Chemistry Reference
In-Depth Information
a
b
11'
1
1'
s
Z-RS'
1
t
Z-P
1'
1
1'
1
a
Z-RR'
1'
1
a
1
a
Z-RR'
1'
a
Z-SS'
Z-SS'
1'
1
1
s
Z-SR'
1'
t
Z-M
1'
1
1'
s
t
E-P
1
E-RR'
1'
1
1'
1
1
a
a
1'
1
a
a
1'
E-RS'
E-SR'
E-RS'
E-SR'
1
1'
1'
1
t
s
E-M
E-SS'
Fig. 24 Schematic mechanisms of the inversion of the
anti
-folded conformation a-
C
2h
(
y
) via (a)
a twisted transition state t-
D
2
or (b)a
syn
-folded transition state s-
C
2v
(
x
)
B
1
or B
3
symmetry would indicate possible transition states t-C
2
(
z
)orts-C
2
(
x
),
respectively (cf. Fig.
13
and Table
9
).
In the second mechanism (Fig.
24b
) the two moieties invert one after the other:
the
anti
-folded conformation is converted to the
syn
-folded transition state via
inversion of one moiety, followed by inversion of the second moiety to give the
inverted
anti
-folded conformation. Either the first or the second moiety may invert
first, leading to two parallel pathways. In this mechanism the transient structures
along the pathways are not twisted and have
C
s
(
xz
) symmetry. An imaginary mode
with B
2
symmetry corresponds to the transition vector for inversion of the
anti
-
folded conformation. In the case where s-
C
2v
(
x
) is a higher order saddle point,
additional modes with A
2
and B
1
symmetry indicate possible lower symmetry
transition states st-C
2
(
x
) and s-C
s
(
xy
), respectively (cf. Fig.
13
and Table
9
).
E
,
Z
-Isomerization with Simultaneous Inversion of One Tricyclic Moiety
of the
anti
-Folded Conformation
Possible point groups for the transition state of the
E
,
Z
-isomerization with simul-
taneous inversion of one moiety may be constructed by combining the symmetry
operators of a-
C
2h
(
y
),
E
, (11
0
)(88
0
)(99
0
), (18)(1
0
8
0
)*, and (18
0
)(81
0
)(99
0
)*, with
operators corresponding to the process of
E
,
Z
-isomerization with inversion of one
moiety: (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
)*, and
(18
0
81
0
)(99
0
)*, and adding the required operators to ensure closure of the resulting
group. Note that, in this case, the smallest group combining all symmetry elements
of both sets is
G
16
. The point group
D
2d
combines some operators from both sets
plus some additions to give a closed set. Table
11
lists all possible groups of
permutation-inversion operators, the corresponding conformations with their point
group, order of the point group
h
TS
,numberofversions
n
TS
,connectivity
C
, number
