Chemistry Reference
In-Depth Information
*
(a)
(b)
600
800
1000
1200
1400
1600
1800
2000
Raman shift (cm
-1
)
FIGURE 7.20.
A comparison of the B3LYP/6-31G
computed normal Raman spectra of the
cis
azo cation (a) PC3a without a hydroxide ion nearby to (b) PC3b with a hydroxide ion close
to the cation. The star marks the Raman band corresponding to the N
N stretching mode.
Source
: Reprinted with permission from Ref. 47. Copyright (2008) American Chemical Society.
denoted with a star symbol becomes much less intense. There are complex
interactions such as hydrogen bonding that will occur between the water molecules
and the reactants and also the intermediate molecules and all of these interactions
occur under actual experimental conditions but they are hard to effectively mimic
employing only our crude theoretical computations here.
The 4-biphenyl nitrenium ion may react with its unphotolyzed precursor,
4-biphenyl azide to form a 4,4
0
-azobisbiphenyl cation intermediate first and then
release H
รพ
to produce a stable 4,4
0
-azobisbiphenyl product. A previous study of the
reaction of the 2-fluorenyl nitrenium ion with 2-fluorenyl azide to form 2,2
0
-
azobisfluorene found that the 2,2
0
-azobisfluorene generation and the 2-fluorenyl
nitrenium ion decay could be fitted by a common time constant, and there was no
apparent intermediates between the nitrenium ion and the azo compound and this
different behavior can be attributed mostly to the rigid structure of the two aromatic
rings in the 2-fluorenyl system compared to the greater flexibility and rotatable
structure of the 4-biphenyl species where the rotation of the aromatic rings in the
4-biphenyl species noticeably stabilizes the geometries as illustrated in Table 7.3
showing the dihedral angles between the two aromatic rings. As the reaction
proceeds from the reactant complex and to the transition states to the intermediates,
the dihedral angles between the two aromatic rings change by about 10
and the
flexibility of the bond between the two phenyl rings results in the 4-biphenyl
nitrenium ion having a smaller iminocyclohexadienyl character compared to the
rigid planar structure of the 2-fluorenyl nitrenium ion.
40
Transient absorption quenching experiments found that the 4-biphenyl nitrenium
ion reacts with the azide anion (N
3
) and water molecules with rate constants in the
diffusion limit and 2
10
6
s
1
, respectively, and was also seen to react with
deoxyguanosine with a rate constant of 2
10
9
M
1
s
1
.
32
Our results indicate
