference
In-Depth Information
6 ns
1 ns
100 ps
20 ps
10 ps
8.5 ps
7 ps
6 ps
5 ps
4 ps
3 ps
2 ps
1 ps
0 ps
800
1000
1200
1400
1600
1800
2000
Raman shift (cm
-1
)
FIGURE 7.7.
An overview is displayed for selected 320 nm ps-KTR
3
spectra acquired after
267 nm photolysis of 4-methoxyphenyl azide in a water-acetonitrile (60:40) solvent system.
The time-delays are indicated to the right of each spectrum.
Source
: Reprinted with
permission from Ref. 43. Copyright (2004) American Chemical Society.
(64:40), and it is clear that the decay of the first species directly correlates to
the formation of the second species so that a simultaneous best-fit by a common
time constant exponential decay and growth functions, respectively (the solid lines in
Fig. 7.9) gave a time constant 5.4 ps in the water-acetonitrile (60:40). Because the
second species is attributed to the singlet 4-methoxyphenyl nitrenium ion and the first
species is formed within the 2 ps time resolution of the ps-KTR
3
apparatus (Fig. 7.8),
the first species is attributed to singlet 4-methoxyphenyl nitrene. This is consistent with
the known photochemistry of aryl azides, such as 4-methoxyphenyl azide, and thewell-
established reactivity of singlet alkoxyaryl nitrenes with water to generate their
corresponding singlet aryl nitrenium ions.
The ultraviolet excitation of 2-fluorenyl azide and 4-methoxyphenyl azide in
mixed water-acetonitrile solvents both generate a transient species within the 2 ps
resolution of the experiments that were tentatively attributed to the 2-fluorenyl
nitrene and 4-methoxyphenyl nitrene, respectively, and in both cases, the aryl
nitrenes react with water to form their respective singlet aryl nitrenium ions
(e.g., the 2-fluorenyl nitrenium and 4-methoxyphenyl nitrenium ions). It is particu-
larly interesting that the decay of the 4-methoxyphenyl nitrene was observed to be
more than 30 times faster than the decay of the 2-fluorenyl nitrene at about the same
water concentration (see Figs. 7.6 and 7.9), and this suggests the reactivity of the two
