akin to the ‘bulky’ R-group. During crystal growth the crystal fractured on cooling
so that structure refinement was not possible. The data had to be collected at 403 K
so that the crystal remained intact for the duration of the collection. It was speculated
that the fracture of the crystal was due to an increase in the disorder on cooling
[23]. Further work investigated the co-crystallisation of 2-methylphenol and
2-chlorophenol due to their isostructurality and demonstrated that both transform
to the linear chain behaviour at high pressure. In the case of 2-chlorophenol, a new
polymorph was discovered at even higher pressure than previous studies accessed
through a failed co-crystallisation attempt with 2-methylphenol at high pressure
[21]. More recently, Barnett and Allen have investigated the crystallisation of
trifluoroethanol and observed two different phases they are composed of twofold
chains. Interestingly, in this example, chemical pressure (substitution of hydrogen
for fluorine) has induced a change in the packing of the molecules at low temperature
to be similar to the high-pressure form of ethanol, whilst the high-pressure polymorph hydrogen bonds in a similar manner, but the packing of the molecules is more
strained.
So there is a wide variety of behaviour that the alcohol systems exhibit. Nine out
of 16 molecules studied so far alter their structure from a packing arrangement for
‘bulky’ R-groups to a packing for ‘thin’ R-groups with the application of pressure.
Hydrogen bonds lengthen to enable the more efficient packing of these R-groups as
the volume is reduced. The fact that methanol crystallises into another linear
arrangement to promote packing that does not conform to ideal twofold symmetry
(Fig. 7) does suggest that we may be able to take these systems and alter their
packing even further by achieving higher pressures. The application of the mixed
PTM techniques in Sect. 2 may be a route that would aid the discovery of new crystal
packing of simple alcohols.
3.2 Halogenated Compounds
Halogen bonding is an area of expanding interest over the last few years. The
manipulation of halogen bonding interactions is a critical tool in the area of crystal
engineering where new molecular complexes are formed via this interaction.
Readers are directed to an extensive review of the area by Cavallo et al. that
highlights many of the advancements in the area [83]. We will provide an overview
of how these interactions may be altered or used to great effect to promote chemical
reactions.
3.2.1 Low-Melting Examples
There have been several studies that have investigated van der Waals halogenated
solids to elucidate how halogen bonds are altered as a function of pressure in a
similar manner to the studies of alcohols above. Some of the simplest molecules
162
S. A. Moggach and I. D. H. Oswald
so that structure refinement was not possible. The data had to be collected at 403 K
so that the crystal remained intact for the duration of the collection. It was speculated
that the fracture of the crystal was due to an increase in the disorder on cooling
[23]. Further work investigated the co-crystallisation of 2-methylphenol and
2-chlorophenol due to their isostructurality and demonstrated that both transform
to the linear chain behaviour at high pressure. In the case of 2-chlorophenol, a new
polymorph was discovered at even higher pressure than previous studies accessed
through a failed co-crystallisation attempt with 2-methylphenol at high pressure
[21]. More recently, Barnett and Allen have investigated the crystallisation of
trifluoroethanol and observed two different phases they are composed of twofold
chains. Interestingly, in this example, chemical pressure (substitution of hydrogen
for fluorine) has induced a change in the packing of the molecules at low temperature
to be similar to the high-pressure form of ethanol, whilst the high-pressure polymorph hydrogen bonds in a similar manner, but the packing of the molecules is more
strained.
So there is a wide variety of behaviour that the alcohol systems exhibit. Nine out
of 16 molecules studied so far alter their structure from a packing arrangement for
‘bulky’ R-groups to a packing for ‘thin’ R-groups with the application of pressure.
Hydrogen bonds lengthen to enable the more efficient packing of these R-groups as
the volume is reduced. The fact that methanol crystallises into another linear
arrangement to promote packing that does not conform to ideal twofold symmetry
(Fig. 7) does suggest that we may be able to take these systems and alter their
packing even further by achieving higher pressures. The application of the mixed
PTM techniques in Sect. 2 may be a route that would aid the discovery of new crystal
packing of simple alcohols.
3.2 Halogenated Compounds
Halogen bonding is an area of expanding interest over the last few years. The
manipulation of halogen bonding interactions is a critical tool in the area of crystal
engineering where new molecular complexes are formed via this interaction.
Readers are directed to an extensive review of the area by Cavallo et al. that
highlights many of the advancements in the area [83]. We will provide an overview
of how these interactions may be altered or used to great effect to promote chemical
reactions.
3.2.1 Low-Melting Examples
There have been several studies that have investigated van der Waals halogenated
solids to elucidate how halogen bonds are altered as a function of pressure in a
similar manner to the studies of alcohols above. Some of the simplest molecules
162
S. A. Moggach and I. D. H. Oswald