TIME-RESOLVED RESONANCE RAMAN SPECTROSCOPIC STUDIES OF ARYL NITRENIUM IONS AND THEIR CHEMICAL REACTIONS - Nitrenes and Nitrenium Ions - page 245

100 µ s
10 µ s
9 µ s
8 µ s

7 s

6 µ s
5

µ

s
4 µ s
3

µ

µ s

2 µ s

1.5 µ s

s
700 ns
500 ns
400 ns
300 ns
200 ns
150 ns
100 ns
80 ns
50 ns

1 µ

10 ns

1100 1200 1300 1400 1500 1600 1700 1800

Raman shift (cm -1 )

FIGURE 7.14. An overview of 354.7 nm probe wavelength TR 3 spectra acquired after
299.1 nm photoexcitation of 1mM 4-biphenyl azide in a water-acetonitrile (60:40) mixed
solvent are shown. Source : Reprinted with permission from Ref. 47. Copyright (2008)
American Chemical Society.

4-biphenyl azide in a water-acetonitrile (60:40). The TR 3 spectra displayed in

Figures 7.14 shows three species between 10 ns and 100 s and the first species has

diagnostic Raman features at 1619, 1541, 1492, and 1333 cm 1 that decay within
several hundred nanoseconds and are a precursor of the other two species. The first
species has been previously attributed to the 4-biphenylnitrenium ion 40 (see Figure 2S
in Ref. 47). The second species has a diagnostic Raman feature at 1590 cm 1 that
overlaps a little with features of the other two species and the second species seems to
first increase and then decreases in intensity. The third species has diagnostic features at
1593, 1455, 1440, and 1142 cm 1 and seems to increase in intensity and does not
appear to decrease in intensity over the several hundred microseconds timescale. In
addition, oxygen purged TR 3 experiments were done and found that all three species
are not sensitive to oxygen.
The third species has two strong resonance Raman bands in the 1400-1500 cm 1
region diagnostic of N

m

N bond stretching vibrational modes. The third species was
determined to be 4,4-azobisbiphenyl from comparison of the TR 3 spectra to DFT-
computed Raman spectra of the 4,4-azobisbiphenyl species and a resonance Raman
spectrum acquired for an authentic sample of 4,4-azobisbiphenyl (see Figure 3S in
Ref. 47). Because cis and trans 4,4 0 -azobisbiphenyl isomers display well-separated
absorption bands in the UV-visible absorption spectrum 71 (see Figure 4S in Ref. 47),