Chemistry Reference
In-Depth Information
Fig. 16 Crystal structure of the BA/PFBA cocrystal viewed along the stacking axis. Interstack
hydrogen bonding involving the carboxylic acid groups is shown as
dashed lines
. Hydrogen atoms
are not shown, and the fluorine atoms of the PFBA molecules are shown in
green
out very recently [
123
], with the structure determined directly from powder XRD
data exploiting the direct-space strategy for structure solution. Under appropriate
conditions, this reaction proceeds to 100% conversion and thus the powder XRD
pattern does not contain residual amounts of the unreacted monomer phase
(although, as the crystal structure of the monomer was already known, structure
determination of the polymer product would still have been feasible if the powder
had comprised a mixture of monomer and polymer phases).
In the crystal structure of the polymer phase (Fig.
17a
), the polymer chains are
aligned along the
c
-axis and the distance (3.71
˚
) between the centres of adjacent
cyclobutane and pyrazine rings corresponds to half the
c
-axis repeat of the unit cell.
For comparison between the monomer and polymer structures, an overlay plot of
these structures is shown in Fig.
17b
. It is clear that the solid-state reaction is
associated with only very small atomic displacements at the site of the [2+2]
photocyclization reaction (the displacement of the carbon atoms of the C
C double
bonds of monomer molecules on forming the cyclobutane ring of the polymer is
only ca. 0.8
˚
for one pair of carbon atoms and ca. 1.6
˚
for the other pair). Such
small displacements are completely in accord with the assignment of this solid-state
reaction as a topochemical transformation [
124
-
127
] (in which the crystal structure
of the reactant monomer phase imposes geometric control on the pathway of the
¼
