O. Franco et al. / Journal of Molecular Structure 649 (2003) 219–230
221
To investigate the crystallization behavior of DPO
and to determine the appropriate conditions for the
precipitation of a single phase, either DPO I or DPO
II, several different experiments have been carried out
carefully. The synthesized DPO was recrystallized
from different solvents like petrolether or ethanol with
varying dilution at ambient or decreased temperature.
Additionally, the products resulting from a distinct
solvent were dissolved in the other solvent and
recrystallized again. The resulting crystals were first
microscopically inspected for their morphology and in
such cases were only ill crystallized material resulted
by Raman spectroscopy to determine the phase
content of the sample. But, in most cases the visual
inspection of well crystallized samples is sufficient to
differentiate between both structures having a unique
habit (needle, plate) as could be concluded from
extensive Raman investigations (see below).
For the high temperature measurement a heated air
stream was directed onto the same crystal also used for
theroomtemperaturemeasurement. ThedataofDPOII
were recorded at 293 K. The structures were solved by
direct methods and refined by full-matrix least-squares
calculations using SHELX-97 [16]. All hydrogen atoms
of both DPO I structures were located by difference
Fourier syntheses and refined isotropically. In the case
of DPO II only small single crystals with relatively
weak diffraction power were available. Therefore, the
atomic coordinates were refined using restraints
demanding nearly equal lengths for corresponding
bonds of the six symmetry independent molecules. The
existence of the space group Cc instead of C2/c was
confirmed by the results of the structure analysis. A
careful inspection of the molecular packing using
CELLGRAF [29] indicated that there are no twofold
axes in the crystal structure. The hydrogen atoms of
DPO II were calculated according to their geometrical
conditions and included in the structure refinement
using the riding model. Atomic coordinates, thermal
parameters, bond lengths and angles have been
deposited at the Cambridge Crystallographic Data
Centre (CCDC) and allocated the deposition number
CCDC 190263 (DPO I at 293 K), CCDC 190264 (DPO
I at 353 K), and CCDC 160022 (DPO II).
In general, it has to be stated that it was rather
impossible to obtain a fully crystallized sample
containing only one single unique DPO phase from
the solution experiments. Always both phases were
present in different ratios at least in traces like small
admixtures, especially few needles of phase I
(
preferentially grown at impurities) in larger amounts
of phase II or very small crystallites grown on the
surface of larger crystals. This is mainly observed if
phase I crystallizes in excess. Then some needles with
very small crystallites of phase II on their surface are
found. In the latter two cases the admixtures are well
below the X-ray detection limit and may best be
detected visually. Therefore, it is concluded, that the
final structure is independent on the DPO synthesis
route as could be the case for instance by the existence
of very small and nearly inextractable traces of
synthesis residues or molecular fragments. So, only
the influence of the solvent and the ambient conditions
have to be taken into account.
Crystal data of DPO I (at 293 K). A colorless
needle shaped crystal with dimensions 1.00 £
3
0.075 £ 0.070 mm was used for data collection.
Crystal system: monoclinic, space group P2 /c (No
1
14), a ¼ 0.51885(6) nm, b ¼ 1.8078(2) nm, c ¼
1.21435(14) nm, b ¼ 93.193(3)8, V ¼ 1.1373(2)
3
3
nm , Z ¼ 4, d(calc.) ¼ 1.298 Mg/m , linear absorp-
2
1
tion coefficient m ¼ 0.083 mm , and Fw ¼ 222.24
for C H N O, F(000) ¼ 464. The total number of
1
4 10 2
reflections in the range 4.08 , 2Q , 46.68 was
10468, 1595 independent, 1154 observed ½I .
2sðIÞꢀ: The number of refined parameters was 195.
Final R (on F) ¼ 0.0486.
The same holds also for sublimated DPO. The
product predominately consists of DPO I needles that
are stronger agglomerated but also contains smaller
admixtures of DPO II stuck together in polycrystalline
grains. Smaller crystallites of the second structure are
also found on the surface of the needles.
Crystal data of DPO I (at 353 K). The same crystal
described above was used for data collection. Crystal
system: monoclinic, space group P2 /c (No 14),
1
a ¼ 0.52131(12) nm, b ¼ 1.8116(5) nm, c ¼ 1.2155(3) nm,
3
b ¼ 92.933(9)8, V ¼ 1.1464(5) nm , Z ¼ 4, d(calc.) ¼
3
X-ray structure analysis. The X-ray data of DPO I
were collected on a SIEMENS SMART diffractometer at
1.288 Mg/m , linear absorption coefficient m ¼
2
1
0.083 mm , and Fw ¼ 222.24 for C H N O,
1
4
10
2
2
0
93 and 353 K using Mo Ka radiation (l ¼
F(000) ¼ 464. The total number of reflections in
the range 4.08 , 2Q , 46.68 was 10685, 1580
˚
.71073 A) monochromatized by a graphite crystal.