Chemistry Reference
In-Depth Information
scattering properties of hydrogen atoms. These properties, comparably weak scat-
tering with limited resolution in reciprocal space, for decades have been known to
cause concern regarding the reliability of properties based on least-squares refined
parameters of H-atoms [
36
]. Since H-atoms are often situated at the molecular
periphery and often far away from the centre of mass, their influence on the
molecular dipole moment can be significant.
2
Accurate X-H bond distances are
therefore imperative. Neutron diffraction experiments are the preferred source of
accurate X-H bond distances; but results for particular molecules or bonding
environments are usually not available due to the considerable additional experi-
mental effort. Favourable developments with the advent of spallation Neutron
sources might change this situation in the future.
Technical improvements help to reduce the problem caused by the scattering
properties of hydrogen. A recent study used external information from periodic
calculations to try to limit the flexibility of the screening parameters [
37
] for C, N
and O. Scattering-factor databases [
27
,
38
,
39
] provide even more accurate
“hybrid” scattering factors, also for hydrogen atoms. For the theoretical databases
[
27
,
39
] these hybrid scattering factors are obtained by combining fixed multipoles
from the database - with the order of the expansion
l
1 - with refined monopole
and dipole populations. In that sense hybrid scattering factors for H-atoms can be
seen in analogy to constraints or restraints, since they reduce the flexibility of the
least-squares refinement model by adding prior chemical information. Furthermore,
X-H distances from geometry optimizations can now be used. They are included in
the invariom database [
27
] and can be retrieved with the program
INVARIOMTOOL
[
40
]. In Sect.
5.1
we show how hybrid hydrogen scattering factors and fixed X-H
bond distances can increase the reliability of the determination of dipole moments
from multipole refinements.
2.2 Experimental Challenge: Data Resolution
Apart from a careful treatment of hydrogen scattering, another requirement for the
determination of reliable dipole moments from multipole refinement of X-ray
diffraction is good quality low-temperature intensity data, preferably extending
high into reciprocal space. These are required in order to refine the large number of
possible least-squares parameters per atom (three positional, six displacement and
up to 25 multipole parameters up to
l
max
¼
4, not counting radial screening
parameters). Low temperature is mandatory, since experimental conditions are
more favourable, e.g. regarding the significant reduction of atomic displacements
and thermal diffuse scattering [
41
]. For further (experimental) requirements
concerning multipole refinements of X-ray diffraction we refer to [
42
].
2
One way to resolve the issue is to choose a sample devoid of hydrogen altogether.
