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Identification of Hydrogen Bond Modes in Polarized Raman Spectra of Single Crystals of Oxalic Acid Dihydrate

Polarized Raman spectra of single crystals of the α -polymorphs of protonated and deuterated oxalic acid dihydrate were recorded. The interpretation of the spectra is assisted by the periodic DFT calculation using the CRYSTAL06 program and by comparison with the infrared spectra of polycrystalline material. The agreement between the calculated and observed band frequencies is fair in the case of low anharmonicity modes, but marked differences appear with the stretching modes that are strongly anharmonic. A very broad feature extending between ~2000 and 1200 cm-1 is attributed to this mode. Notable is the topping of this feature by distinct bands that can be attributed to C=O stretchings, H2O scissorings and COH bendings coupled to C-O stretching. The assignments are supported by isotope effects. However, deuteration does not notably affect the frequency limits of the broad OH stretching band ; this suggests that the potential governing the proton dynamics is of the asymmetric double minimum type with a very low barrier. The calculated normal coordinates show a strong participation of the bending modes of water molecules in almost all internal acid motions as well as in the external phonons.

strong hydrogen bond ; Raman ; oxalic acid dihydrate ; CRYSTAL06

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