Analysis of the polarized IR reflectance spectra of the monoclinic α-oxalic acid dihydrate

In contrast to the extensive molecular and crystal structure investigations on oxalic acid dihydrate (C2H2O4 center dot 2H(2)O, alpha-POX) and its deuterated analogues (alpha-DOX), stands the absence of a complete vibrational spectra analysis. Such analysis is desirable in view of the proton dynamics in alpha-POX crystals. In this communication we report the room temperature polarized IR reflectance spectra of a single crystal of alpha-POX recorded from the ac crystal plane, and from the plane containing the b-crystallographic axis, with polarization along the axis. Dispersion analysis of the reflectance spectra of both B-u and A(u) symmetry type modes, using model dielectric and reflectance function valid for the monoclinic case, have been performed and the results are discussed. Some aspects of the obtained fit results for some of the spectral regions and the peculiar change of the reflectance function with polarization angle are also discussed in this work. A correlation between the crystal structure and measured spectra, together with the results of the performed dispersion analysis, gave answers to some of the problems concerned with the orientation of the transition dipole moments of the IR active modes. The assignment of modes is assisted by DFT calculations. Another aspect covered in this work is the model reflectance functions using different averaging theories that have been applied in obtaining the reflectance spectrum of a polycrystalline sample. The results of the comparison between these spectra and the recorded reflectance spectra from a polycrystalline sample were further discussed. (c) 2019 Published by Elsevier B.V.