Chemistry Reference
In-Depth Information
Δ
D
0.10
2
0.05
1
0
3
-0.05
-0.10
ν/cm
-1
600
900
1200
1500
FIGURE 12.9.
Changes in the IR spectrum (
1
) recorded after additional irradiation at 313 nm
(8min) of the sample of
1a
irradiated at 254 nm (2min). The IR spectra of isocyanate
2a
(
2
)
and singlet species
1
3a
(
3
) calculated by the B3LYP/6-31G(
d
) method and scaled by 0.97.
19
Source
: Reprinted with permission from Ref. 19. Copyright 2005 Springer Science
þ
Business Media.
1765 cm
1
for
1
3a
(Fig. 12.9, spectrum 3). Thus, the two bands observed in Ar
matrices at 1727 and 1765 cm
1
is due to site splitting or a Fermi resonance. The
displacement vectors for this vibration show that it consists mainly of C
N stretch
(Fig. 12.8b).
The electronic absorption spectrum of
has two maxima at 269 and 277 nm
76b
in
agreement with the absence of appreciable absorptions of the final products above
280 nm (Fig. 12.7). Thus, the absorption band at about 300 nm belongs exclusively to
the singlet species
1
3a
2a
. The electronic absorption spectrum of
1
3a
was calculated
using the CASSF/CASPT2 procedure
17
and found to fit well with the experimental
spectrum (Fig. 12.7). The active space used in the calculations consisted of the eleven
MOs, seven of which are depicted in Figure 12.10. The two intense transitions in the
near-UV spectrum of
1
3a
calculated by the CASSCF/CASPT2 method are at 306 and
290 nm, respectively (Fig. 12.7, open bars). The most intense transition at 306 nm
consists mainly of electron promotion from the
3a
00
p
orbital (HOMO-2) to the
6a
00
). In agreement with previously reported time-dependent DFT
calculations,
16
a very weak transition to the first excited state is predicted at
513 nm.
17,19
It consists mainly of electron promotion from an allylic type
LUMO
p
orbital (
p
orbital
27a
0
) of the very weak NO bond.
17
Figure 12.10 demonstrates that the highest a
0
-type
4a
00
, HOMO-1) to the low-lying
s
orbital (
(
orbital and the lowest a
0
-type
s
s
s
NO bond orbitals (Figs. 12.10, MO
26a
0
and
orbital are the corresponding
s
and
27a
0
). Thus, the singlet species
1
3a
has indeed the electronic structure of oxazirene,
although its NO bond is very weak.
On the contrary, the electronic structure of the triplet benzoyl nitrene
3
3a
is typical
of nitrenes.
13-15
In this species, one of the unpaired electrons occupies the
p
-type
