title: Structure, hydrogen bonding and thermal expansion of ammonium carbonate monohydrate.
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.
authors.affiliation: Department of Earth and Planetary Sciences, Birbeck, University of London, Malet Street, London WC1E 7HX, England.
authors.forename: A Dominic
authors.initials: AD
authors.lastname: Fortes
authors.affiliation: Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, England.
authors.forename: Ian G
authors.initials: IG
authors.lastname: Wood
authors.affiliation: Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, England.
authors.forename: Dario
authors.initials: D
authors.lastname: Alfè
authors.affiliation: Instituto de Ciencia de Materiales de Madrid, Campus de Cantoblanco, 28049 Madrid, Spain.
authors.forename: Eduardo R
authors.initials: ER
authors.lastname: Hernández
authors.affiliation: ISIS Facility, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0QX, England.
authors.forename: Matthias J
authors.initials: MJ
authors.lastname: Gutmann
authors.affiliation: School of Chemisty, University of Bristol, Bristol BS8 1TS, England.
authors.forename: Hazel A
authors.initials: HA
authors.lastname: Sparkes
mesh_terms: D002254:Carbonates / Q000737:chemistry*; D003460:Crystallization; D018360:Crystallography, X-Ray; D004583:Electrons; D006860:Hydrogen Bonding; D015394:Molecular Structure; D011789:Quantum Theory; D013696:Temperature; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D002254:Carbonates; D014867:Water; C040502:ammonium carbonate
keywords: Raman spectroscopy; ammonium carbonate; density functional theory; neutron diffraction