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Are Z’=1 Crystal Structures More Energetically Favorable Than Z’=2?
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DOI：10.1021/acs.cgd.6c00348.png)
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Abstract
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Z’ denotes the number of formula units in an asymmetric part of a crystallographic unit cell. We evaluated computationally, at the DFT-D theory level, 635 polymorphic pairs (1080 unique crystal structures) having at least one Z’ = 1 and one Z’ = 2 polymorph. This allowed for the comparison of the energetic preferences of Z’ = 1 and 2 data sets, including differences in total energy, as well as intra- and intermolecular contributions to the total energy. Overall, the observed differences between pairs of polymorphs were in the range of ±5 kJ/mol for 82% of cases. On average, Z’ = 1 polymorphs were slightly more often energetically stable, and this was true for 56.7% of entries (360 polymorphic pairs). This energy difference was mostly due to the difference in intermolecular contributions to the total energy, while intramolecular energy contributions were comparable for both sets, with only very minor preference for Z’ = 2 structures to accommodate lower-energy conformations (50.8% of polymorphic pairs). We analyzed all outlier structures, with an exceptionally high difference in either total energy or intramolecular contributions to it. We found that, in the case of total energy, this was either indicative of one of the polymorphs being a high-pressure structure or of an incorrect structure solution (e.g., in terms of hydrogen atom positions). In the cases of the outliers in the intramolecular energy difference, it was always the case of breaking an intramolecular hydrogen bond to form favorable intermolecular interactions.
Keywords:
Chemical structure
Conformation
Crystal structure
Molecular structure
Thermodynamic properties
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