Chemistry Reference
In-Depth Information
Table 6 Dipole moments D in Debye from Hirshfeld-atom refinement (
D
HAR
) and from X-ray
constrained wavefunctions (
D
XCW
) from both Hartree-Fock and Density Functional Theory using
the in-cluster HAR geometry and the DZP basis set
Structure
HF
DFT
D
HAR
3
D
HAR
3
D
XCW
Dm
/[%]
D
XCW
Dm
/[%]
L
-Alanine
12.6
13.4
+6
11.0
11.9
+8
L
-Cysteine
11.7
12.3
+5
10.1
10.7
+6
L
-Glutamine
12.7
14.2
+12
11.2
12.8
+14
D,L
-Serine
14.0
15.1
+8
12.2
13.1
+7
L
-Threonine
11.2
12.9
+15
10.0
12.1
+21
D,L
-Aspartic Acid
11.6
12.8
+10
10.4
11.4
+9
D,L
-Histidine
16.1
17.1
+6
14.1
15.1
+7
5.2 Molecular Dipole Moments and Their Enhancement from
Hirshfeld-Atom Refinement and Wavefunction Fitting
Wavefunction fitting [
8
,
15
-
18
] can be expected to yield better accuracy for
properties derived from experimental Bragg data than those derived from the
multipole model, since a basis-set description of chosen sophistication can be
used to model the electron density. We have chosen the DZP basis [
63
] already
used in Sect.
4.1
(see Table
4
). Wavefunction fitting requires a weighting of
the experimental data with a multiplier [
8
] to extract the information content
of the individual experimental observations and their standard uncertainties.
Hence, the fitting procedure is more demanding than a single-point cluster calcula-
tion and needs several repetitions, gradually increasing the multiplier. Geometries
obtained from Hirshfeld-atom refinement with cluster charges and dipoles were
used and kept fixed. Geometries were assured to be consistent with the basis set this
way, which would not have been achieved had invarioms geometries been used.
Also, effects on the geometry due to small changes in the dipole moment are
avoided.
3
In Table
6
dipole moments obtained are given together with the
isolated-molecule result already reported in Table
4
. Since the same geometry is
used, an enhancement or decrease is reported. A direct comparison to dipole-
moment enhancements derived using the Hansen/Coppens multipole model
(Table
5
) is possible. Analogous to the multipole-model result a strong increase
of the in-crystal dipole moment is not observed as it was predicted from theory.
Trends from wavefunction fitting hence confirm the results obtained from the
multipole model.
3
A change in dipole moment due to small adjustments of the geometry between Hirshfeld-atom
and invariom refinement can be studied by comparing the dipole to the value given in Table
4
,
where the invariom geometry was used as input. It is found to be insignificant, with the largest
difference being 0.1 Debye for Alanine.
