Two polymorphic forms of a conformationally flexible molecule, 5-[(Diphenylphosphoryl)methyl]-4-(prop-2-en-1-yl)-2,4-dihydro-3H-1,2,4-triazole-3-thione, were obtained by crystallization and characterized by X-ray diffraction analysis and differential scanning calorimetry. The relative stability of polymorphic forms was estimated with DFT calculations of crystal structures and isolated molecules. It turns out, that in the first more dense polymorph with higher cohesion energy and crystal lattice energy, the molecule adopts an energetically unfavorable conformation, and forms dimers with lower H-bond strength, as compared to the second polymorph. On the other hand, in the second polymorph, the molecule adopts almost the lowest-energy conformation and forms infinite chains via strong H-bonds. The first form that seems to be more thermodynamically stable at room temperature transforms into the second form via two endothermic phase transitions; the apparent irreversibility of the transition is due to high energy difference between the molecular conformations in crystals.

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Melting points were determined on a melting point apparatus and are uncorrected. The 1 H NMR spectra were recorded on Bruker AVANCE 400 (Bruker BioSpin, Rheinstetten, Germany) NMR spectrometer (400.13 MHz) using tetramethylsilane as an internal standard and the chemical shifts are reported in δ units. The IR spectra were recorded on a TENSOR 27 Fourier spectrometer (Bruker Optik GmbH, Karlsruhe, Germany) in the range 400-4000 сm -1 . Crystalline samples were studied as emulsions in vaseline oil. Elemental analyses were performed on a CHNS-O Elemental Analyser EuroEA3028-HT-OM (EuroVector S.p.A., Milan, Italy) with an accuracy ±0.4% for C, H and N. Most chemicals were purchased from commercial sources. Solvents were of analytical grade and were used without further purification unless otherwise stated. MALDI-TOF experiments were performed with an Ultraflex III TOF/TOF (Bruker Daltonics, Germany) mass spectrometer equipped with a Nd:YAG laser. The spectra were measured in the positive ion linear modе. MS data were processed using the software FlexAnalysis 3.0 from Bruker Daltonics.2-(Diphenylphosphoryl)-2- (3',5'-di-tert-butyl-4'-hydroxybenzyl)methyl acetate (5, С 30 H 37 O 4 Р) A solution of 1.0 g benzyl acetate 3 (3.6 mmol), 0.9 g ether 4 (3.3 mmol) and 2 cm 3 of triethylamine in 30 cm 3 of DMF was kept at 25 °C for 5 days. The reaction mixture was poured into saturated NaCl solution. The precipitate was filtered off, washed with water and dried in air to constant weight. The dry precipitate was washed with hot hexane 3 times. Yield 0.4 g (45%), the product is white powder.