Abstract
4-Methylbenzophenone crystallizes in a monoclinic stable form (mp 59°C) with a = 5.70 A ̊ , b = 13.89 A ̊ , c = 14.08 A ̊ , β = 95.18° and space group P2 1 c , and a trigonal metastable form (mp 55°C) with a = 9.12 A ̊ , c = 11.28 A ̊ and space groups P3 1 or P3 2. The metastable phase is obtained only in highly supercooled melts at −25° to −35°C. It can undergo a monotropic phase transition into the stable form. Large and optically homogeneous crystals of both phases were grown from melts slightly supercooled below their melting temperatures. The structures of both crystals were determined by X-ray diffraction methods. The metastable phase contains molecules of only one chirality, packed into columns along three-fold screw axes. The centrosymmetric stable structure is racemic. Both structures and their different nucleation properties in supercooled melts are discussed.
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