Raman, infrared and force field studies of K212C2O4•H2O and K213C2O4•H2O in the solid state and in aqueous solution, and of (NH4)212C2O4•H2O and (NH4)212C2O4•H2O in the solid state

The Raman and infrared spectra of solid (K2C2O4)-C-12 (.) H2O are reported together with, for the first time, the corresponding Raman and infrared spectra of solid (K2C2O4)-C-13 (.) H2O. Raman spectra of aqueous solutions of both isotopomers are also reported, In the solid state the oxalate anion is planar with D-2h symmetry in this salt, whereas in aqueous solution the Raman spectra of the anion are best interpreted on the basis of D-2h symmetry. The Raman spectra of solid (NH4)(2)(C2O4)-C-12 (.) H2O and (NH4)(2)(C2O4)-C-13 (.) H2O, in which the oxalate anion is twisted from planarity by 28degrees about the CC bond, have also been recorded. Several reassignments have been made. The harmonic force field for the oxalate anion in the D-2h. D-2 and D-2d geometries has been determined in part, and approximate values of key valence force constants determined. All the observed band wavenumbers and C-12/C-13 isotopic shifts are well reproduced by the force fields. The potential energy distribution of the totally symmetric normal modes of planar oxalate indicates that each mode consists of extensively mixed symmetry corrdinates and that the labels previously used for the bands seen here at 475 and 879 cm(-1) would better be described as v(CC) and delta(s)(CO2), respectively, putting them in the same wavenumber order as v(NN) and delta(s)(NO2) for the isoelectronic and isostructural molecule N2O4. The stretching force constants of N2O4 and planar C2O42- are established to be in the order f(NN) < f(CC) and f(NO) > f(CO) consistent with the known relative bond lengths. (C) 2002 Elsevier Science B.V. All rights reserved.