Chemistry Reference
In-Depth Information
0
500
1000
1500
2000
Time delay (ps)
FIGURE 7.6.
Plots of the integrated areas of the strong 1550-1600 cm
1
Raman bands due to
the first species in the 342 nm ps-KTR
3
spectra (open squares and circles) and the strong
1630 cm
1
Raman bands due to the second species in the 400 nm ps-KTR
3
spectra (solid
squares and circles). Data are displayed for ps-KTR
3
spectra acquired in water-acetonitrile
(25:75) (squares) and water-acetonitrile (50:50) solvents (circles) and the lines show the best-
fit exponential decay and appearance curves to the data.
Source
: Reprinted with permission
from Ref. 43. Copyright (2004) American Chemical Society.
Figure 7.7 shows an overview of selected 320 nm ps-KTR
3
spectra acquired
following 267 nm photolysis of 4-methoxyphenyl azide in a water-acetonitrile
(60:40) and it can be seen in which one species appears at very short times and
then decays and forms a second species similar to the analogous experiments done
for photolysis of 2-fluorenyl azide in mixed aqueous solvents. Figure 7.8 compares a
ps-KTR
3
spectrum (3 ns) to that acquired for the 320 nm ns-TR
3
singlet 4-methox-
yphenyl nitrenium ion reported in Ref. 41. There is excellent agreement between the
two spectra presented in Figure 7.8 taking into account the broader spectral
bandwidth for the ps-KTR
3
spectrum than the ns-TR
3
spectrum and this identifies
the second species seen in the ps-KTR
3
experiments as the singlet 4-methoxyphenyl
nitrenium ion. The Raman band vibrational frequencies for the ps-KTR
3
spectrum at
3 ns were compared to those previously observed for the 320 nm ns-TR
3
spectrum
and DFT-computed frequencies (see Table S2 in Ref. 43).
43
This comparison
provides further support for the second species seen in the ps-KTR
3
spectra being
attributed to the singlet 4-methoxyphenyl nitrenium ion intermediate.
The strong 1460-1470 cm
1
Raman band that ismostly due to the 4-methoxyphenyl
nitrene (320 nm ps-KTR
3
spectra) was integrated to monitor its decay and the
strong 1632 cm
1
band that is mostly due to the 4-methoxyphenyl nitrenium ion
(320 nm ps-KTR
3
) was integrated to monitor its formation. Figure 7.9 displays the
intensities of these Raman band(s) as a function of delay time in water-acetonitrile
