Chemistry Reference
In-Depth Information
states. It was found that protonation did not make any difference in the ordering of
the excited states except for some small geometrical changes around the azide
unit.
150
2.6 STRUCTURE AND PHOTOCHEMISTRY OF HETERO-AZIDES
2.6.1 Carbonyl Azides
Attachment of a carbonyl unit to the azide group changes both geometrical as well as
photochemical parameters. First, there are two possibilities for the azide unit to
orient relative to the carbonyl unit that is
cis
(
syn
) and
trans
(
anti
); additionally, a
cyclic isomer can also exist. Additionally, there are three dominant resonance
structures for carbonyl azides (Fig. 2.12). Both B3LYP and MP2 calculations
indicate that the
cis
conformer is favored by about 3 kcal/mol for formyl azide
(HC(
O)N
3
),
151
which is in agreement with Raman and infrared vibrational data
along with electron diffraction analysis.
152,153
The energy barrier for conversion to
the
trans
conformer is estimated to be 12 kcal/mol.
151
The
cis
conformer is about
26.7 kcal/mol lower in energy than the cyclic form of formyl azide at the MP4(SDQ)/
6-31G(
d
) level of theory.
154
Interestingly, the cyclic tautomer was found to be about
8.8 kcal/mol more stable for thioformyl azide when compared with the
cis
con-
former, presumably due to the electronic differences of the C
O and C
S
moieties.
154
These parameters lead to the conclusion that the resonance structures
A
and
B
are dominant, while
C
does not have a significant contribution to the overall
structure (Fig. 2.12).
With regard to photochemical behavior, the carbonyl group introduces the (
n
,
p
)
p
) states of the C
p
and (
O unit into the excited state manifold. Carbonyl azides
also generate carbonylnitrenes like other azide precursors discussed earlier; more-
over, these nitrenes may proceed through rearrangements to give the corresponding
,
(a)
-
O
O
O
N
+
N
N
N
R
N
N
+
N
-
R
N
R
N
trans
cis
Cyclic
(b)
-
-
O
O
O
+
+
N
N
+
N
N
N
N
-
R
N
R
N
R
N
(A)
(B)
(C)
FIGURE 2.12.
(a) The
trans
(
anti
) and
cis
(
syn
) conformers of the azide unit relative to the
carbonyl moiety as well as the
cyclic
structural isomer. (b) Three possible resonance structures
of a carbonyl azide are shown.
