Structure, hydrogen bonding and thermal expansion of ammonium carbonate monohydrate

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)ų [ρ_calc = 1281.8(7)kgm^-3] at 10K. The single-crystal data collected at 10 and 100K are complemented by X-ray powder diffraction data measured from 245 to 273K, Raman spectra measured from 80 to 263K 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 273K; above 273K, 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 (E HB ≃ 30-40kJmol^-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 ∼36kJmol^-1. The c-axis exhibits negative thermal expansion, but the thermal expansion behaviour of the a and b axes is ormal.