Chemistry Reference
In-Depth Information
TABLE 12.5. Function of the Solvent Polarity (
f
ð
e
;
n
Þ) and Time Constants (
t
1
,
t
2
)
Describing the Biexponential Decay of the S
1
State of 2-Naphthoyl Azide (7) Obtained
by fs TRIR and fs UV-Vis Techniques upon 270 nm Excitation, as well as Time
Constant (
t
) Describing the Monoexponential Decay of the S
1
State Upon the 350 nm
Excitation
36b
fs TRIR
fs UV-Vis
Solvent
f
ð
e
;
n
Þ
t
1
(ps)
t
2
(ps)
t (ps)
t
1
(ps)
t
2
(ps)
t (ps)
0.0007 14.0
1.6 580
60 800
120 ND
a
Cyclohexane
ND
ND
CCl
4
0.0116
8.6
0.8 570
50 690
60 ND
ND
425
12
CHCl
3
0.1550
6.8
1.3 120
30 133
66
2
130
20 ND
CH
2
Cl
2
0.2305
6.5
1
70
6
ND
ND
ND
ND
CH
3
OH
0.3327
3.6
0.9
19
3
ND
2.0
0.2 24
3 D
CH
3
CN
0.3303
3.0
0.7
19
48
53
1
23
46
4
a
ND—the decay was not detected.
the lifetime of the thermally relaxed S
1
state of
decreases significantly (Table 12.5).
Moreover, this lifetime depends linearly on the function of the solvent polarity—
f
7
n
2
2
n
2
. As the decay of
S
1
state is con-
trolled at least by three processes (isocyanate formation, intersystem crossing, and
nitrene formation), there is no simple explanation of the observed trend.
In all solvents under study, the formation of isocyanate
ð
e
;
n
Þ¼ð
e
1
Þ=ð
2
e
þ
1
Þð
1
Þ=ð
þ
1
Þ
correlates with the decay
of the azide S
1
state.
36
Thus, upon 270 nm excitation the isocyanates arise from both
the hot and thermally relaxed lowest excited singlet state of the azides. On the
contrary, the nitrene IR band was formed within the laser pulse (
8
300 fs) upon
excitation at 270 nm.
36
The IR band of the S
1
state at 2100 cm
1
was detected on both
type of excitations. It was demonstrated that only about 50% of the molecules in the
S
n
state (
n
2) produce the S
1
state.
36b
This could be explained by the formation of
the nitrene
1
13
in the upper excited singlet states (S
n
,
n
2).
Recently,
37
Kubicki et al. have also studied photochemistry of pivalolyl, benzoyl,
4-phenylbenzoyl, and 2-anthroyl azides in chloroform and acetonitrile using femto-
second time-resolved transient absorption and IR spectroscopy with 270 nm exci-
tation, and computational chemistry. Similar results have been observed for all the
acyl azides in Table 12.6. As for azide
, the S
1
state decay was best described by a
bi-exponential function and the decay was significantly faster in the more polar
solvent acetonitrile than in chloroform. It was also demonstrated that the isocyanate
formation correlates with the decay of the S
1
state of the azides.
37
Therefore, results of matrix isolation spectroscopy in combination with modern
computational chemistry unambiguously proved that the aroyl nitrenes have a singlet
ground state.
17
Geometries of the CON-fragments of singlet carbonyl nitrenes are
intermediate between that of nitrenes and oxazirenes.
16-20
The electronic structure of
these singlet species is indeed the electronic structure of oxazirene with a very weak
NO bond. These singlet species were also directly detected in the nanosecond
17,25,26
and femtosecond
36,37
TRIR experiments. The singlet nitrene band at about
1750 cm
1
was observed upon 270 nm fs excitation of a series of carbonyl
7
