engleski

Evidence of Strong Hydrogen Bonding in Monocrystals of Oxalic Acid Dihydrate

Effects of surroundings on proton transfer reactions are of paramount importance in many biological processes. However, spectroscopic investigations of highly complex systems are difficult and therefore model systems are useful. Oxalic acid dihydrate, α – (COOH) 2 • H2O, with its short hydrogen bond (2.48 Å , ref [1]), observed proton mobility [2] and temperature dependent electrical conductivity [1] due to proton transfer from the -COOH to water molecules, lends itself useful for such studies. Although many diffraction studies were performed on oxalic acid dihydrate, no detailed spectroscopic study has been published so far. We undertook Raman spectra of α – (COOH) 2 • H2O monocrystals in the temperature interval from 10 K to 300 K and room temperature infrared spectra were compared with the frequencies of normal modes in crystal calculated using Car-Parrinello method. Monocrystals of prismatic habits with elongated faces along crystallographic b axes were grown from water solution, and the sample sealed in a capillary tube. Orientation of the crystal was such that laboratory y axes coincided with the crystallographic b axis and z axes coincided with the crystal c axes. In Fig. 1 low temperature Raman spectra obtained when the laser beam polarized in y direction entered the crystal along the z direction are shown when the light was observed in x direction. Most interesting feature observed in Raman spectra in the interval 1800-1900 cm-1 is the broad band attributed to ν (O-H) stretching vibration of the proton donor carboxylic OH group. Ebisuzaki and Angel reported large uncertainty in the position of this Raman band, its center lying from 1900 cm-1 to 2000 cm-1 [4].

oxalic acid dihydrate; Raman; hydrogen bonding; low temperature

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