STABLE AND METASTABLE CRYSTAL PHASES OF 4-METHYLBENZOPHENONE

4-Methylbenzophenone crystallizes in a monoclinic stable form (mp 59 d egrees C) with a = 5.70 Angstrom, b = 13.89 Angstrom, c = 14.08 Angstr om, beta = 95.18 degrees and space group P2(1)/c, and a trigonal metas table form (mp 55 degrees C) with a = 9.12 Angstrom, c = 11.28 Angstro m and space groups P3(1) or P3(2). The metastable phase is obtained on ly in highly supercooled melts at -25 degrees to -35 degrees C. It can undergo a monotropic phase transition into the stable form. Large and optically homogeneous crystals of both phases were grown from melts s lightly supercooled below their melting temperatures. The structures o f both crystals were determined by X-ray diffraction methods. The meta stable phase contains molecules of only one chirality, packed into col umns along three-fold screw axes. The centrosymmetric stable structure is racemic. Both structures and their different nucleation properties in supercooled melts are discussed.