Raman, infrared and force field studies of K 212C 2O 4 · H 2O and K 213C 2O 4 · H 2O in the solid state and in aqueous solution, and of (NH 4) 212C 2O 4 · H 2O and (NH 4) 213C 2O 4 · H 2O in the solid state

Abstract

The Raman and infrared spectra of solid K 2 12C 2O 4 · H 2O are reported together with, for the first time, the corresponding Raman and infrared spectra of solid K 2 13C 2O 4 · H 2O. Raman spectra of aqueous solutions of both isotopomers are also reported. In the solid state the oxalate anion is planar with D 2 h symmetry in this salt, whereas in aqueous solution the Raman spectra of the anion are best interpreted on the basis of D 2 d symmetry. The Raman spectra of solid (NH 4) 2 12C 2O 4 · H 2O and (NH 4) 2 13C 2O 4 · H 2O, in which the oxalate anion is twisted from planarity by 28° about the CC bond, have also been recorded. Several reassignments have been made. The harmonic force field for the oxalate anion in the D 2 h , D 2 and D 2 d geometries has been determined in part, and approximate values of key valence force constants determined. All the observed band wavenumbers and 12C/ 13C 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 ν(CC) and δ s(CO 2), respectively, putting them in the same wavenumber order as ν(NN) and δ s(NO 2) for the isoelectronic and isostructural molecule N 2O 4. The stretching force constants of N 2O 4 and planar C 2O 4 2− are established to be in the order f NN< f CC and f NO> f CO, consistent with the known relative bond lengths.