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In-Depth Information
reflect the diverse nature of HBs certainly represented two good motivations behind
such a test [
9
]. Enhanced SF contributions to the HB bcp density from the atoms
most directly involved in the H-bond (D-H
A) and a parallel decrease of those
from the remaining atoms, with increasing energy, covalency, and local character
of the HB, could perhaps be anticipated. However, the extremely varying role of the
H atom involved in the HB with change in the HB nature [
9
], or the existence of
specific signatures for the low-barrier hydrogen bonds (LBHB) [
47
] and for the
-
bond cooperativity mechanism controlling resonance-assisted hydrogen bonds
(RAHBs) [
9
,
48
] were certainly less trivial and predictable outcomes.
In the following, we review the main results derived from the only five HB
studies [
9
,
13
,
47
,
49
,
50
] that, to the best of our knowledge, have thus far made use
of the SF approach. We also add a number of new results, especially for the RAHBs
systems, that come from closer examination of or from comparison among these
studies. Not surprisingly, in view of the postulated relevant role of the LBHBs in
enzymatic catalysis, the first paper due to Overgaard et al. [
47
] concerned the
comparison of the SF contributions for two small molecules taken as examples of
a low-barrier and of a single-well HB (benzoylacetone and nitromalonamide,
respectively). Appreciation of the results of this pioneering study is facilitated by
reviewing first the main outcome of a systematic study on a number of paradigmatic
HB systems carried on by Gatti et al. [
9
].
Evaluation of the SF contributions to the density at the HB bcp along the reaction
path for two water molecules, which approach each other within the linear dimer C
s
constraint, served as a model to study the effect on the localizability or spread of
atomic sources when the H
p
O distances change from the values typical
of the weak isolated HBs to those characteristic of the charge-assisted H-bonds
[
48
]. Gatti et al. [
9
] found that, despite an almost constant and comparable source
function contribution from the donor and the acceptor water molecules, the atomic
S% contributions vary radically along the bond path (Table
6
). The S%(H) clearly
O and O
Table 6 Source contributions to the hydrogen-bond bcp density in a number of prototypical
hydrogen-bonded complexes
a
System
b
R
O
O
,
˚
R
H
O
(
R
O-H
),
˚
2
S%
(H
þ
D
þ
A)
1
þ
(CAHB) 2.409 1.204 (1.204)
0.415 31.4 9.6 51.7 83.1 92.7
2
- (CAHB) 2.430 1.216 (1.214)
0.392 32.1 8.3 49.9 82.0 90.3
4
(RAHB)
*c
2.370 1.209 (1.209)
0.425 32.2 8.5 48.8 80.9 89.5
3
(RAHB) 2.538 1.639 (1.008) 0.148 2.1 34.7 34.0 36.1 70.8
5
(PAHB) 2.749 1.850 (0.984) 0.092
14.4 53.1 31.0 16.6 69.7
6
(IHB) 3.020 2.077 (0.943) 0.067
72.3 106.6 18.7
53.7 53.0
2.750
d
1.809 (0.941) 0.124
35.5 74.2 32.2
3.2 71.0
2.500
d
1.564 (0.936) 0.216
12.5 55.4 41.1 28.6 83.9
2.250
d
1.327 (0.923) 0.333
þ
2.8 39.6 46.2 49.1 88.7
2.000
d
1.110 (0.890) 0.208
þ
13.1 29.6 50.8 63.8 93.5
a
Data from [
9
]. If not otherwise stated, all quantities in au. D and A are the H-donor and H-
acceptor oxygen atoms, while H is the hydrogen atom involved in the H-bond
b
H-bonded complexes are numbered as in Fig.
4c
and classified according to [
48
]
c
Transition state for the H atom migration in malonaldeyde
d
Points along the reaction path for the approach of two water molecules within the linear dimer C
s
constraint
r
r
b
S%
(H)
S%
(D)
S%
(A)
S%
(H
þ
A)
