CH···N Bonds and Dynamics in Isostructural Pyrimidine Polymorphs

In the crystalline pyrimidine, the most basic building block of biochemical systems, the changes of entropy and enthalpy combine into a series of discrete structural transitions that have defied detection until now. The counterintuitive fully isostructural polymorphs of pyrimidine differ mainly by entropy, while the varied space occupied by molecules in partly isostructural polymorphs can be connected to the molecular dynamics, too. The interplay of thermodynamic and structural features affects the molecular interactions and environment and is most relevant to the functions of all organic compounds in the living tissue. The single crystals of four pyrimidine polymorphs have been grown at isobaric, isothermal, and isochoric conditions, and their structures have been determined by X-ray diffraction.