Pressure Effect on D,L-Mandelic Acid Racemate Crystallization

Mandelic acid, C6H5CHOHCOOH, (MA) is one of frequent exempts from the Wallach's rule, as its racemate DL-MA is less dense than the enantiomers. This relation appears to be unfavorable for the racemate stability at high pressure; however, the racemate remains more stable than the conglomerate of enantiomers up to 1.36 GPa at least. The isochoric crystallization of DL-MA yields its orthorhombic form I, space group Pbca, and above 0.65 GPa, another centrosymmetric polymorph of monoclinic DL-MA form II, space group P2(1)/c, becomes stable. Their structures have been determined by X-ray diffraction of the single crystals in situ grown in a diamond-anvil cell up to 1.36 GPa. Lattice-energy calculations by the semiempirical PIXEL method demonstrate that DL-MA form II is more stable than DL-MA form I, and both of these racemates are more stable than their enantiomer counterpart L-MA.