High-Pressure and Low-Temperature Polymorphism in C–H···F–C Hydrogen Bonded Monofluorotoluenes

Three fluorotoluene isomers, all
liquids under ambient conditions,
have been crystallized using two distinct in situ methods: cryo-crystallization
(by decreasing the temperature below the freezing point) and through
the application of high-pressure. In all three isomers, crystallization
from high-pressure results in different polymorphic forms to those
observed at low-temperature: cryo-crystallization and compression
of the pure liquid results in the formation of two distinct polymorphs
of both 3- and 4-fluorotoluene, but only one polymorph of 2-fluorotoluene.
However, compression of a 1:1 mixture of 2- and 3-fluorotoluenes results
in a different polymorph of 2-fluorotoluene to that seen when crystallizing
the pure liquid. The effects of temperature and pressure on the crystallization
process in systems that have only weak C–H···F–C
hydrogen bonds are confirmed to be very different.