Chemistry Reference
In-Depth Information
Table 20 Analysis of the ED at the BCP
A
within a single molecule (a) and at the BCP
B
of the
hydrogen bond linking two molecules (b)
r BCP
ð
Þ
r
2
r
ð
BCP
Þ
Source function contributions to the ED at the
BCP
Dimer
1st shell
2nd shell
(a)
Dimer
2.07
46.68
2.07
46.68
100
Dimer + 1st shell
2.06
46.45
2.06
46.45
98
Dimer + 1st and 2nd shell 2.06
46.46
2.06
46.46
98
(b)
Dimer
0.34
3.16
100
-
-
Dimer + 1st shell
0.34
3.04
90
11
0
Dimer + 1st and 2nd shell 0.34
3.01
90
10
2
EDs are given in (e/
˚
3
), Laplacians in (e/
˚
5
), and the contributions in percent
The datasets are collected in Table
20
. It summarizes the computed densities,
Laplacians, and contributions for model 1, i.e., pure gas phase calculations, only
considering the atoms of the given cluster. We refrain from giving the values for
models 2 (using COSMO) and 3 (using point charges) since the values do not differ
significantly from those obtained in model 1. As expected, the EDs at the covalent
BCP
A
(about 2.09 e/
˚
3
) are almost six times higher than those at the H-bond BCP
B
(about 0.34 e/
˚
3
). This reflects the different nature of the two bonds. The deviations
of these values along the series of clusters are almost negligible (
0.07 e/
˚
3
for
0.01 e/
˚
3
for BCP
B
), although the surrounding changes
dramatically from a complete coverage in the largest clusters to the bare dimer.
This is in line with the low sensitivity of the density that is generally observed with
respect to environmental changes. Unexpectedly, the same also holds for the
Laplacians.
The source function was used to obtain a deeper insight into possible differences
of the environmental influences on the ED. As for the hydrocarbon chains, more
than 95% of the total contributions to BCP
A
originate from the two atoms (O1 and
H2) which form the bond. Even the first shell of surrounding atoms contributes less
than 5%.
The ED at the H-bonded BCP
B
behaves somewhat different. In contrast to the
covalent bond, the ED at this point is mainly determined by the heavy atoms in the
vicinity, namely O1 and O4 (see Fig.
19a
). Each of them contributes roughly 40%
to the ED. Although the atom H2 is the nearest atom to the BCP
B
and even though it
is directly involved in forming the bond, its contribution to the ED is very small, in
most cases even negative. This is in agreement with the results of Gatti [
47
] for the
pure dimer, where the corresponding hydrogen atom H2 has also been found to act
as a sink in the range of the equilibrium distance.
For the BCP
B
of the hydrogen bond, the first shell of the surrounding water
molecules contributes about 10% of the ED. Hence, describing the electronic
structure in the hydrogen bonding network between enzyme and inhibitor requires
only the residues of the protein to be included in the QM part which are directly
involved in forming the hydrogen bond.
BCP
A
and less than
