Infrared spectra of the hydrogen bond in Pimelic acid (cas 111-16-0) crystals: polarization and temperature effects

Add time:08/22/2019    Source:sciencedirect.com

This paper deals with the polarized IR spectra of the hydrogen bond in Pimelic acid (cas 111-16-0) crystals and their quantitative interpretation. The spectra were measured for the νOH and νOD band frequency regions, at temperatures of 298 and 77 K, for the ‘alpha’ crystalline form of pimelic acid. Two kinds of transmission spectra were obtained, for the beam perpendicular to the two different crystalline faces: ‘ac’ and ‘ab’. The spectra of the hydrogen and the deuterium bonded systems were quantitatively reproduced, with sufficient accuracy, on the basis of the ‘strong-coupling’ model, assuming the (COOH)2 cycles to be the structural units responsible for the basic spectral properties. It was found that the spectra could be reproduced only, when assuming spectral activity in the IR of the totally symmetric proton stretching vibrations in centrosymmetric cyclic dimers of hydrogen bonds. The polarization effects in the crystal spectra were interpreted as directly connected with the hydrogen bond orientation in the crystal. However, in the pimelic acid crystalline spectra there were not observed another polarization effects characteristic for another carboxylic acid crystals, depending on differentiation of the long- and of the short-wave branch properties of each of the νOH and νOD bands. The temperature variation only affect the intensity ratio between the lower- and the higher-frequency branches of the νOH and νOD bands. These spectral effects were ascribed to less strained hydrogen bond structures in the pimelic acid lattices of the ‘alpha’ crystalline form and a relatively low concentration of defects in the lattices.

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