Crystal Structure Basis for the Absence of Thermal Mesomorphism in p-Hydroxy-trans-cinnamic Acid

Abstract

The molecular packing in p-hydroxy-cinnamic acid has been determined by single-crystal x-ray analysis in order to establish the role played by hydrogen bonding in inhibiting mesophase formation. The molecules exist in the crystal as centrosymmetric hydrogen-bonded dimers whose long axes are inclined at ± 84.7° to the crystal b-axis and lie almost in the (104) crystal planes. Rows of dimers lying parallel to b are linked into infinite sheets coincident with the (104) planes by head-to-tail hydrogen bonds involving the phenolic –OH groups. These sheets are interlocked by the alternating ring orientations of neighboring molecules in adjacent sheets.

Crystals belong to the monoclinic system, space group P2₁/c, with a = 8.707(3), b = 5.256(2), c = 17.207(6) Å, β = 99.67(2)°, and Z = 4. The structure was solved by direct methods, and least-squares refinement gave R = 0.057 for 800 independent reflections whose intensities were measured by counter diffractometry.