low-temperature devices [75]. To dovetail with these experimental efforts, in 1994
Brock and Duncan used the Cambridge Structural Database (CSD) to investigate the
unusual distribution of space groups in which monoalcohols crystallised [76]. They
were able to identify common structural motifs, e.g. crystallisation in Z
0
> 1 in
low-symmetry space groups or Z
0
¼ 1 in high-symmetry space groups. This departure from common packing motifs was attributed to the size of the R-groups attached
to the alcohol moiety. Even within the dataset, they observed different behaviour
depending on the relative size of the R-group. For small R-groups, there was
propensity to crystallise around twofold chains, but as the size of the alkyl group
increased, higher fold chains, helices or discrete dimers were observed. Due to the
effects of group size and the role of packing efficiencies in these systems, high
pressure has been explored as a method to tune the molecular packing [14–16, 23,
77–80].
Initial studies on methanol [15], ethanol [14], phenol [81] and cyclobutanol [77]
confirmed the hypothesis that by applying pressure to these systems, the packing of
the molecules changed from being that of bulky alcohols to smaller alcohols:
i.e. threefold axes changed to be linear twofold chains in the case of cyclobutanol;
phenol is reduced from a pseudo threefold axis with Z
0
¼ 3 to a 2 1 -screw with Z
0
¼ 1
(Fig. 7). 2-Butanol crystallises around a 2 1 -screw axis at 2.14 GPa [82] which fits the
hypothesis, but unfortunately it has not been explored at low temperature to confirm
if it has the packing of a ‘bulky’ R-group. Other alcohols, such as 2-chlorophenol
and 4-fluorophenol, have also followed a similar pattern, but it was apparent that this
is not universal amongst all alcohols [80].
The change from cyclobutanol to cyclopentanol [78] sees a change in the
behaviour as the latter adopts a flattened fourfold chain at 1.5 GPa that is attributed
to a bulky alcohol rather than interaction as a twofold chain. However, the
low-temperature behaviour of cyclopentanol is rather interesting as it exhibits four
low-temperature phases that have yet to be fully characterised. The highesttemperature phases are orientationally disordered in hexagonal symmetry; hence,
from the perspective of Brock and Duncan’s rules, a change to a structure in which
the molecules adopt a static position will be lessening the ‘bulk’ of the R-group;
hence, one could say that pressure has altered the behaviour. Isopropyl alcohol [16]
crystallises with unusual eight-membered hydrogen-bonded rings at 1.1 GPa with
Z
0
¼ 4, and t-butanol [79] does not show any change in phase with respect to
pressure to 0.85 GPa.
Building on the work on the alcohols, we investigated halogen-containing phenols to extend the size of the R-group and understand whether position of substitution would affect the outcome of the crystallisation [21, 23, 80]. At the time there
was an increase in activity around organic fluorine which provided an interesting
subtext to our research. Using the high-pressure recrystallisation techniques in
combination with low-temperature studies and crystal structure prediction methods,
we explored a range of monoalcohols, e.g. 2-, 3- and 4-fluorophenol and 2-, 3- and
4-chlorophenol. The behaviour varied markedly over the series of compounds and
did not necessarily fit the observations that had been made previously regarding the
change in packing at high pressure. 2-Chloro- and 4-fluorophenol showed behaviour
160
S. A. Moggach and I. D. H. Oswald
Brock and Duncan used the Cambridge Structural Database (CSD) to investigate the
unusual distribution of space groups in which monoalcohols crystallised [76]. They
were able to identify common structural motifs, e.g. crystallisation in Z
0
> 1 in
low-symmetry space groups or Z
0
¼ 1 in high-symmetry space groups. This departure from common packing motifs was attributed to the size of the R-groups attached
to the alcohol moiety. Even within the dataset, they observed different behaviour
depending on the relative size of the R-group. For small R-groups, there was
propensity to crystallise around twofold chains, but as the size of the alkyl group
increased, higher fold chains, helices or discrete dimers were observed. Due to the
effects of group size and the role of packing efficiencies in these systems, high
pressure has been explored as a method to tune the molecular packing [14–16, 23,
77–80].
Initial studies on methanol [15], ethanol [14], phenol [81] and cyclobutanol [77]
confirmed the hypothesis that by applying pressure to these systems, the packing of
the molecules changed from being that of bulky alcohols to smaller alcohols:
i.e. threefold axes changed to be linear twofold chains in the case of cyclobutanol;
phenol is reduced from a pseudo threefold axis with Z
0
¼ 3 to a 2 1 -screw with Z
0
¼ 1
(Fig. 7). 2-Butanol crystallises around a 2 1 -screw axis at 2.14 GPa [82] which fits the
hypothesis, but unfortunately it has not been explored at low temperature to confirm
if it has the packing of a ‘bulky’ R-group. Other alcohols, such as 2-chlorophenol
and 4-fluorophenol, have also followed a similar pattern, but it was apparent that this
is not universal amongst all alcohols [80].
The change from cyclobutanol to cyclopentanol [78] sees a change in the
behaviour as the latter adopts a flattened fourfold chain at 1.5 GPa that is attributed
to a bulky alcohol rather than interaction as a twofold chain. However, the
low-temperature behaviour of cyclopentanol is rather interesting as it exhibits four
low-temperature phases that have yet to be fully characterised. The highesttemperature phases are orientationally disordered in hexagonal symmetry; hence,
from the perspective of Brock and Duncan’s rules, a change to a structure in which
the molecules adopt a static position will be lessening the ‘bulk’ of the R-group;
hence, one could say that pressure has altered the behaviour. Isopropyl alcohol [16]
crystallises with unusual eight-membered hydrogen-bonded rings at 1.1 GPa with
Z
0
¼ 4, and t-butanol [79] does not show any change in phase with respect to
pressure to 0.85 GPa.
Building on the work on the alcohols, we investigated halogen-containing phenols to extend the size of the R-group and understand whether position of substitution would affect the outcome of the crystallisation [21, 23, 80]. At the time there
was an increase in activity around organic fluorine which provided an interesting
subtext to our research. Using the high-pressure recrystallisation techniques in
combination with low-temperature studies and crystal structure prediction methods,
we explored a range of monoalcohols, e.g. 2-, 3- and 4-fluorophenol and 2-, 3- and
4-chlorophenol. The behaviour varied markedly over the series of compounds and
did not necessarily fit the observations that had been made previously regarding the
change in packing at high pressure. 2-Chloro- and 4-fluorophenol showed behaviour
160
S. A. Moggach and I. D. H. Oswald