A new polymorph of oxindole (termed as “δ-form”) has been found and characterized by single-crystal X-ray diffraction. Its structural properties were compared with those of the previously reported polymorphs (the α-, β- and γ-forms). This polymorphic form was crystallized for the first time but could not be reproduced. Thus, the δ-form can serve as an example of a “disappearing polymorph”. As a result, the three previously reported α-, β- and γ-forms have now been characterized by powder X-ray diffraction, vibrational spectroscopy (infrared and Raman) and calorimetry (differential scanning calorimetry). In the structures of the earlier known α- and β-forms, N–H⋯O hydrogen bonds connect the molecules into dimers, while the crystals of the γ- and δ-forms contain hydrogen-bonded chains of associated molecules. An analysis of the Hirshfeld surfaces and fingerprint plots has also been performed to study the intermolecular interactions in the crystal network of each polymorph. The spectroscopic and thermal investigations showed that the polymorphs convert into the α-form, which indicates that the α-form is the most stable at ambient temperature. Additionally, the α- and β-forms are enantiotropically-related phases, whereas the γ-form is monotropically related to the α-form. Unexpectedly, the β- and γ-forms were found to be unstable under Cu Kα radiation and both gradually transformed into the α-form with the γ-phase being more stable.

中文翻译：

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羟吲哚的多晶型物是生物活性化合物的核心结构†

已经发现了新的羟吲哚多晶型物（称为“δ形式”），并通过单晶X射线衍射进行了表征。将其结构性质与先前报道的多晶型物（α-，β-和γ-形式）进行了比较。这种多晶型形式是第一次结晶，但无法复制。因此，δ形式可以用作“消失的多晶型物”的例子。结果，现已通过粉末X射线衍射，振动光谱法（红外和拉曼光谱）和量热法（差示扫描量热法）对三种先前报道的α-，β-和γ形式进行了表征。在较早已知的α和β形式的结构中，N–H⋯O氢键将分子连接成二聚体，而γ和δ形式的晶体包含与相关分子氢键连接的链。还对Hirshfeld表面和指纹图进行了分析，以研究每种多晶型物在晶体网络中的分子间相互作用。光谱和热学研究表明，多晶型物转变为α-形式，这表明α-形式在环境温度下是最稳定的。另外，α-和β-形式是与对映体有关的相，而γ-形式与α-形式是单向相关的。出乎意料的是，发现在CuKα辐射下，β和γ形式不稳定，并且都逐渐转变为α形式，而γ相更稳定。这表明α-形式在环境温度下最稳定。另外，α-和β-形式是与对映体有关的相，而γ-形式与α-形式是单向相关的。出乎意料的是，发现在CuKα辐射下，β和γ形式不稳定，并且都逐渐转变为α形式，而γ相更稳定。这表明α-形式在环境温度下最稳定。另外，α-和β-形式是与对映体有关的相，而γ-形式与α-形式是单向相关的。出乎意料的是，发现在CuKα辐射下，β和γ形式不稳定，并且都逐渐转变为α形式，而γ相更稳定。