Abstract
With the intention of producing the most comprehensive treatment of the prevalence of crystal polymorphism among structurally characterized materials, all polymorphic compounds flagged as such within the Cambridge Structural Database (CSD) are analysed and a list of crystallographically characterized organic polymorphic compounds is assembled. Classifying these structures into subclasses of anhydrates, salts, hydrates, non-hydrated solvates and cocrystals reveals that there are significant variations in polymorphism prevalence as a function of crystal type, a fact which has not previously been recognized in the literature. It is also shown that, as a percentage, polymorphic entries are decreasing temporally within the CSD, with the notable exception of cocrystals, which continue to rise at a rate that is a constant fraction of the overall entries. Some phenomena identified that require additional scrutiny include the relative prevalence of temperature-induced phase transitions among organic salts and the paucity of polymorphism in crystals with three or more chemical components.
Highlights
Polymorphism is a concept that has been well known within the crystallization field since Mitscherlich discovered different crystal forms of the same phosphate salt in the early 1800s (Bernstein, 2002)
We are not alone in this endeavour; a search for the term ‘polymorph’ in the journal Crystal Growth & Design shows that, on average, 18% of the research articles and communications published in the last 15 years contain this term. (It is notable that searching for this term in Crystal Growth & Design leads to very few false hits involving genetic polymorphism or other meanings of the term; see section S1 of the supporting information for more details.) funds are limited and researcher time is in high demand, so scrutinizing
The remaining hits were found to have other entries present in the Cambridge Structural Database (CSD), albeit only by removing the search parameter of having threedimensional coordinates available. This parameter was chosen in order to select only those compounds with full structural proof of polymorphism, and these 347 entries are excluded from the overall listing
Summary
Polymorphism is a concept that has been well known within the crystallization field since Mitscherlich discovered different crystal forms of the same phosphate salt in the early 1800s (Bernstein, 2002). It was not until the mid-1960s that McCrone (1965) presented a review of the relevance of this concept in the field of pharmaceuticals, where it would eventually become one of the most studied topics in solid-state organic chemistry. We examine organic polymorphs deposited in the Cambridge Structural Database (CSD) to determine the trends in prevalence as a function of time and crystal type, providing an overview of research activity and progress in the field
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