Chemistry Reference
In-Depth Information
topological approach was also highlighted. The reader is addressed to the original
paper [
9
] for details.
Gatti et al. [
9
] also examined HB systems where either the D or the A atom, or
both of them are no longer O atoms. Although based on a limited number of cases, it
was shown that the relative weights of the SF contributions from atoms in the HB
complexes are related not only to the D and A atoms distances but also to the
specific nature of the H-donor and H-acceptor atoms. The conclusion that “the
source function seems sensitive enough to discriminate between different donor-
acceptor pairs, despite similar donor to acceptor distances and that it constitutes a
very suitable tool to get insights into the hetero-nuclear H-bonds” [
9
] is likely to be
a valid one, but certainly would warrant a more systematic investigation. A fortiori
this holds true for more subtle aspects like the discussed capability of the SF tool to
disclose the signature of
p
-bond mechanisms leading to RAHBs, for which a
comparative study of systems showing
-bond cooperativity or anticooperativity
effects [
57
] could be definitely more compelling. Equally important would be
exploring in some detail whether also the mechanisms of
p
-bond cooperativity
can be revealed by the SF tool, as the noticeable source contribution to the H-bond
density from an hydrogen atom involved in a different HB of the homodromic water
trimer would seemingly suggest (Fig.
4c
).
We are now ready to review those results of the paper by Overgaard et al. [
47
]
that are of relevance for the application of the SF tool to HBs. In their effort to
provide an answer to the very important question of what causes the formation of an
LBHB, the authors investigated three short NH
s
O HBs in a cocrystallized complex
of betaine, imidazole, and picric acid, which serves as a model for the active site
(the catalytic triad) in the serine protease class of enzymes. The occurrence of an
LBHB had in the past been postulated as a condition for TS stabilization in the
catalytic triad, which is in turn necessary to increase the proteases rate constant
[
47
]. As anticipated earlier, Overgaard et al. also studied the OH
O bond in
benzoylacetone (bza) and nitromalonamide (nma) as simple examples of an
LBHB and of a single-well HB, respectively. This spectrum of HBs thus covered
the complete range of strong HBs, that is “localized”
10
HB (the H atom was indeed
found to be firmly localized in the “nitrogen wells” in all the three NH
O bonds of
the catalytic triad), LBHB (bza), and single-well HB (nma). Clearly, it is not simply
the donor-acceptor distance that determines the HB type, since, for instance, the
authors noticed that citrinin has a localized HB, while bza exhibits a LBHB,
notwithstanding their almost identical O
O separation. Moreover, based on the
H
O distance and the topological properties at bcps, one HB in the catalytic triad
complex was proved to be stronger and more covalent than the other two NH
O
bonds, despite its largest O
O distance. The authors thus speculate that there must
be differences in the chemical environment that change the potential energy surface
10
Note that this definition of localized HBs refers to the shape of the DH
A potential well and to
the consequent localization of the H close to the D atom, rather than to a “localization” of sources
and related H
A covalency as described earlier for the strongest prototypical HBs.
