Crystal structures and phase behaviour of acetaldehyde-d₄: A study by high-resolution neutron powder diffraction and calorimetry

High-resolution neutron powder diffraction is particularly suited for structure solution and refinement of low-melting point organic crystals of moderate complexity. Here we present the ab initio structure solution and refinement of stable-phase and metastable-phase perdeuterated acetaldehyde (m.p.: 151 K). In the stable phase, the molecule crystallises in the space group P2₁/c (no. 14) with a = 3.9069(1) Å b = 5.4224(1) Å, c = 12,1868(1) Å, β = 94.2970(2)°at 5 K, Z = 4. In the metastable phase, the molecule crystallises in the space group Pna2₁ (no. 33) with a = 5.1973(1) Å, b = 6.9791(1) Å, c = 6.9712(1) Å at 5 K, Z = 4. The metastable to stable phase transition is characterised by heat capacity measurements. (C) 2000 Elsevier Science B.V.