Chemistry Reference
In-Depth Information
O atoms in
1
and in malonaldeyde (Fig.
4
) reveals how the SF tool reflects the
departure of
1
from a real RAHB system. As discussed earlier, the contribution
from the carbonyl C atom in malonaldeyde is quite significant, S%(C)
8.3, while
that of C11 in
1
is only 5.0%. Analogously, the contributions from the C linked to
the O donor atom are as big as 3.8% in malonaldeyde and only 1.0% for
1
.
Clearly the SF analysis alone is not able to provide an answer to what causes a
strong O3-H1 bond in
1
despite the partial lack of the usual RAHB mechanisms.
12
Nor it may offer alone an answer to the occurrence of two quite different O
¼
H
interactions in the two six-membered rings of
1
. Yet, despite these limitations, it
sheds important light on and quantifies the different involvement of the various atoms
in determining the densities at the two HB bcps. One may assume these differences
be related to the environmental influence on the HB geometry and strength.
The very recent study by Schmidtmann et al. [
50
] on polymorphic isocotinami-
de-oxalic acid molecular complexes with strong O
N hydrogen bonds has the
great merit of applying, for the first time for HBs, the SF tool to both experimental
and theoretical Laplacian distributions. To avoid inconsistencies between the two
distributions due to the shortcomings of the multipole model, the theoretical
structure factors obtained from the periodic calculation were refined using the
same multipole model adopted for the experimental data. SF calculations were
then performed with the XD2006 package [
59
], taking as input the parameters
refined from either the experimental or the theoretical structure factors. Figure
7
displays experimental and theoretical
L
(r) plots for the O
H
H
N SSHB between
Fig. 7 Plots of the experimental (
left
) and theoretical (
right
) negative Laplacian
L
(r) in the
C1-O1-N1 planes of the O1···H1···N1 SSHB in the stable polymorph of the isonicotinamide-
oxalic acid molecular complex. Source function percentage contributions to the density at the
H1···N1 bcp are reported for the O1, H1, and N1 atoms (adapted from Figs. 4 and 5, with
permission from [
50
], Copyright 2009, American Chemical Society)
12
The decrease of the carbonyl atom contribution with respect to malonaldeyde could be compen-
sated for by a distributed contribution from the carbon atoms of the conjugated naphthalene ring.
