As explained above, the cusp-type catastrophe between SSD-III and SSD-IV
accounts for the V(H6) and V 1 (O5) monosynaptic basins being replaced by a single
disynaptic basin V(O5,H6), while the turning point between SSD-IV and SSD-V
accounts for the change in the synaptic order of the disynaptic V(C3,O5) basin to a
new monosynaptic V 3 (O5) basin. Hence, the whole population of the monosynaptic
basins on O5, represented as V(O5) in Fig. 10.8, decreases in the SSD-III to
SSD-IV passage and increases back in the SSD-IV to SSD-V turning point.
The topological change between SSD-V and SSD-VI corresponds to the splitting
of the V(C2,O1) basin, whose population was increasing from the very beginning
of the process. From this turning point onwards, two disynaptic basins, V 1 (C2,O1)
and V 2 (C2,O1) are observed between C2 and O1 accounting for its double bond
character. Finally, when SSD-VII is reached, the new V(C3,O4) basin appears
accounting for the closure of the lactone ring. The population of this new disynaptic
basin is quite low: slightly increases at the beginning of the SSD-VII, but soon its
population stabilizes around a 0.78 e value.
10.5.2.2 Mechanism A, Second Step
As explained above, the second step of the mechanism A accounts for the lactone
intermediate opening to form carbon monoxide and formaldehyde. The energy
profile along the IRC path down from TS4 to the α-lactone intermediate in one side,
and to carbon monoxide plus formaldehyde in the other side, is reported in
Fig. 10.9, together with the SSDs found. These have been shown in Scheme 10.3,
which as explained has been depicted from the perspective of the ELF analysis,
following the same code.
At the α-lactone, the first point of the SSD-I, fifteen basins have been found,
corresponding with the four core basins, five disynaptic basins accounting for the
0
1
2
3
4
5
6
7
-6
-4
-2
0
2
4
6
Basin population
Reaction coordinate
V(C3,O4)
V(O1,H6)
V(O5,H6)
V(H6)
V(C3,O5)
V(O1)
V(O5)
V1(C2,O1)
V1(C2,O4)
V2(C2,O4)
V2(C2,O1)
Fig. 10.8 Population evolution of some selected basins along the IRC path from TS1. Dashed
lines separate the structural stability domains found, which are indicated. V(O1) and V(O5) account
for the total population of the monosynaptic basins on the O1 and O5 atoms, respectively
10 Quantum Chemical Topology Approach …
275
accounts for the V(H6) and V 1 (O5) monosynaptic basins being replaced by a single
disynaptic basin V(O5,H6), while the turning point between SSD-IV and SSD-V
accounts for the change in the synaptic order of the disynaptic V(C3,O5) basin to a
new monosynaptic V 3 (O5) basin. Hence, the whole population of the monosynaptic
basins on O5, represented as V(O5) in Fig. 10.8, decreases in the SSD-III to
SSD-IV passage and increases back in the SSD-IV to SSD-V turning point.
The topological change between SSD-V and SSD-VI corresponds to the splitting
of the V(C2,O1) basin, whose population was increasing from the very beginning
of the process. From this turning point onwards, two disynaptic basins, V 1 (C2,O1)
and V 2 (C2,O1) are observed between C2 and O1 accounting for its double bond
character. Finally, when SSD-VII is reached, the new V(C3,O4) basin appears
accounting for the closure of the lactone ring. The population of this new disynaptic
basin is quite low: slightly increases at the beginning of the SSD-VII, but soon its
population stabilizes around a 0.78 e value.
10.5.2.2 Mechanism A, Second Step
As explained above, the second step of the mechanism A accounts for the lactone
intermediate opening to form carbon monoxide and formaldehyde. The energy
profile along the IRC path down from TS4 to the α-lactone intermediate in one side,
and to carbon monoxide plus formaldehyde in the other side, is reported in
Fig. 10.9, together with the SSDs found. These have been shown in Scheme 10.3,
which as explained has been depicted from the perspective of the ELF analysis,
following the same code.
At the α-lactone, the first point of the SSD-I, fifteen basins have been found,
corresponding with the four core basins, five disynaptic basins accounting for the
0
1
2
3
4
5
6
7
-6
-4
-2
0
2
4
6
Basin population
Reaction coordinate
V(C3,O4)
V(O1,H6)
V(O5,H6)
V(H6)
V(C3,O5)
V(O1)
V(O5)
V1(C2,O1)
V1(C2,O4)
V2(C2,O4)
V2(C2,O1)
Fig. 10.8 Population evolution of some selected basins along the IRC path from TS1. Dashed
lines separate the structural stability domains found, which are indicated. V(O1) and V(O5) account
for the total population of the monosynaptic basins on the O1 and O5 atoms, respectively
10 Quantum Chemical Topology Approach …
275