and HP phase and isolate the new form. In each of the phases, the CH. . .F interaction
is favoured.
Anioła et al. have probed chlorinated and bromonated meta-disubstituted benzenes by mixing the material with methanol to help to achieve higher pressures than
would be able using the pure forms similar to our previous example (mixed PTM
method) [86]. m-Dichloro- and dibromobenzene were investigated individually as
well as a solid solution. The pure phases observed at high pressure were the same as
those crystallised under cooling. An interesting observation from a methodological
point of view was that in an attempt to co-crystallise the two solids together, solid
solutions are formed from methanol solutions but only at pressures greater than
1.02 GPa; below this the pure m-dichlorobenzene phase is favoured. If, however, a
mixture of the two pure materials is used, without methanol, then the solid solution
can be isolated. In this case, the difference in the solubilities of the pure materials in
methanol is an important property at lower pressures, i.e. the m-dichlorobenzene
precipitates before the dibromobenzene, but at higher pressures the difference in this
physical property becomes negligible; hence, the solid solution can be crystallised.
The latter study of m-dichlorobenzene, together with the 2-chlorophenol [21],
2-fluorotoluene [85] and the acrylic acids [31, 33, 34], demonstrates that even if
there are no apparent phase transitions at high pressure, there may be potential for the
isolation of new forms through alternate methodologies. It is worthwhile to explore
mixing the compound of interest with another solvent or PTM to investigate whether
the system can be pushed to higher pressures and potentially over phase boundaries
to new unidentified phases.
3.2.2 Compression of Solids
The next few examples explore the effect of pressure on heteromeric intermolecular
interactions whether they are in a single pure compound or whether they are in a
multicomponent form (salt or co-crystal).
4-Iodobenzonitrile is a simple disubstituted benzene that possesses an
intermolecular interaction between a nitrile group and the iodine [87]. The solid is
constructed through chains of molecules, exploiting this interaction, that lie antiparallel with their neighbours. The compression of this phase demonstrates the importance of the non-directional π. . .π and weak H. . .N interactions rather than the
visually recognisable N. . .I halogen bond. Giordano et al. observed that the π. . .π
interaction becomes very repulsive on compression until a phase transition occurs at
5.5 GPa from monoclinic I2/a to P-1. The lower symmetry enables the separation of
the π. . .π interaction into two symmetry independent interactions so that one of these
interactions increases in length, stabilising it with respect to the lower pressure phase
(21 kJmol
À1 cf. 28 kJmol
À1 ), whilst the other continues to be compressed (Fig. 9).
The authors identified that it is the change in these ‘weaker’ interactions that
promotes the phase transition at pressure. So even in these halogenated compounds,
the packing efficiency is key, sacrificing the length of the more recognisable N. . .I
interaction for a closer packing of the main benzene backbone.
164
S. A. Moggach and I. D. H. Oswald
is favoured.
Anioła et al. have probed chlorinated and bromonated meta-disubstituted benzenes by mixing the material with methanol to help to achieve higher pressures than
would be able using the pure forms similar to our previous example (mixed PTM
method) [86]. m-Dichloro- and dibromobenzene were investigated individually as
well as a solid solution. The pure phases observed at high pressure were the same as
those crystallised under cooling. An interesting observation from a methodological
point of view was that in an attempt to co-crystallise the two solids together, solid
solutions are formed from methanol solutions but only at pressures greater than
1.02 GPa; below this the pure m-dichlorobenzene phase is favoured. If, however, a
mixture of the two pure materials is used, without methanol, then the solid solution
can be isolated. In this case, the difference in the solubilities of the pure materials in
methanol is an important property at lower pressures, i.e. the m-dichlorobenzene
precipitates before the dibromobenzene, but at higher pressures the difference in this
physical property becomes negligible; hence, the solid solution can be crystallised.
The latter study of m-dichlorobenzene, together with the 2-chlorophenol [21],
2-fluorotoluene [85] and the acrylic acids [31, 33, 34], demonstrates that even if
there are no apparent phase transitions at high pressure, there may be potential for the
isolation of new forms through alternate methodologies. It is worthwhile to explore
mixing the compound of interest with another solvent or PTM to investigate whether
the system can be pushed to higher pressures and potentially over phase boundaries
to new unidentified phases.
3.2.2 Compression of Solids
The next few examples explore the effect of pressure on heteromeric intermolecular
interactions whether they are in a single pure compound or whether they are in a
multicomponent form (salt or co-crystal).
4-Iodobenzonitrile is a simple disubstituted benzene that possesses an
intermolecular interaction between a nitrile group and the iodine [87]. The solid is
constructed through chains of molecules, exploiting this interaction, that lie antiparallel with their neighbours. The compression of this phase demonstrates the importance of the non-directional π. . .π and weak H. . .N interactions rather than the
visually recognisable N. . .I halogen bond. Giordano et al. observed that the π. . .π
interaction becomes very repulsive on compression until a phase transition occurs at
5.5 GPa from monoclinic I2/a to P-1. The lower symmetry enables the separation of
the π. . .π interaction into two symmetry independent interactions so that one of these
interactions increases in length, stabilising it with respect to the lower pressure phase
(21 kJmol
À1 cf. 28 kJmol
À1 ), whilst the other continues to be compressed (Fig. 9).
The authors identified that it is the change in these ‘weaker’ interactions that
promotes the phase transition at pressure. So even in these halogenated compounds,
the packing efficiency is key, sacrificing the length of the more recognisable N. . .I
interaction for a closer packing of the main benzene backbone.
164
S. A. Moggach and I. D. H. Oswald