Chemistry Reference
In-Depth Information
The significant advances that have been made in the last 17 years or so in the
capability of determining crystal structures of organic molecular solids from powder
XRD data have been catalyzed largely by the availability of direct-space techniques
for structure solution, both because structure determination of molecular materials is
particularly well suited to these techniques and because of improvements in computer
power during this time. To date, most reported crystal structure determination of
organic molecular solids from powder XRD data has used the direct-space strategy,
although there have also been several reports of successful structure determination
of such materials using the traditional approach (see examples discussed in [
3
,
7
]).
The first demonstration [
83
] of structure solution of a molecular crystal from powder
XRD data was for the
previously known
structure of cimetidine using the traditional
approach for structure solution (direct methods) and using data recorded at a
synchrotron radiation source. The first
previously unknown
equal-atom molecular
crystal structure to be solved from powder XRD data was formylurea, again using
the traditional approach (direct methods) but from laboratory powder XRD data [
84
].
The first material of unknown crystal structure to be solved using a direct-space
strategy was
p
-BrC
6
H
4
CH
2
CO
2
H[
2
] using the Monte Carlo method, followed by
other examples (including 3-chloro-
trans
-cinnamic acid [
38
] and 1-methylfluorene
[
85
]) using the same technique.
For structure solution by the direct-space approach, the complexity of the
structure solution problem is dictated largely by the dimensionality of the hyper-
surface to be explored (i.e. the total number of structural variables in the set
)
rather than the number of atoms in the asymmetric unit. Thus, the greatest
challenges in the application of direct-space techniques arise when the number of
structural variables is large; this situation occurs when there is considerable molec-
ular flexibility (i.e. when the molecular geometry is defined by a large number of
variable torsion angles) and/or when there are several independent molecules in the
asymmetric unit.
We note that for molecular solids or other materials constructed from
well-defined modular building units (such as metal-organic-framework materials),
the direct-space strategy represents a particularly suitable approach for structure
solution, given the wide availability of reliable information on the geometries of the
molecular building units. For other types of materials, for which the peak overlap
problem is less severe or for which there is insufficient prior knowledge of the
geometry of a suitable structural fragment for use in direct-space calculations,
the traditional approach would generally be the favoured approach for structure
solution.
G
2.7 Structure Refinement
In Rietveld refinement [
22
,
27
,
28
], the variables that define the powder XRD
profile (i.e. the variables (1)-(4) in Sect.
2.5
) and the variables defining the structural
model (which are used to determine the relative peak intensities in the calculated
