Nearly Isostructural Polymorphs of Ethynylbenzene:  Resolution of ⋮CH···π(arene) and Cooperative ⋮CH···π(C⋮C) Interactions by Pressure Freezing

In ethynylbenzene, elevated pressure favors cooperative ⋮CH···π(C⋮C) hydrogen bonds compared to ⋮CH···π(arene) and leads to the ordered polymorph nearly isostructural form with the crystals frozen by cooling. The high-pressure polymorph is more resistant to polymerization than the liquid and, in this respect, is a manifestation of Le Chatelier principle explained at the microscopic level for this compound. A new type of the polymorphic relation has been described.