Chemistry Reference
In-Depth Information
R
R
+
-
-
+
N
1
N
2
N
3
N
1
N
2
N
3
y
R
R
x
N
N
N
z
FIGURE 2.5.
Resonance structures for the azide unit and the involved
p
-orbitals.
flash photolysis (UV-Vis and IR detection), and more recently with ultrafast
(femtosecond) techniques. Moreover, the interplay between the different electronic
states of the nitrenes and the possible photoproducts from different routes
(Scheme 2.2) has also attracted theorists to perform advanced electronic structure
theory calculations. However, the excited states of azide precursors comparatively
remained unexplored until recently, when Platz et al. detected the singlet excited
state of the
o
-biphenyl and
p
-biphenyl azides.
89,90
On a theoretical front, a few reports have been published recently speculating on
the mechanism for the dissociation of azides and have explored the possible
structures of the excited states via semiempirical methods.
91-93
Nevertheless, the
application of femtosecond spectroscopy has increased the curiosity of theorists to
explore the excited-state potential energy surfaces since a wealth of experimental
information can be obtained about these transient species. In this section, we will
begin with the geometry and electronic structure of the azide precursors and discuss
the excited states of alkyl, aryl, carbonyl, and phosphoryl azides, and then we will
end with the generation of the corresponding nitrenes from a theoretical standpoint.
2.4.2 Geometry and Electronic Structure of Azides
The azide unit consists of three nitrogen atoms of which all of them can be
considered in different electronic environments. Two possible resonance structures
of the azide group are shown in Figure 2.5. The N1
1
has
sp
2
hybridization and the
R
N2 angle is generally about 110-120
. Conventional wisdom suggests that
the N1
N2
N3 angle would be 180
as the N2 atom is thought to be
sp
hybridized,
similar to the central carbon of an allene. However, both experiments and calcula-
tions have concluded that the azide unit is bent and the N1
N1
N3 angle is about
170
.
92,94-98
Why does the azido moiety have a bent structure? A possible explan-
ation can be found if one analyzes the orbitals of the left structure of Figure 2.5. Since
N3 is accepting the lone pair from N2 in the
p
x
orbital, the
p
y
(N2) orbital is forced to
have two electrons, which in turn feels repulsion from the
N2
b
(N1
p
N2) orbital and
thus the azide unit deviates from the ideal linear geometry.
The azide unit has a characteristic stretching vibrational mode at about
2100 cm
1
, which suggests that the N
3
unit has triple bond character for the azide
unit. Another IR signature of azides occurs between 1210 and 1290 cm
1
.
99
Two
1
The numbering of the azide group, R
N1
N2
N3, will be used consistently in this report.
