Identification of hydrogen bond modes in polarized Raman spectra of single crystals of α‐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 periodic DFT calculations using the CRYSTAL06 program and by comparison with the infrared spectra of the polycrystalline material. The agreement between the calculated and observed band wavenumbers is fair in the case of low‐anharmonicity modes, but marked differences appear for the stretching modes that are strongly anharmonic. A very broad feature, extending between ∼2000 and 1200 cm⁻¹, is attributed to OH stretching. Notable is the topping of this feature by distinct bands that can be attributed to CO stretching, H₂O scissoring and COH bending coupled to C O stretching. The assignments are supported by isotope effects. However, deuteration does not notably affect the wavenumber limits of the broad OH stretching band, which 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. Copyright © 2009 John Wiley & Sons, Ltd.