Chemistry Reference
In-Depth Information
Fig. 5 Portraits of the negative Laplacian,
L
(r), in a series of HBs covering the complete range of
strong HBs: localized HB (
top
: HB1, HB2, and HB3 in the cocrystallized complex of betaine,
imidazole, and picric acid), single-well HB (
bottom left
nitromalonaldeyde, nma), and Low Barrier
HBs (
bottom right
benzoylacetone, bza). The different
L
(r) portraits around the H atom involved
in the HB lead to different S(H) and S%(H) values for the diverse HB types (see Table
7
). The
L
(r)
portraits are obtained from ab initio computations, analogously to the source function contribu-
tions data reported in Table
7
(adapted from Fig. 4.1.4 of [
58
] and Fig. 4 with permission from
[
47
], Copyright 2001, Wiley-VCH Verlag GmbH & Co, KGaA)
the stronger and more covalent character assigned to this bond on the basis of the
bcp topological properties. Overgaard et al
.
[
47
] could thus conclude that HB2 can
be characterized as an intermediate between localized HBs and delocalized LBHBs.
The merit of their study was to show that the nature and strength of an HB are not
“unambiguously determined by the geometrical parameters which define the sys-
tem” [
47
], and that the formation of an LBHB is revealed by a dramatic increase in
the H atom source contribution to the HB bcp density, whereas the changes in the
contributions from the surrounding seem to be more subtle. While the SF neatly
summarizes the changes in the
L
(r) distribution around the H atom due to the
formation of an LBHB, it yet appears not to be able to reveal the causes leading
to the onset of an LBHB potential well. Further studies in this direction are surely
required.
Another more recent study by S
orensen et al
.
[
49
] concerned the environmental
influence on the electronic character of the HBs in a
€
b
-diketone, 2-acetyl-1,
