Uniaxial Negative Thermal Expansion in Polymorphic 2-Bromobenzophenone, Due to Aromatic Interactions?
Résumé
The structure of the metastable form II of 2-bromobenzophenone, obtained by crystallization from the melt, has been determined by powder X-ray diffraction. Form II has been solved in the centrosymmetric monoclinic space group P21/c with a = 8.4896(19) Å, b = 6.5438(8) Å, c = 20.253(1) Å; β = 104.452(6)°, and Z = 4 (Z′ = 1) at 200 K. Both form I and form II contain a multitude of aromatic interactions, and the strength and direction of these interactions could only be interpreted with the support of the thermal-expansion tensors. Both forms exhibit, unexpectedly, uniaxial negative thermal expansion, while hydrogen bonding does not play a significant role in either of these two structures. It appears to be the first time in the literature that uniaxial negative thermal expansion may be caused by aromatic interactions. Thermodynamic properties at normal and high pressure have been determined for the stable and metastable phases, and a pressure–temperature phase diagram has been constructed. While the metastable form behaves monotropically with respect to the stable form under ambient conditions, the phase relationship becomes enantiotropic at high pressure, providing a clear example of phase behavior in which the densest form is not the most stable form under ambient conditions.
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