Chemistry Reference
In-Depth Information
Δ
D
Δ
D
200x10
-6
100x10
-6
1760 cm
-1
k
obs
= 1.1 x10
6
s
-1
1680 cm
-1
50
100
k
obs
= 1.0 x10
6
s
-1
0
0
0
5
10
15
μ
s
0
5
10
15
μ
s
2x10
-3
Δ
D
2265 cm
-1
1
k
obs
> 2 x10
7
s
-1
0
0
5
10
15
s
μ
FIGURE 12.13.
Kinetic traces observed at 1760, 1680, and 2265 cm
1
following 266 nm
laser photolysis (5 ns, 2mJ) of azide
1a
(3.3mM) in argon saturated cyclohexane. The dotted
curves are experimental data; the solid curves are the calculated best fits to a single exponential
function.
17
Source
: Reprinted with permission from Ref. 17. Copyright 2003 RSC Publishing.
Note that the DFT calculations at the B3LYP level considerably underestimate the
free energy of activation (13.9 kcal/mol).
18
The barrier for this reaction was also
calculated by the PBE/TZ2P method, but its value (18.9 kcal/mol)
45
cannot be
directly compared with the free energy of activation.
Theoretical data are also available for the rearrangement of formyl nitrene
1
.
45,95,101,102
The barrier (without ZPE correction) for the rearrangement was
found to be 29.9,
95
23.4,
101
and 20.9 kcal/mol
45
at the G2, G2(MP2,SVP), and
PBE//TZ2P levels of theory, respectively. It is clear that the PBE method signifi-
cantly underestimates the barrier for rearrangement of the nitrene to the isocyanate.
Only the data from PBE calculations are available for the rearrangement of the
singlet benzoyl nitrene
1
3a
3c
. Its barrier was calculated to be 13.6 kcal/mol,
45
and we
can estimate a rate constant of rearrangement as
10
3
s
1
. As the PBE method
evidently underestimates the barrier, the rearrangement of benzoyl nitrene
1
3a
is
even slower and does not compete with its bimolecular reactions.
Therefore, all data discussed in this chapter unambiguously prove that both the
singlet carbonylnirenes (in our opinion, oxazirene is more convenient name) and the
isocyanates are the primary products of the acyl azide photolysis, and the carbon-
ylnirenes do not rearrange to isocyanates on a timescale of the chemical experiments.
However, isocyanates can arise from an excited state of azides or from electronically
