Chemistry Reference
In-Depth Information
N
N
N
N
N
N
N
N
N
N
N
N
H
H
H
H
FIGURE 2.7.
Nitrene production in the excited state of the azide and rotational motion on the
nitrogen molecule.
When the azide is relaxing on the S
1
surface, the N3 atom has an angular momentum
along the N
N angle's rotational vector which makes the azide angle angular;
after which the proximal N
N
N bond starts breaking and this process along with the
angular momentum of the N3 atom generates an angular momentum relative to the
N2 atom, and in the opposite direction. Accordingly, the nitrogen molecule would
extrude with high rotational motion, whereas the imidogen part has only some
vibrational motion along the H
N bond (Fig. 2.7).
In the case of methyl azide (CH
3
N
3
), another interesting route comes into
consideration, which is hydrogen-atom transfer in the corresponding nitrene, that
is, formation of an imine (CH
2
NH) from methylnitrene (CH
3
N) via a hydrogen-
atom shift.
114
The frontier orbitals for methyl azide are the same as for hydrazoic
acid, and there is also significant evidence that the S
1
state generates the singlet
nitrene.
Abramovitch and Kyba studied several alkyl azides photochemically
115,116
and
found that imine formation is the only possible path; moreover, decomposition
proceeds via a concerted mechanismwithout forming the nitrene. In addition, nitrene
insertion products were observed from the pyrolysis of the alkyl azides, which
suggests that the vibrationally hot ground-state surface generates the singlet nitrene.
On the other hand, the authors suggested that the singlet excited-state surface has
presumably no bound state for formation of singlet nitrene and the excited azide
converts into the corresponding imine in a barrierless manner.
From a theoretical standpoint, there has been some debate as to whether
methylnitrene is a bound state or imine formation is a concerted process. If the
process is stepwise, then the transition state would be H
3
C
N
N and will not
involve any significant motion of the methyl group while the C
N
N
N and
N
N vibrational mode would be affected the most. In the other scenario,
when the photochemistry is concerted, all of the vibrational modes would have
significant movements including the umbrella motion of the methyl group. A Raman
spectroscopic study
114
suggested that all of the vibrational signatures are changed
significantly upon excitation. Shang et al. noted that the methyl group rotates and the
C
N
H bond length is increased significantly in the excited state; thus, they concluded
their study by suggesting that the process is concerted and extrusion of molecular
nitrogen and the 1,2-H shift happens simultaneously. This experimental observation
is in good accordance with the computational work by both Demuynck et al.
117
and
Pople et al.
118
where they found that there is no barrier for 1,2-H shift in methyl
nitrene. Later on, Schaefer et al., using higher-level calculations, reported that the
lowest singlet excited state of methyl nitrene,
1
E state, splits into 1A
0
and 1A
00
states
