Chemistry Reference
In-Depth Information
-50 ps
5 ps
13 ps
30 ps
47 ps
120 ps
100
50
0
1890
1880
1870
1860
1850
Wavenumber (cm
-1
)
75
1866 cm
-1
50
25
0
100
75
50
1876 cm
-1
25
0
-100
0
100
200
300
400
Time delay (ps)
FIGURE 1.8.
(a) Time-resolved IR spectra produced on ultrafast photolysis of
ortho
-
biphenylyl azide in acetonitrile (270 nm excitation). (b) Selected kinetic traces recorded at
1866 and 1876 cm
1
.
Source
: Reprinted with permission from the American Chemical
Society,
J. Am. Chem. Soc
. 2006,
128
, 14804.
nitrene to form both isocarbazole and ketenimine. Picosecond IR studies again show
the rise of ketenimine. Initially, its absorption band is broad and then sharpens and
blue shifts over a 10-50 ps time window (Fig. 1.8a). The ketenimine is formed with a
time constant consistent with the decay of the singlet nitrene (
10 ps, Fig. 1.8b)
determined by ultrafast UV-Vis transient absorption spectroscopy.
In contrast to
o
-biphenylyl nitrene, singlet
p
-biphenylyl nitrene does not exhibit
any significant population decay on the 100 ps timescale (lifetime is
9 ns). Ultrafast
UV-Vis studies demonstrated that it is formed thermally excited and undergoes
vibrational cooling in 13 ps.
26,27
Attempts to measure the ketenimine formation from
p
-biphenylyl nitrene over a 100 ps time windowwith IR detection failed. We attribute
this to the relatively large rearrangement barrier (6.8 kcal/mol for
p
-biphenylyl
nitrene vs. 5.6 kcal/mol for singlet phenyl nitrene), which effectively prohibits
isomerization of the corresponding hot nitrene.
