Chemistry Reference
In-Depth Information
Table 7 Geometrical data and source contributions to the hydrogen-bond (HB) bcp density in a
number of systems with strong HBs
a
System
R
H
A
(
R
D-H
),
˚
R
D
A
,
˚
HB
b
r
b
S%(D)
S%(D+A)
HB1
¼
N1A-H1A···O1A 1.630 (1.046)
Betaine, imidazole
and picric acid
complex [
47
]
2.614
0.051
1.1
57.9
HB2
¼
N3A-H3A···O8
1.635 (1.057)
2.684
0.054
4.9
56.6
HB3
¼
N1B-H1B···O1B 1.681 (1.048)
2.676
0.046
2.3
57.3
nma
c
O1-H(
X
)···O3
1.308 (1.140)
2.391
0.122
23.4
61.1
bza
c
O2-H(
X
1)···O1 1.329 (1.245) 2.502 0.129 30.4 55.5
a
Data from [
47
]. If not otherwise stated, all quantities in au. D and A are the H-donor and
H-acceptor, while H is the hydrogen atom involved in the H-bond
b
Labeling of atoms involved in the HBs according to Fig.
5
c
nma and bza are nitromalonaldeyde and benzoylacetone, respectively
of the hydrogen atom, and that such a surrounding effect on the HB character could
be possibly traced out using the SF tool. Table
7
reports geometrical data for the five
strong HBs investigated and compares their S% atomic contributions to the electron
density at the HB bcp, while Fig.
5
displays the
L
(r) portraits leading to such
contributions.
The LBHB in bza, having an HB distance only slightly larger, 1.329
˚
, than that
of the very elongated O-H bond, 1.245
˚
, exhibits a quite symmetric
L
(r) distribu-
tion around the H atom, which translates in an impressively large SF contribution
from H to the HB bcp density. Its value is 38% larger than in nma and represents
30.4% of the bcp density, to be compared with a corresponding value of only 23.4%
for nma, despite this latter molecule exhibits much shorter R
O
...
O
and slightly
shorter HB distances (Table
7
).
11
The LBHB is thus characterized by an enor-
mously increased H atom source contribution, while the contributions from the
atoms not directly involved in the HB are almost similar in the two systems (14.1%
and 15.5% in bza and nma, respectively). The diminished percentage contribution
from the donor-acceptor atom [S%(D)
¼
9% and 17% in bza and nma] compen-
sates for the higher S%(H) contribution in bza due to the presence of an LBHB.
The strong, but localized HBs in the catalytic triad complex show, instead, the
usual asymmetric
L
(r) distribution around the H atom, with a shared region of
negative Laplacian between the donor N atom and its covalently bonded H atom. As
a consequence, the source percentage contribution from H at the HB bcp density is
small, close to zero and even negative for HB3 (Table
7
). HB energy wells leading
to strong localized HBs are thus characterized by a much smaller source from H
than for single-well or low barrier HBs and by a more than doubled, about 40%, SF
percentage contribution from the surroundings (atoms other than D, A, or H).
Despite its largest N
O separation, 2.684
˚
, HB2 was found to have an
L
(r)
distribution slightly more polarized toward the acceptor atom and the largest source
contribution from H, both in magnitude and percentage value, in agreement with
¼
2.538
˚
) and
11
Note also that S%(H) in bza is dramatically larger than in malonaldeyde
3
(
R
O
O
2.5
˚
, despite a quite similar donor to acceptor separation,
R
O
O
(bza)
in water dimer at
R
O
O
¼
2.502
˚
.
¼
