Abstract
We have determined the crystal structure of ammonium carbonate monohydrate, (NH4)2CO3·H2O, using Laue single-crystal diffraction methods with pulsed neutron radiation. The crystal is orthorhombic, space group Pnma (Z = 4), with unit-cell dimensions a = 12.047 (3), b = 4.453 (1), c = 11.023 (3) Å and V = 591.3 (3) Å(3) [ρcalc = 1281.8 (7) kg m(-3)] at 10 K. The single-crystal data collected at 10 and 100 K are complemented by X-ray powder diffraction data measured from 245 to 273 K, Raman spectra measured from 80 to 263 K and an athermal zero-pressure calculation of the electronic structure and phonon spectrum carried out using density functional theory (DFT). We find no evidence of a phase transition between 10 and 273 K; above 273 K, however, the title compound transforms first to ammonium sesquicarbonate monohydrate and subsequently to ammonium bicarbonate. The crystallographic and spectroscopic data and the calculations reveal a quite strongly hydrogen-bonded structure (EHB ≃ 30-40 kJ mol(-1)), on the basis of H...O bond lengths and the topology of the electron density at the bond critical points, in which there is no free rotation of the ammonium cation at any temperature. The barrier to free rotation of the ammonium ions is estimated from the observed librational frequency to be ∼ 36 kJ mol(-1). The c-axis exhibits negative thermal expansion, but the thermal expansion behaviour of the a and b axes is ormal.
Highlights
Interactions between the most simple of molecules are of fundamental interest across diverse areas of the physical sciences, as well as underpinning a number of important industrial and biological processes; the ternary system H2O– CO2–NH3 is no exception
Measurements were carried out on large single generalized gradient corrected functional (Perdew et al, 1996, 1997). This form of the generalized gradient approximation (GGA) has been demonstrated to yield results of comparable crystals of ammonium carbonate monohydrate in our cold accuracy to higher-level quantum chemical methods, such as room using the BC100 fibre-optic coupled Raman probe
Zero-pressure density functional theory (DFT) structural relaxation are in close agreement with the observed structure
Summary
Interactions between the most simple of molecules are of fundamental interest across diverse areas of the physical sciences, as well as underpinning a number of important industrial and biological processes; the ternary system H2O– CO2–NH3 is no exception. Interaction of CO2 with aqueous ammonia during the accretion or differentiation of icy planetary bodies is likely to have sequestered any free ammonia in the form of solid ammonium carbonates (Kargel, 1992), and this is the leading hypothesis for the lack of any appreciable ammonia or ammonia hydrates on planetary surfaces It is plausible, that ammonium carbonates are major ‘rock-forming’ minerals in the outer solar system. Ammonium carbonate may be synthesized in situ on the surface of Saturn’s giant satellite Titan It is known from laboratory analogue experiments that the organic molecules produced photochemically in Titan’s dense N2/CH4 atmosphere may be hydrolyzed in aqueous ammonia to form both urea and amino acids (Poch et al, 2012). (i) to provide data necessary to identify this material in planetary environments, whether by in situ X-ray diffractometry (Fortes et al, 2009) or by in situ Raman scattering (e.g. Jehlicka et al, 2010), and (ii) to lay the foundations for measuring the thermoelastic properties necessary to include ammonium carbonate in structural and evolutionary models of icy planetary body interiors
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