Chemistry Reference
In-Depth Information
FIGURE 2.10.
(a) Difference density calculation using points A and B; (b) difference density
plot corresponding to the S
1
state of
p
-biphenyl azide.
Franck-Condon excited state (point B, Fig. 2.10) which in turn yields the electron
density on the excited state, from which the ground-state electron density (point A,
Fig. 2.10) is subtracted. The subtraction yields a plot with positive and negative
electron density regions. Conventionally, we describe the positive region with a
green color, which indicates the accumulation of electron density upon vertical
excitation to the excited state; conversely, the negative electron density is represented
with a red color, which indicates the depletion of electron density in the excited state
as compared to the ground state. Such approaches have been utilized successfully to
understand the photochemistry of N-confused porphyrins,
145
naphthalimides,
146
nitroaromatics
147
as well as inorganic complexes.
148
A difference density plot for the S
1
state of
p
-biphenyl azide is shown in
Figure 2.10. It is clear that the electron density is depleted along the N1
N2
p
in-plane
orbital at the
p
bond, while accumulation of electron density occurs in the
N2
N2 bond upon
relaxation on the S
1
potential energy surface. Using coupled-cluster (RI-CC2/TZVP)
calculations, we identified that 1-naphthyl,
o
-biphenyl and
p
-biphenyl azides have an
S
1
state with (
N3 bond. This indicates an elongation or breaking of the N1
p
in-plane
) character localized on the azide unit, while the S
2
state is a
p
,
p
) state localized on the aromatic unit.
90
Since the S
1
state involves transfer of
electron density from an out-of-plane to an in-plane orbital, the S
1
excited state had a
very small oscillator strength. On the other hand, the S
2
state (
(
p
,
p
) is an allowed
transition, and the oscillator strength was 100-1000-fold higher. Thus, the prediction
was that the aryl azides are initially excited to the S
2
state upon photolysis by 266 nm
radiation.
The optimized geometries indicated the elongation of the N1
p
,
N2 bond to about
1.45 A
in the S
1
state,
90
which is significantly longer than what was predicted by less
