Anisotropy of intermolecular interactions and its relation to mechanical properties of organic molecular crystals
and classification of crystal structures. The first one was used for
prediction of cleavage planes in molecular crystals. In all the cases
cleavage planes coincided with the planes corresponding to layers
with highest X value. The second proposed descriptor is more general,
but both of them are needed both for systematics of molecular crystals
and for the development of expert systems in materials science.
Relationship between structure and mechanical properties of molecular crystals are of great interest for many applications in microelectronics, technology of high-energetic materials and especially in pharmaceutical industry. Anisotropy of molecular forces plays a leading role in determining the solid’s response on the applied mechanical stress but by now, only few schemes were proposed for description of interaction energies in molecular crystals, e.g. analysis of dimensionality if intermolecular interactions .
We propose two such descriptors for crystal structure – X parameter and anisotropy index (AI) – that characterize anisotropy of intermolecular interactions. The information on the strength of intermolecular interactions was obtained with the PIXEL method , which is suitable for these purposes because of its accuracy and relatively low demands in computational resources. The physical meaning of the first descriptor is related to intermolecular interaction energy anisotropy that is the share of cohesive energy in the given layer compared to the total cohesive energy in the cluster, representing the whole crystal in PIXEL calculations. The correlation was found between magnitude of this parameter and presence of cleavage planes in molecular crystal . For instance, for primary (010) and for secondary (111) cleavage planes in b-alanine crystal, corresponding values are X(010)=0.902, X(111)=0.685, i.e. larger than X values for other crystal planes. The proposed scheme was tested on 13 crystals, which were grown and subsequently tested for its mechanical properties. In all the cases cleavage planes coincided with the planes corresponding to layers with highest X value.
The second proposed descriptor evaluates anisotropy of interactions taking into account only the first coordination shell of molecule. Its value is between zero and one and in contrast from the first descriptor this one is more general (value of the X parameter is specific for a given molecular layer). Anisotropy index can be helpful in crystal structure analysis, since new crystal structure descriptors are needed both for systematics of molecular crystals and for the development of expert systems in materials science.
The work was supported by the Russian government (Grant 14.B25.31.0005).
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