AMMONIUM CARBONATE, AMMONIUM BICARBONATE, AND AMMONIUM CARBAMATE EQUILIBRIA - A RAMAN-STUDY

Raman spectra of aqueous solutions of ammonium carbonate, ammonium bicarbonate, ammonium carbamate, and ammonium chloride with sodium carbonate gave very similar spectra, A factor analysis method has been applied to the digitized Raman spectra for a series of solutions of different concentrations in the region between 965 and 1099.5 cm(-1) The results indicated the presence of three scattering components in this band envelope. Subsequent band resolution gave three components which are centered at 1017, 1034, and 1065 cm(-1) Further Raman and C-13-NMR spectroscopic studies showed that the scattering components are due to nu(5)(A') of HCO3-, nu(5) of H2NCOO-, and nu(1)(A(1)') of CO32-, respectively. A rather complex equilibrium has been revealed among the species bicarbonate, carbonate, carbamate, ammonium, and ammonia. Carbamic acid was found to be unstable with respect to decomposition into CO2 and NH3. There was no evidence to indicate urea formation in the ammonium carbamate solutions. Quantitative Raman intensity studies of aqueous solutions of ammonium carbonate have been performed in the spectral region between 850 and 1100 cm(-1), where bands at 1017, 1034, and 1065 cm(-1) have been used to follow the relative concentrations of HCO3-(aq), H2NCOO-(aq), and CO32-(aq), respectively, as a function of concentration and temperature. The corresponding equilibrium constants and thermodynamic parameters have been estimated, Relative stabilities and optimum geometries for the species H2NCOO-, H2NCOOH, and (H3NCOO-)-H-+ were calculated by ab initio methods. The results indicated that the zwitterion ion was the less stable form of carbamic acid and therefore the most probable intermediate for the decomposition of carbamic acid to ammonia and carbon dioxide.