Chemistry Reference
In-Depth Information
Table 14 Possible point groups and conformations of the transition state for
E
,
Z
-isomerization
with simultaneous inversion of one moiety of the
syn
-folded conformation s-
C
2v
(
x
)
Group of permutation-inversion operators
h
TS
n
TS
Cp
a
b
TS
{
E
, (18)(1
0
8
0
), (11
0
)(88
0
)(99
0
), (18
0
)(81
0
)(99
0
),
(18)*, (1
0
8
0
)*, (11
0
88
0
)(99
0
)*, (18
0
81
0
)(99
0
)*}
-
c
C
2
(
x
)-
d
t
⊥
-
D
2d
8
2
2
{
E
, (18)(1
0
8
0
), (11
0
88
0
)(99
0
)*, (18
0
81
0
)(99
0
)*} t
⊥
-
S
4
4 4 2 1
C
1
-
d
{
E
, (18)(1
0
8
0
), (18)*, (1
0
8
0
)*} t
⊥
-
C
2v
(
d
) 4 421
C
1
-
d
{
E
, (18
0
)(81
0
)(99
0
), (18)(1
0
8
0
)*, (11
0
)(88
0
)(99
0
)*} s-
C
2v
(
x
)
e
4 4 2 1
C
2v
(
x
)A
1
{
E
, (18
0
)(81
0
)(99
0
)} ts-
C
2
(
x
)
e, f
2 8 2 2
C
2
(
x
)A
{
E
, (18)(1
0
8
0
)*} f-
C
s
(
xz
)
e
2 8 2 2
C
s
(
xz
)A
0
{
E
, (11
0
)(88
0
)(99
0
)*} s-
C
s
(
xy
)
e
2 8 2 2
C
s
(
xy
)A
0
{
E
, (18)*} ft
⊥
-
C
s
(
d
) 2 822
C
1
A
00
{
E
,(1
0
8
0
)*} ft
⊥
-
C
s
(
d
0
) 2 822
C
1
A
00
{
E
} ft-
C
1
1624
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
])
c
There is no integer
p
which would satisfy (
4
)
d
The point group along the pathway does not correspond to any non-degenerate irreducible
representation of the transition state point group
e
For s-
C
2v
(
x
), ts-
C
2
(
x
), f-
C
s
(
xz
), and s-
C
s
(
xy
) there is no pathway for
E
,
Z
-isomerization without
leaving this point group
f
See Sect.
4.3.7
E
,
Z
-isomerizations with simultaneous inversion of one moiety have low symmetry:
ft
⊥
-
C
s
(
d
)orft
⊥
-
C
s
(
d
0
) and ft-
C
1
.The
C
s
symmetric transition states ft
⊥
-
C
s
(
d
) and
ft
⊥
-
C
s
(
d
0
) have a point group order
h
TS
¼
2 and
n
TS
¼
8 versions. The connec-
tivity in this process is
C ¼
2 with
p ¼
2 parallel pathways connecting each pair of
minima. The mechanism is schematically shown in Fig.
29
.
In each of the automerization reactions one moiety remains basically unchanged,
while the other moiety is unfolded, rotated by 180
either clockwise or counter-
clockwise (
p
2), and refolded in the opposite direction. Rotating the first or
second moiety gives different products. Hence, the connectivity
C
¼
2. The tran-
sient structures along the steepest descent paths from the transition states to
reactants and products have
C
1
symmetry. The transition vector has A” symmetry.
This mechanism facilitates an inversion of the
syn
-folded conformation in a
two-step process. Enantiomeric (labeled) versions may be found on opposite sides
of the center of Fig.
29
. Note that the same type of transition state has been
considered for the
E
,
Z
-isomerization of the
anti
-folded conformation (Fig.
25
).
The low symmetry of the pathways (
C
1
) does not allow assigning the reactant and
product of this transition state unambiguously from the symmetry species of the
vibrational mode. Only explicit calculations of the steepest descent path will allow
a decision.
¼
