Polymorphism and Negative Linear Compressibility in Pyrazine-d4.

Pyrazine (1,4-diazine) is a widely used linker in coordination polymers, molecular magnets, and metal-organic frameworks, yet its solid-state polymorphism is controlled by weak C-H···N interactions. Using variable-temperature and high-pressure neutron diffraction, a previously unrecognized low-temperature transition to phase IV is identified, showing that the form stabilized under pressure can also be accessed below ∼90 K at ambient pressure in the perdeuterated compound. The transition from phase III → IV involves a symmetry-breaking lattice distortion driven by the reorganization of C-H···N hydrogen bonds, accompanied by negative linear compressibility, which is primarily accommodated through the reduction in void-space within the structure. Structural analysis reveals that while the overall monoclinic framework of phase IV is preserved under both temperature and pressure routes, the evolution of hydrogen-bonded network is distinct depending on the pathway taken. These results establish pyrazine as a model molecular solid for studying pressure-temperature polymorphism and demonstrate how weak directional interactions can produce anomalous mechanical responses in organic crystals.