atoms while Ag2 recover the five coordination at N = 4. Simultaneously, in passing
from N = 0 to N = 4 Ag2–Ag3 distance decreases from 5.725 to 4.853 Å. In
addition, a comparison of the two pictures of Fig. 10.5 reveals that the electron
density distribution is enhanced between Ag2 and Ag3 for N = 4.
Ag1 center, that forms the [AgO 6 ] clusters, shows a pronounced charge density
decrease up to N = 2, according to the change of the coordination number from 6 to
2. Therefore, Fig. 10.4 reveals that the extra electron density added to the material is
transferred from one cluster to another through the lattice network, in particular
between Ag1 and Ag3 arrangements, which behave similarly. At N = 4 both are
practically reduced and are coordinated only to two O atoms, being Ag1–Ag3
distance of adjacent cells is reduced to 2.745 Å.
Finally it is worth noting that the charge density for the Ag4, that forms [AgO 7 ]
cluster, as well as for V atoms remains almost unaltered.
10.5.2 Glycolic acid decomposition
The gas phase decompositions of several carboxylic acids and related compounds
have been theoretically characterized by us in the past [171–176], reproducing the
experimentally observed kinetics of such processes. In particular, the decomposition of glycolic acid takes place through a homogeneous, unimolecular reaction
following a first order rate law. We demonstrated [177] that three competitive
reaction mechanisms could exist, being a two-step process the more favorable
reaction pathway. For this pathway, the first step was associated with the water
elimination, thus giving rise to the formation of an α-lactone intermediate by means
of the nucleophilic attack of the carbonyl oxygen atom. The second step was the
ring opening to obtain carbon monoxide and formaldehyde (mechanism A).
A second two-step mechanism was found to be also possible, with a first step also
describing the water elimination with formation of the α-lactone intermediate, but in
this case by means of the nucleophilic attack of the hydroxylic oxygen atom of the
carboxyl group (mechanism B, which shares the second step with mechanism A).
Finally, a third pathway was also described, consisting on a one-step process in
which the decomposition of the glycolic acid would take place in a concerted
fashion to form carbon monoxide, water and formaldehyde in a unique step
(mechanism C). The three proposed mechanisms are sketched in Scheme 10.1 along
with the atom numbering used.
In the present work, we revisited that study by using a topological approach, as
an example of the usefulness of this methodology to describe chemical reactions,
and in particular to characterize in detail the chemical events that make possible the
glycolic acid decomposition. The study has been done by using the Gaussian 09
program [178] at the MP2/6-31++G** theoretical level, that has previously proven
to be adequate to reproduce the experimental values of the rate constants [177].
To gain a deeper insight in the description of the decomposition process we have
used the TopMod package [87] to obtain the ELF function values. We have used a
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J. Andrés et al.
from N = 0 to N = 4 Ag2–Ag3 distance decreases from 5.725 to 4.853 Å. In
addition, a comparison of the two pictures of Fig. 10.5 reveals that the electron
density distribution is enhanced between Ag2 and Ag3 for N = 4.
Ag1 center, that forms the [AgO 6 ] clusters, shows a pronounced charge density
decrease up to N = 2, according to the change of the coordination number from 6 to
2. Therefore, Fig. 10.4 reveals that the extra electron density added to the material is
transferred from one cluster to another through the lattice network, in particular
between Ag1 and Ag3 arrangements, which behave similarly. At N = 4 both are
practically reduced and are coordinated only to two O atoms, being Ag1–Ag3
distance of adjacent cells is reduced to 2.745 Å.
Finally it is worth noting that the charge density for the Ag4, that forms [AgO 7 ]
cluster, as well as for V atoms remains almost unaltered.
10.5.2 Glycolic acid decomposition
The gas phase decompositions of several carboxylic acids and related compounds
have been theoretically characterized by us in the past [171–176], reproducing the
experimentally observed kinetics of such processes. In particular, the decomposition of glycolic acid takes place through a homogeneous, unimolecular reaction
following a first order rate law. We demonstrated [177] that three competitive
reaction mechanisms could exist, being a two-step process the more favorable
reaction pathway. For this pathway, the first step was associated with the water
elimination, thus giving rise to the formation of an α-lactone intermediate by means
of the nucleophilic attack of the carbonyl oxygen atom. The second step was the
ring opening to obtain carbon monoxide and formaldehyde (mechanism A).
A second two-step mechanism was found to be also possible, with a first step also
describing the water elimination with formation of the α-lactone intermediate, but in
this case by means of the nucleophilic attack of the hydroxylic oxygen atom of the
carboxyl group (mechanism B, which shares the second step with mechanism A).
Finally, a third pathway was also described, consisting on a one-step process in
which the decomposition of the glycolic acid would take place in a concerted
fashion to form carbon monoxide, water and formaldehyde in a unique step
(mechanism C). The three proposed mechanisms are sketched in Scheme 10.1 along
with the atom numbering used.
In the present work, we revisited that study by using a topological approach, as
an example of the usefulness of this methodology to describe chemical reactions,
and in particular to characterize in detail the chemical events that make possible the
glycolic acid decomposition. The study has been done by using the Gaussian 09
program [178] at the MP2/6-31++G** theoretical level, that has previously proven
to be adequate to reproduce the experimental values of the rate constants [177].
To gain a deeper insight in the description of the decomposition process we have
used the TopMod package [87] to obtain the ELF function values. We have used a
270
J. Andrés et al.