Can identical crystals melt into two distinct liquids?

Seemingly identical molecular crystals of acetaldehyde phenylhydrazone (APH), C8H10N2, are reported to melt to liquids with distinctly different structures, and with different melting points of between 60 and 100 C. The low- or high-melting forms can be reliably prepared by exposure of crystals to trace acid or base (as little as 1/1000th molar equivalent) respectively, with no detectable change to the crystal structure. Although rare, this remarkable and strange behavior demands further investigation. It has implications for our fundamental understanding of phase transitions (kinetics and thermodynamics), the structure of liquids and relaxation phenomena in general. This experiment will provide new information on the intermolecular interactions at play, possible temperature dependence of the isomer ratio and the purported E-isomer crystal form.

据报道，两组外观完全一致的乙醛苯腙（acetaldehyde phenylhydrazone, APH，分子式C₈H₁₀N₂）分子晶体，熔融后可形成结构截然不同的液体，且二者熔点介于60至100℃之间且互不相同。通过分别将晶体暴露于痕量酸或碱（摩尔当量仅为1/1000），可稳定制备低熔点或高熔点晶型，且该过程不会引起晶体结构的可检测变化。尽管该现象较为罕见，但这种非同寻常的奇特行为仍需开展进一步研究。其对我们在相变（动力学与热力学）、液体结构以及普遍弛豫现象领域的基础认知具有重要启示。本实验将为探究所涉及的分子间相互作用、异构体比例的潜在温度依赖性，以及此前所报道的E-异构体（E-isomer）晶型提供全新的研究信息。