Chemistry Reference
In-Depth Information
Δ
D
3
1
0.1
0
2
-0.1
275
300
325
350
λ/nm
FIGURE 12.7.
Difference electronic absorption spectra recorded upon irradiation of
1a
at
254 nm for 2min in an argon matrix at 12 K (
1
) and after further irradiation at 313 nm for 8min
(
2
).
19
The positions and relative intensities of the absorption bands calculated for
1
3a
at the
CASSCF/CASPT2 level are indicated by vertical bars.
Source
: Reprinted with permission from
Ref. 19. Copyright 2005 Springer Science
þ
Business Media.
9 kcal/mol)
17
(
into account drops the
D
E
ST
for
3a
to a negative value (about
4 kcal/mol) in agreement with the findings of Schuster et al.
51-54
Having understood the nature of the singlet acyl nitrenes, Pritchina and cow-
orkers
17,19
studied the photolysis of
and its
para
-acetyl-substituted analogue in
argon matrices at cryogenic temperature in order to detect this species directly with
spectroscopy. After 2min of photolysis at 254 nm in Ar matrices, the optical
spectrum of
1a
was replaced by a new band with a maximum around 300 nm
(Fig. 12.7, spectrum 1). Simultaneously, the IR bands of
1a
, in particular the strong
N
3
stretching vibration at 2145 cm
1
and characteristic C
1a
O stretch at 1706 cm
1
,
were almost completely depleted (Fig. 12.8a,b) and gave way to a set of new peaks,
notably a strong band around 2270 cm
1
(Figs. 12.8 and 12.9).
Subsequent irradiation at 313 nm led to the disappearance of the newly formed UV
band (Fig. 12.7, spectrum 2). Concomitantly, some of the newly formed IR peaks
diminished significantly (Fig. 12.8b, blue curve; Fig. 12.9, spectrum 1, negative
peaks), whereas some other bands (Fig. 12.9, spectrum 1, positive peaks), notably the
intense band around 2270 cm
1
(Fig. 12.8, spectrum 2), continued to grow. This
experiment indicates very clearly the formation of at least two products at 254 nm
photolysis. The first of these products has an absorption maximum at
300 nm and
rearranges to the other primary product on exposure to 313 nm light. The latter
product has an IR spectrum characteristic of isocyanate
(Fig. 12.9, spectrum 1,
positive peaks, and calculated spectrum 2). The IR spectrum of the former product
(Fig. 12.9, negative peaks) is in very good agreement with the calculated spectrum of
singlet species
1
3a
2a
with a structure resembling that of cyclic oxazirene (Fig. 12.9,
spectrum 3).
17,19
Note that
the B3LYP calculation predicts a single peak at
