The optimized geometries, energies of the possible conformers of allantoin (2,5-dioxo-4-imidazolidinyl urea, the diureide of glyoxylic acid) as well as the barriers for conformational interconversion have been calculated using the density functional theory [DFT(B3LYP)/6-311++G(d,p)] method. The calculations predicted the existence of four conformers (gC, tT, g'C, and g'T; where the first and second symbols in the name of the conformers designate the conformation around the exocyclic NHC-NHCO and CNH-CO axes, respectively), with the gC form contributing to more than 98% of the population in gas phase at room temperature. This conformer is different from that corresponding to the monomeric unit found in crystalline RS-allantoin (g'C; Mootz, D. Acta Crystallogr. 1965, 19, 726), Stressing the importance of intermolecular H-bonding in determining the structure of the crystal. Upon sublimation under vacuum (10(-6) mbar), the compound was found to undergo extensive decomposition to urea, isocyanic acid, NH3, and carbon. The identification of the decomposition products Was made by using matrix isolation infrared spectroscopy. In consonance with the theoretical predictions, the allantoin molecules surviving thermal decomposition were found to undergo conformational isomerization and be present in the cryogenic argon matrix in both the gC and gC conformations. The solid state room temperature infrared spectrum of allantoin was also investigated and assigned. (C) 2009 Elsevier Ltd. All rights reserved.