Chemistry Reference
In-Depth Information
Table 5 Dipole moments
D
in Debye from invariom refinement (
D
inv
) and from a refinement of
multipole parameters (
D
exp
) using the same multipole model and geometry
Structure
D
inv
D
exp
Dm
[%]
Theory
Multipole projection
Dm
[%]
L
-Alanine
12.1
12.5
+3
11.4
(9.9)
+9
L
-Cysteine
11.2
11.2
0.0
10.5
(9.4)
+6
L
-Glutamine
13.1
13.4
+2
11.5
(10.8)
+14
D,L
-Serine
13.5
12.9
4
12.5
(11.1)
+3
L
-Threonine
11.9
12.0
+1
10.0
(9.2)
+17
D,L
-Aspartic Acid
13 10.6 (8.8) +7
D,L
-Histidine 15.7 17.9 +14 14.5 (12.3) +19
Results of a DFT single-point calculation (“Theory”) with the method/basis-set B3LYP/D95
++(3df,3pd) are given in the right column for comparison. Results from a multipole projection
of the DFT density are found to be systematically lower than the single-point results. Hence both
single-point (and even more so multipole projection) gives a more pronounced enhancement than
the invariom-database [
27
] fragments
13.1
11.4
(gas-phase) reference value from theory. A convenient choice for obtaining refer-
ence dipole moments for results from experimental multipole refinements is
invariom modelling, since it allows a dipole-moment estimate even for large
molecules at negligible computational cost. By calculating the difference between
experimental dipole moment and the invariom result (
1
) an enhancement is
obtained. To allow a fair comparison between dipole moments from experimental
multipole refinement and invariom model we use the same multipole model [
7
] (i.e.
the same local atomic site symmetry and chemical constraints) in both cases. This
will be detailed below in the following section.
Invariom modelling is an attempt to apply the benefits of a scattering model
that is superior to the independent atom model (IAM) to general small-molecule
[
33
,
59
] and ultra-high-resolution macromolecular crystallography [
27
,
60
,
61
].
Similar scattering-factor databases are available [
38
,
39
]. In contrast to the experi-
mental multipole refinement, in invariom refinement theoretically predicted multi-
pole populations are kept fixed, so that the number of refinable parameters does
not increase with respect to the IAM. Like in the IAM, only positional and
displacement parameters are adjusted to the experimental Bragg data.
To put the following results into perspective we need to be aware that both
invariom modelling and experimental multipole refinement only permit to obtain
the molecular dipole moment within the accuracy the multipole model is capable to
provide, as discussed in Sect.
3
.
5.1 Molecular Dipole Moments and Their Enhancement in
the Solid State from Experimental Multipole Refinement
and Invariom Refinement
An invariom refinement was performed for the seven datasets considered (Table
1
).
The input files for invariom refinement were generated by the program
INVARIOMTOOL
