Chemistry Reference
In-Depth Information
Fig. 23 Schematic
mechanism of the inversion
of the
anti
-folded
conformation a-
C
2h
(
y
) via
a planar transition state
p-
D
2h
1
1'
p
Z
1'
1
a
1'
1
a
Z-RR'
Z-SS'
1'
1
p
1'
1
1'
a
a
1
E-RS'
E-SR'
Table 10 Symmetry species of the vibrational modes of the p-
D
2h
planar conformation and
resulting conformations and point groups
Symmetry
species
A
g
B
1g
B
2g
B
3g
A
u
B
1u
B
2u
B
3u
Conformation p-
D
2h
pt-
C
2h
(
z
) a-
C
2h
(
y
) p-
C
2h
(
x
)
t-
D
2
p-
C
2v
(
z
) p-
C
2v
(
y
)
s-
C
2v
(
x
)
The mechanism corresponds to a synchronous reduction of the degree of folding
of both moieties of the
anti
-folded conformation leading to the planar transition
state, followed by folding in the opposite direction to give the inverted version.
Due to the extreme intramolecular overcrowding in a planar p-
D
2h
conforma-
tion, this is most likely not the lowest energy transition state. It is probably a higher
order saddle point with more than one imaginary vibrational frequency. The B
2g
mode corresponds to the transition vector of the conformational inversion of the
anti
-folded conformation. Additional modes with imaginary frequencies indicate
the possibility of transition states (local minima or higher order saddle points) with
lower symmetry. The conformations and point group symmetries resulting from
deformations of p-
D
2h
along the various modes are listed in Table
10
(cf. Fig.
13
or
[
279
]).
More feasible mechanisms for conformational inversion of the
anti
-folded
conformation may proceed via the twisted conformation t-
D
2
or via the
syn
-folded
conformation s-
C
2v
(
x
) as transition states. In both cases there are
n
TS
¼
4 versions
of this transition state with point group order
h
TS
¼
4. The connectivities
C
¼
1
lead to the conclusion that there must be
p
¼
2 parallel pathways. The connectiv-
ities and mechanisms are schematically shown in Fig.
24
. The versions with
Z
- and
E
-configuration interconvert independently via analogous mechanisms.
In the first inversion mechanism (Fig.
24a
) the
anti
-folded conformation is
twisted and both moieties are unfolded. Twist may be introduced in two directions,
(
P
) and (
M
), leading to the two parallel pathways. Transient structures along the
pathway have
C
2
(
y
) symmetry, the common subgroup of
C
2h
(
y
) and
D
2
. The
transition vector has B
2
symmetry. It should be noted that t-
D
2
is the midpoint of
the reaction paths, and because of its symmetry it has to be a stationary point.
However, it may be a local minimum with no imaginary frequency, leading to a
two-step process, which will be discussed later (Sect.
4.3.4
). Alternatively, it could
be a higher order saddle point. This question can only be solved by calculating the
transition state and its vibrational frequencies. Additional imaginary frequencies of
