4-(1,2,4-Triazole-5-one-4-yl)-3-thiourea carboxylic acid ethyl ester was synthesized by mixing 4-amino-1,2,4-triazole-5-one (ATO), potassium thiocyanate and ethyl chloroformate in ethyl acetate. Single crystals suitable for X-ray measurement were obtained by slow evaporation of the solvent dimethylformamide at room temperature. The crystal belongs to rhombohedral symmetry with space group R-3 and crystal parameters of a＝2.60524(7) nm, b＝2.60524(7) nm, c＝0.82579(6) nm, γ＝120°, V＝4.8540(4) nm³, D_c＝1.442 g/cm³, μ＝0.300 mm^－1, F(000)＝2190, Z＝18, R₁＝0.0569, wR₂＝0.1424. A crystal unit of the title compound was selected as the initial structure, and it was fully optimized by HF/6-311G, MP2/6-311G and B3LYP/6-311G methods in Gaussian 03 package, and the atomic charges and natural bond orbital (NBO) analysis were also discussed.

Key words: 4-amino-1,2,4-triazole-5-one, ethoxylacylthiourea, crystal structure, theoretical calculation, natural bond orbital