Raman study of the high‐temperature phase transition of malonic acid

The Raman spectra of the high‐temperature (α) phase of crystalline malonic acid, CH 2 (COOH) 2 , and its deuteriated derivative, CD 2 (COOD) 2 , have been investigated at 370 K in the wavenumber shift range 0–4000 cm −1 . An assignment of the internal and external vibrations is given. Comparison of the infrared and Raman spectra of the α and β phases shows that the high‐temperature phase (α) consists of quivalent centrosymmetric dimer rings and that the corresponding space group is C  C2/c with molecules occupyging C 2 sites. The temperature dependence of various Raman lines, in particular those due to lattice modes, indicates that the phase transition is of first order and that during the transition, reorientation of the molecules about the c axis is strongly coupled with the low‐frequency torsional (γOH…O) and librational modes.