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correct. However, the triplet excited state of N
2
is about 6.22 eV higher
132
than the
ground state.
133
Therefore, the first mechanism seems improbable.
The involvement of solvent molecules can also be ruled out to help produce the
nitrene in a concerted manner, as solvation effects were negligible on the quantum
yields.
134-137
Reiser argued that although bond orders of the N1
N2 bond were
lower in the excited state relative to that in the ground state, the N1
N2 bond is not
ruptured completely. In addition, in his view, changes in the
p
-electron density will
not affect the
bond and therefore the second mechanism should also be excluded.
Accordingly, only the third mechanism remains to be considered and Reiser
proposed that internal conversion takes place from higher lying singlet excited
states to the hot ground state, which in turn generates the nitrene.
129
Contradicting
the second mechanism, Nielsen and co workers
138
as well as several later exper-
imental and computational reports confirmed that singlet nitrene formation takes
place on the singlet excited state of the precursor azide.
s
2.5.3.3 Computational and Experimental Efforts to Understand the Excited
States.
While azides were always studied as precursors to the nitrene, the focus
of the theoretical work has largely been on chemistry of the nitrenes. In 1997,
Budyka and Zyubina
139
reported semiempirical and
ab initio
molecular orbital
theory calculations for the ground state, lowest excited state (S
1
and T
1
), and radical
anions (D
0
and D
1
) of a few aryl azides. Their PM3 calculations suggested that the
p
in-plane
orbital is the LUMO for the case of HN
3
. Moreover, for aryl azides with
extended conjugation, like aryl azides, the
p
in-plane
orbital becomes the LUMO
þ
1
p
orbital of the aromatic ring becomes the LUMO, as shown in Figure 2.8. The
azide unit was observed to be angular in the S
1
state with the N1
and a
N2 bond length
being increased from 1.26 A
in S
0
to 1.36 A
in S
1
. In addition, they observed that the
triplet state of aryl azides has an azide unit which is bent with a N
N
N bond
127
. When these data for the neutral aryl azides were compared to the
corresponding values for the radical anionic states of the aryl azides, these authors
found that the anionic ground state (D
0
) looked quite similar to the S
1
state, while the
lowest anionic excited state (D
1
) looked similar to the S
0
state. This observation was
also explained through the nature of the HOMO.
angle of
π
*
π
*
in-plane
π
*
in-plane
π
*
E
π
π
p
-NO
2
p
-CH
3
CO PhN
3
Py-N
3
HN
3
FIGURE 2.8.
Frontier orbitals for
para
-nitrophenyl azide,
para
-azidoacetophenone, phenyl
azide, pyridinyl azide (Py-N
3
), and hydrazoic acid (from left to right).
