between C3 and O4 that is observed. Snapshots of the ELF basins for some selected
points along the IRC are shown in Fig. 10.10.
The population evolution of some basins directly related with the changes taking
place in the reaction is reported in Fig. 10.11. As can be seen, the reaction begins
with a very acute diminution of the population of the V(C2,O4) disynaptic basin,
that very soon disappears in the catastrophe between SSD-I and SSD-II, thus
making apparent that the ring opening process begins in this bond, taking place the
breaking of the C2–C3 bond at a later stage. The population of this disappearing
basin is assumed by the monosynaptic basins V(O4), represented as a whole in
Fig. 10.11, whose population increase as the V(C2,C3) population decrease, and
suddenly increases at the turning point between SSD-I and SSD-II. The populations
of the monosynaptic basins on O1 (initially two basins, only one after the turning
point between SSD-II and SSD-III is reached) are also represented as a whole in
Fig. 10.11, and labeled V(O1). As can be seen, either the V(O4) population as well
as the V(O1) one constantly decrease from the SSD-I to SSD-II turning point on,
until stable values of ca 5.22 and ca 4.18 e, respectively, are reached.
Concomitantly, the population of V 1 (C3,O4) disynaptic basin constantly increases
until it is splitted in the turning point between SSD-V and SSD-VI, into two
disynaptic basins accounting for the double C3–O4 bond character. It can also be
noticed that the V(C2,C3) disynaptic basin at the end of SSD-III becomes
monosynaptic V(C2), with the same population that had the V(C2,C3) basin, when
the turning point between SSD-III and SSD-IV is reached. On the other hand, the
populations of the disynaptic basins V 1,2 (C2,O1) slightly increase and finally merge
in the turning point between SSD-IV and SSD-V.
As can be seen, there is only one basin between the C2 and the O1 atoms by the
end of the process, and the populations of the basins V(C2), V(O1) and V(C2,O1) at
the last point of the IRC are 2.39, 4.18 and 3.23 e, respectively. Therefore, the
topological description of the carbon monoxide moiety shows that the ten valence
electrons are greatly displaced towards the oxygen atom, and in fact the populations
of the V(O1) basins are greater than the population of the V(C2,O1) bonding basin,
thus reflecting the electronic displacement due to the large oxygen electronegativity.
10.5.2.3 Mechanism B, First Step
As explained above, an alternative mechanism was suggested for the water elimination from glycolic acid to yield the α-lactone intermediate, by means of the
nucleophilic attack of the hydroxylic oxygen (O1) of the carboxyl group on the αcarbon atom (C3). The energy profile along the IRC path down from TS2 to the
glycolic acid in one side and to the α-lactone intermediate on the other is reported in
Fig. 10.12, along with the SSDs found. In Scheme 10.4 these domains have been
sketched following the same aforementioned code.
The beginning of the reaction takes place in the same way as already explained
when studying the mechanism A: starting at the glycolic acid, with nineteen basins,
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points along the IRC are shown in Fig. 10.10.
The population evolution of some basins directly related with the changes taking
place in the reaction is reported in Fig. 10.11. As can be seen, the reaction begins
with a very acute diminution of the population of the V(C2,O4) disynaptic basin,
that very soon disappears in the catastrophe between SSD-I and SSD-II, thus
making apparent that the ring opening process begins in this bond, taking place the
breaking of the C2–C3 bond at a later stage. The population of this disappearing
basin is assumed by the monosynaptic basins V(O4), represented as a whole in
Fig. 10.11, whose population increase as the V(C2,C3) population decrease, and
suddenly increases at the turning point between SSD-I and SSD-II. The populations
of the monosynaptic basins on O1 (initially two basins, only one after the turning
point between SSD-II and SSD-III is reached) are also represented as a whole in
Fig. 10.11, and labeled V(O1). As can be seen, either the V(O4) population as well
as the V(O1) one constantly decrease from the SSD-I to SSD-II turning point on,
until stable values of ca 5.22 and ca 4.18 e, respectively, are reached.
Concomitantly, the population of V 1 (C3,O4) disynaptic basin constantly increases
until it is splitted in the turning point between SSD-V and SSD-VI, into two
disynaptic basins accounting for the double C3–O4 bond character. It can also be
noticed that the V(C2,C3) disynaptic basin at the end of SSD-III becomes
monosynaptic V(C2), with the same population that had the V(C2,C3) basin, when
the turning point between SSD-III and SSD-IV is reached. On the other hand, the
populations of the disynaptic basins V 1,2 (C2,O1) slightly increase and finally merge
in the turning point between SSD-IV and SSD-V.
As can be seen, there is only one basin between the C2 and the O1 atoms by the
end of the process, and the populations of the basins V(C2), V(O1) and V(C2,O1) at
the last point of the IRC are 2.39, 4.18 and 3.23 e, respectively. Therefore, the
topological description of the carbon monoxide moiety shows that the ten valence
electrons are greatly displaced towards the oxygen atom, and in fact the populations
of the V(O1) basins are greater than the population of the V(C2,O1) bonding basin,
thus reflecting the electronic displacement due to the large oxygen electronegativity.
10.5.2.3 Mechanism B, First Step
As explained above, an alternative mechanism was suggested for the water elimination from glycolic acid to yield the α-lactone intermediate, by means of the
nucleophilic attack of the hydroxylic oxygen (O1) of the carboxyl group on the αcarbon atom (C3). The energy profile along the IRC path down from TS2 to the
glycolic acid in one side and to the α-lactone intermediate on the other is reported in
Fig. 10.12, along with the SSDs found. In Scheme 10.4 these domains have been
sketched following the same aforementioned code.
The beginning of the reaction takes place in the same way as already explained
when studying the mechanism A: starting at the glycolic acid, with nineteen basins,
278
J. Andrés et al.