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 P21/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.