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.

三种氟甲苯（fluorotoluene）异构体在常温常压下均为液态，本研究采用两种不同的原位结晶方法对其开展结晶实验：低温结晶（cryo-crystallization，即通过将温度降至冰点以下实现结晶）与高压加压结晶。对于三种氟甲苯异构体而言，高压结晶所得的多晶型与低温结晶所得的多晶型存在显著差异：对纯液体样品进行低温结晶与加压结晶时，间氟甲苯（3-fluorotoluene）与对氟甲苯（4-fluorotoluene）均可生成两种不同的多晶型，而邻氟甲苯（2-fluorotoluene）仅能生成一种多晶型。然而，将邻氟甲苯与间氟甲苯以1:1的比例混合后进行加压结晶，所得邻氟甲苯的多晶型与纯邻氟甲苯液体结晶所得的多晶型完全不同。研究证实，在仅存在弱C–H···F–C氢键作用的体系中，温度与压力对结晶过程的影响存在显著差异。