Chemistry Reference
In-Depth Information
a
b
11'
1'
1
1
a
Z-RR'
t
Z-P
1'
1
1'
1
1'
1'
s
Z-RS'
s
1
s
Z-RS'
s
1
Z-SR'
Z-SR'
1
1
1'
1'
a
Z-SS'
t
Z-M
1'
1
1'
t
1
a
E-P
E-RS'
1'
1'
1
1
1'
1'
s
s
1
1
s
s
E-RR'
E-SS'
E-RR'
E-SS'
1'
1
1
1'
a
t
E-M
E-SR'
Fig. 28 Schematic mechanisms of the inversion of the
syn
-folded conformation s-
C
2v
(
x
) via (a)a
twisted transition state t-
D
2
or (b)an
anti
-folded transition state a-
C
2h
(
y
)
higher order saddle points. The conformations corresponding to distortions of p-
D
2h
along vibrational modes with imaginary frequency of various symmetry species are
indicated in Table
10
.
The twisted conformation t-
D
2
and the
anti
-folded conformation a-
C
2h
(
y
) are
less overcrowded candidates for a transition state for inversion of the
syn
-folded
conformation. In each case there are
n
TS
¼
4 versions of the transition state with
point group order
h
TS
¼
4. The connectivities
C ¼
1 of the two parallel pathways
(
p ¼
2) and the schematic mechanisms are shown in Fig.
28
. The versions with
E
-
and
Z
-configuration interconvert independently via analogous mechanisms.
In the first mechanism (Fig.
28a
), the
syn
-folded conformation is twisted and the
two moieties are unfolded to reach the twisted transition state. Folding in the
opposite direction and untwisting lead to the inverted
syn
-folded conformation.
The two parallel pathways are distinguished by positive (
P
) or negative twist (
M
)of
intermediate structures. The transient structures along the pathways have
C
2
(
x
)
symmetry. The symmetry species of the transition vector is B
3
. Note that t-
D
2
was
also considered as a transition state for the inversion of the
anti
-folded conforma-
tion (Fig.
24a
). The two mechanisms are mutually exclusive and may be distin-
guished by the symmetry of the vibrational mode of t-
D
2
that has an imaginary
frequency. Distortions of t-
D
2
along the modes B
1
,B
2
, and B
3
lead to conforma-
tions t-
C
2
(
z
), ta-
C
2
(
y
), and ts-
C
2
(
x
), respectively. Alternatively, the conformation
t-
D
2
may be a local minimum and an intermediate in one or both of the conforma-
tional inversion processes (see Sects.
4.3.4
and
4.3.5
), or a higher order saddle
point.
In the second mechanism (Fig.
28b
), the two tricyclic moieties are unfolded and
inverted one after the other, leading from a
syn
-folded minimum via an
anti
-folded
transition state to the inverted
syn
-folded minimum. Either of the two moieties may
