Chemistry Reference
In-Depth Information
Table 2 Significant multipole populations of hydrogen atoms averaged over equivalents.
N
obs
is
the number of entries for each set of different H-atom types
00
11
22
22
þ
N
obs
10
30
CH
0.953(7)
0.072(2)
43
CH
2
0.951(14)
0.071(4)
30
CH
3
0.957(8)
0.069(3)
21
NH
0.881(9)
0.085(3)
0.011(3)
5
0.013(3)
NH
2
pyramidal
0.893(6)
0.084(2)
14
NH
2
planar
0.868(7)
0.091(2)
0.008(1)
20
NH
3
0.820(13)
0.089(5)
18
COOH
0.802(4)
0.012(1)
0.099(2)
0.011(1)
5
OH
0.833(9)
0.008(2)
0.096(1)
0.009(2)
0.014(1)
7
0.8
a
b
2
0.7
aspirin
α
-cyanoacrylic acid
N-acetylglycine
4-nitrobenzoic acid
DL-aspartic acid
0
0.6
0.5
-2
2-aminophenol
3-aminophenol
α
0.4
-cyanoacetohydrazide
1,3-diaminourea / N
1
1,3-diaminourea / N
2
2-amino-2-hydro-6-hydroxypyridazin-3-one
2-methyl-4-nitroaniline
-4
0.3
-6
0.2
r
(angstrom)
0.1
-8
r
(angstrom)
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Fig. 5 Selected radial function for chemically similar stockholder atoms before averaging:
(a) H_R
11
[COOH], (b) N_R
11
[NH
2
]
quadrupolar, that of the H-atoms is of dipolar, while that of the N-atom is of
octupolar symmetry.
Figure
5
depicts the
H_R
11-
[COOH]
and
N_R
11-
[NH
2
]
functions for which the
largest discrepancies are found before averaging. The peak-height differences
observed for some of the SPA-RDFs for “equivalent” atoms suggests that the
stockholder decomposition is quite sensitive to fine details in the chemical environ-
ment. The consistency in the peak locations, on the other hand, might imply that
these environmental differences manifest themselves only locally.
In general, the monopole SPA-RDFs show a high degree of chemical transfer-
ability. Even for the H-atoms, only relatively small variations of the
H_R
00
[X-H]
functions with the neighboring atom type (
X
) are observed, but only near to the
proton's position. Figure
6
displays SPA-RDFs averaged over chemical equivalents
and corresponding to the dominating deformation densities for each atom. The
averaged
H_R
10
[X-H]
functions associated with the bond-directed dipolar defor-
mation of the H-atoms have quite similar shape for all
X-H
bonds (Fig.
6a
). The
location of the maxima (~0.25
˚
) seems to be independent of the bonding situation,
but the peak-height scatters in a wide range (0.6-1.6) and correlates with the
