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In-Depth Information
phase, the solvent effect decreases the barrier and reaction energies significantly,
particularly the pathway through TS1 and TS2 and all of the reactions are exothermic
in the aqueous phase indicating the reaction of the 4-biphenyl nitrenium ion with
4-biphenyl azide to form 4,4-azobisbiphenyl can probably take place more easily in
an aqueous solution. Figure 7.18 shows the relative energy profile in the aqueous
phase and there appears to be no great difference between the activation barriers for
the first part of the
cis
and
trans
pathways (6.8 and 7.3 kcal/mol, respectively) and
within the calculation error, the
cis
and
trans
pathways are likely parallel and
competing with each other although the reaction mechanisms on the two pathways
are different. The
cis
pathway reaction mechanism presented in Figure 7.18 displays
anewN
N bond generation between the azide and the nitrenium ion taking place and
then the N
2
and H
þ
species are released in a stepwise process with two intermediates,
PC1 and PC3, respectively, where PC1 is not stable and it readily releases N
2
to form
the azo cation (PC3) as the barrier for the reaction is small in aqueous solution. The PC3
species with the help of a hydroxide ion and the driving force of the dihedral angle
changes can release H
þ
to produce the
cis
4,4-azobisbiphenyl species. On the
trans
pathway, The N
N bond formation between the azide and nitrenium ion and the N
2
release occur concurrently and thus make only one intermediate, PC2 and with
hydroxide ions and water molecules nearby, this PC2 can release H
þ
to form the
trans
4,4-azobisbiphenyl even though no clear transition states were determined from
the DFT computations because our limited model appears inadequate compared to the
complexity of this process.
The first step of the
cis
and
trans
pathways are similar to one other and both probably
take place to some extent and the
cis
and
trans
isomers of the azo cation and 4,4-
azobisbiphenyl species may both be generated in the TR
3
experiments and the relative
T
S1
10.1
T
S
2
7.3
PC1
T
S4
4.5
RC1
4.6
T
S3
3.2
2.6
RC2
PC3b
0.0
0.0
PC3a
- 60.5
PC4
-37.8
PC2
-73.2
Reaction coordinate
FIGURE 7.18.
The relative energy profiles (in kcal/mol) determined from the B3LYP/6-31G
computations are shown. The values of PC3b, TS4, and PC4 were determined relative to PC3b,
the others relative to RC2.
Source
: Reprinted with permission from Ref. 47. Copyright (2008)
American Chemical Society.
