TIME-RESOLVED RESONANCE RAMAN SPECTROSCOPIC STUDIES OF ARYL NITRENIUM IONS AND THEIR CHEMICAL REACTIONS - Nitrenes and Nitrenium Ions - page 224

(a) (b)

3 ns
2 ns
1 ns
500 ps
300 ps
200 ps
150 ps
100 ps
50 ps
20 ps
10 ps
5 ps
3 ps
2 ps
1 ps
0 ps

-1 ps

800 1000 1200 1400 1600 1800 1000 1200 1400 1600 1800 2000

Raman shift (cm -1 ) Raman shift (cm -1 )

FIGURE 7.4. Shown is an overview of selected ps-KTR 3 spectra acquired after 267 nm
photolysis of 2-fluorenyl azide in water-acetonitrile (50:50) solvent with 342 nm (a) and
400 nm (b) probe excitation wavelengths. The time-delays are indicated to the right of each
spectrum. Source : Reprinted with permission from Ref. 43. Copyright (2004) American
Chemical Society.

for the first species seen after 267 nm photolysis of the 2-fluorenyl azide in a water-
acetonitrile (50:50) solvent system. Inspection of Figure 7.5 shows that the 341.5 nm
ns-TR 3 spectrum is in reasonable agreement with the 342 nm ps-KTR 3 spectrum
considering the lower spectral resolution of the ps-KTR 3 spectrum and this indicates
the ps-KTR 3 spectra of the first species is likely the 2-fluorenyl nitrene that has been
previously observed in ns-TR 3 experiments done in pure acetonitrile solvent. 51 In
Figure 7.5b are compared the 416 nm ns-TR 3 spectrum of the 2-fluorenylitrenium ion
in water-acetonitrile (75:25) solvent 39 to the 400 nm ps-KTR 3 spectrum in water-
acetonitrile (50:50) at 3 ns. These spectra are very similar considering the lower
spectral resolution and moderately different probe wavelength of the ps-KTR 3
spectrum and this indicates the second species seen in the 400 nm ps-KTR 3 spectra
is due to the singlet 2-fluorenyl nitrenium ion. The Raman band vibrational
frequencies for the ps-KTR 3 spectrum were compared to those previously reported
for the 416 nm ns-TR 3 spectrum 39 and DFT calculated frequencies (Table S1 of the
supporting information in Ref. 43) 43 and this comparison provides further support for
the assignment of the second species seen in the ps-KTR 3

spectra to the singlet

2-fluorenyl nitrenium ion.
The strong 1550-1600 cm 1 Raman bands due to 2-fluorenyl nitrene (342 nm ps-
KTR 3 spectra) were integrated to monitor their decay. Similarly, the strong
1630 cm 1 region Raman bands (400 nm ps-KTR 3 ) were integrated to monitor
the appearance of singlet 2-fluorenyl nitrenium ion. Figure 7.6 displays the