Chemistry Reference
In-Depth Information
-bond
cooperativity. It relies on the positive synergism between HB strengthening and
RAHBs are known [
40
,
48
] to be characterized by the mechanism of
p
-
delocalization enhancement that occurs when the H-donor and the H-acceptor
atoms are connected by a short chain of conjugated single and double bonds.
Such a
p
-bond cooperativity is revealed in
3
by a particularly large source function
percentage contribution, S%(C)
p
8.3, from the carbon atom linked by a conjugated
formal double bond to the acceptor oxygen atom. It is this large S% value, also
clearly evident in Fig.
4c
, the main cause of the decreased percentage SF contribu-
tion from the triad of atoms directly involved in the H-bond. The source contribu-
tions from the C atom linked to the oxygen donor is, instead, less than half as large,
S%(C)
¼
3.8, despite the two C atoms are almost equally distant (3.3 and 3.8
˚
,
respectively) from the HB bcp. The different source function contributions from
these two carbon atoms seem to clearly reflect the diverse role these two atoms have
in the
¼
-delocalization mechanism leading to the tautomer of
3
through TS
4
. The C
atom with greater S%(C) value releases its shared
p
-electron pair to form the O-H
bond in the tautomer of
3
, while the other carbon, with an halved S%(C) value, can
contribute to this electron pair shift only in an indirect way through
p
p
-electron
conjugation. Although the HB bcp lies in the nodal plane of the
p
distribution, this
distribution has an indirect effect on the
density, hence on the charge density at
the HB bcp, as already documented for the conjugated hydrocarbon systems in this
review (cf. Sect.
3.2.2
). Also in this case, the analysis of SF contributions along a
line perpendicular to the molecular plane and passing through the HB bcp could
magnify the enhancement of delocalization of sources, which is induced by the
p
s
-electron delocalization mechanisms leading to RAHBs.
Based on the results obtained from the investigated series of hydrogen-bonded
prototypical complexes, Gatti et al. [
9
] proposed the following classification of
OH
O interactions, in terms of the SF tool: (a) IHBs have typically a highly
negative SF contribution from H, S%(H)
70, highly positive S%(D) value,
much larger than S%(A), and SF percentage contributions as big as about 50% from
atoms other than the D,H,A triad, in agreement with a dominant electrostatic nature
for such bonds; (b) PAHBs still exhibit a negative S%(H) value, but almost halved
with respect to IHBs, and are characterized by a S%(D) value greater than the S%
(A) value, but less than twice as big, and by a substantially smaller external
percentage source contribution than found for IHBs; (c) RAHBs are distinguished
by a very small, but positive source from H, comparable sources from the donor and
acceptor oxygen atoms, and in particular by a SF contribution from atoms other
than the D
<
A triad that, being similar to that found for PAHBs despite the
significantly smaller O
þ
H
þ
-electron
delocalization mechanisms proposed [
52
,
53
] for such peculiar class of HBs; (d)
O separation in RAHBs, nicely agrees with the
p
CAHB show large and positive S%(H) values (about 30%), an SF contribution
from the acceptor oxygen atom which is about five times as large as that from the
donor oxygen atom, and extremely high SF contributions from the two atoms linked
in the H-bond, S(A
þ
H)
>
80%, consistently with the localized and largely covalent
nature of the H
A interaction in these charged complexes. The correspondence
between this HB classification and the one proposed [
45
] using the ELF [
54
-
56
]
