Evidence of Polaron Excitations in Low Temperature Raman Spectra of Oxalic Acid Dihydrate

Low temperature Raman spectra of oxalic acid dihydrate (8K - 300 K) both for polycrystalline and single crystal phase show strong variation with temperature in the interval from 1200 to 2000 cm-1. Previous low temperature diffraction studies all confirmed the stability of the crystal P21/n phase with no indications of any phase transition, reporting the existence of a strong hydrogen bond between the oxalic acid and a water molecule. A new group of Raman bands in the 1200 – 1300 cm-1 interval below 90 K is observed, caused by possible loss of the centre of inversion. This in turn could originate either due to disorder in hydroxyl proton positions, or due to proton transfer from carboxylic group to water molecule. The hypothesis of proton transfer is further supported by the emergence of new bands centered at 1600 cm-1 and 1813 cm-1, which can be explained with vibrations of H3O+ ions. The broad band at 1600 cm-1 looses intensity, while the band at 1813 cm-1 gains intensity on cooling. The agreement between quantum calculations of vibrational spectra and experimentally observed Raman bands of hydronium ions in oxalic acid sesquihydrate crystal corroborate this hypothesis

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

The Raman spectra of the high-temperature (\alpha ) 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 \alpha and \beta phases shows that the high-temperature phase ( \alpha) consists of quivalent centrosymmetric dimer rings and that the corresponding space group is $C^5_{2h}- C2/c$ 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 ( \gamma OH.. .O) and librational modes

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<SUB>2</SUB>(COOH)<SUB>2</SUB>, and its deuteriated derivative, CD<SUB>2</SUB>(COOD)<SUB>2</SUB>, have been investigated at 370 K in the wavenumber shift range 0-4000 cm<SUP>−1</SUP>. 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<SUP>6</SUP><SUB>2h</SUB>—C2/c with molecules occupyging C<SUB>2</SUB> 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

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