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Molecular Shape and Crystal Packing: a Study of C 12 H 12 Isomers, Real and Imaginary
Author(s) -
Dunitz Jack D.,
Filippini Giuseppe,
Gavezzotti Angelo
Publication year - 2000
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/1522-2675(20000906)83:9<2317::aid-hlca2317>3.0.co;2-r
Subject(s) - chemistry , lattice energy , molecule , crystal structure , crystallography , lattice (music) , crystal (programming language) , lattice constant , chemical physics , computational chemistry , molecular physics , physics , organic chemistry , optics , computer science , programming language , acoustics , diffraction
Isomeric C 12 H 12 hydrocarbon molecules with widely different constitution and shape are analysed for their packing ability. Some correspond to known compounds with known crystal structures, but some are invented hypothetical molecules designed to have low packing efficiency. For each isomer, a large number of close‐packed, low‐energy crystal structures was generated by computer, with lattice energies within a range of a few kJ mol −1 . Molecules with linear chains, triple bonds and Me groups tend to have larger molecular volume, lower lattice energy and lower crystal density than cyclic or cage isomers. The calculated crystal structures for each isomer show an inverse relationship between packing energy and cell volume. Although the slope d E /d V varies from molecule to molecule, the product of slope and free space stays roughly constant; less efficient crystal packings thus appear to be less sensitive to an increase in cell volume. Lattice‐vibrational frequencies and the corresponding contributions to thermal vibrational entropy were estimated for real and virtual crystal structures. For a given isomer, as expected, a higher entropy goes with a larger cell volume, but different isomers show different entropy/volume relationships. At 300 K, T Δ S differences among computational polymorphs may compete with Δ H differences, thus making the lattice‐vibrational entropy estimation a relevant factor in crystal‐structure prediction.