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 center dot center dot center dot F-C hydrogen bonds are confirmed to be very different.