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

Characterization of new cocrystals by raman spectroscopy, powder X-ray diffraction, differential scanning calorimetry, and transmission raman spectroscopy

Cocrystals have been increasingly recognized as an attractive alternative delivery form for solids of drug products. In this work, salicylic acid was employed as a cocrystal former with the nicotinic acid, dl-phenylalanine, and 6-hydroxynicotinic acid (6HNA). Also, 3,4-dihydroxybenzoic acid with oxalic acid was studied. The cocrystals in all cases were prepared by slow evaporation from ethanol followed by characterization using Raman spectroscopy, powder X-ray diffraction, transmission Raman spectroscopy (TRS), and differential scanning calorimetry. Full understanding of the effects of formation on the vibrational modes of motion was obtained by the complete assignment of the spectra of the starting materials and of the cocrystal components. The results show that all the cocrystals, prepared in a 1:1 molar ratio, possess unique thermal, spectroscopic, and X-ray diffraction properties. Raman and TRS spectra showed that the vibrational modes of the cocrystal were different from those of the starting materials, suggesting that Raman spectroscopy and TRS are effective tools to evaluate cocrystal formation through interaction of their components. In addition, we have used a synthetic standard containing a 1:1:1 mixture of KNO 3 and raw material for which each sample was analyzed at seven random positions, with each point sampled twice. We have done the same with all cocrystals (1:1 KNO 3 and cocrystal), the ratios confirming that the cocrystal components (were in a 1:1 molar ratio). © 2010 American Chemical Society