Chemistry Reference
In-Depth Information
Table 24 Possible point groups and conformations of the transition state for interconversion of
the a-
C
2h
(
y
)
anti
-folded and the s-
C
2v
(
x
)
syn
-folded conformations
Group of permutation-inversion operators
a
b
TS
h
TS
n
TS
{
E
, (18)(1
0
8
0
)*} f-
C
s
(
xz
) 2 8
C
s
(
xz
)
0
{
E
} ft-
C
1
1 16
C
1
A
a
Point group symmetry along pathway from transition state to reactant or product, i.e., maximum
common subgroup of transition state and reactant or product
b
Symmetry species of the mode of the transition vector (using the conventional setting of the
transition state point group [
279
])
Fig. 39 Schematic
mechanism for the
interconversion of the a-
C
2h
(
y
)
anti
-folded and s-
C
2v
(
x
)
syn
-folded conformations
via a transition state f-
C
s
(
xz
)
1'
1
1'
1
1'
f
f
Z-RS'
1
s
Z-RS'
Z-RS'
1'
1
1'
1
a
a
Z-SS'
Z-RR'
1
1'
1
1'
1
1'
s
f
f
Z-SR'
Z-SR'
Z-SR'
1'
1
1
1'
1'
f
f
E-RR'
E-RR'
1
s
E-RR'
1
1'
1'
1
a
a
E-RS'
E-SR'
1
1
1'
1'
1'
1
s
f
f
E-SS'
E-SS'
E-SS'
The largest common subgroup, {
E
, (18)(1
0
8
0
)*}, corresponds to the point group
C
s
(
xz
). According to Table
4
, conformations with
C
s
(
xz
) symmetry may be
classified as unequally folded: f-
C
s
(
xz
) (not specifying the relative direction of
folding (
syn
/
anti
); cf. Sect.
“Folded Conformations”
). An ft-
C
1
transition state is
also possible, as shown in Table
24
.
In conformations with point group
C
s
(
xz
), the two moieties may have different
degrees of folding; however, twisting of the central ethylene group is not allowed.
There are
n
TS
¼
8 versions of the f-
C
s
(
xz
) transition state: four with
Z
-configura-
tion and four with
E
-configuration. The four
Z
-versions convert the two
Z
-versions
of the a-
C
2h
(
y
) conformation into the two
Z
-versions of the s-
C
2v
(
x
) conformation
and vice versa as outlined Fig.
39
. The corresponding
E
-versions interconvert via an
analogous mechanism.
It should be noted that in Fig.
39
it is assumed that s-
C
2v
(
x
) has a higher energy
than a-
C
2h
(
y
) and thus, according to the Hammond principle, the transition state
f-
C
s
(
xz
)is
syn
-folded (su-
C
s
(
xz
)). Symmetry predicts unequal degrees of folding
