Acta Crystallographica Section B

Structural Science

Volume 57, Part 6 (December 2001)

research papers

Acta Cryst. (2001). B57, 791-799 [ doi:10.1107/S0108768101014987 ]

Irreversible single-crystal to polycrystal and reversible single-crystal to single-crystal phase transformations in cyanurates

M. Kaftory, M. Botoshansky, M. Kapon and V. Shteiman

Abstract: 4,6-Dimethoxy-3-methyldihydrotriazine-2-one (1) undergoes a single-crystal to single-crystal reversible phase transformation at 319 K. The low-temperature phase crystallizes in monoclinic space group P2₁/n with two crystallographically independent molecules in the asymmetric unit. The high-temperature phase is obtained by heating a single crystal of the low-temperature phase. This phase is orthorhombic, space group Pnma, with the molecules occupying a crystallographic mirror plane. The enthalpy of the transformation is 1.34 kJ mol^-1. The small energy difference between the two phases and the minimal atomic movement facilitate the single-crystal to single-crystal reversible phase transformation with no destruction of the crystal lattice. On further heating, the high-temperature phase undergoes methyl rearrangement in the solid state. 2,4,6-Trimethoxy-1,3,5-triazine (3), on the other hand, undergoes an irreversible phase transformation from single-crystal to polycrystalline material at 340 K with an enthalpy of 3.9 kJ mol^-1; upon further heating it melts and methyl rearrangement takes place.

Keywords: phase transformations; cyanurates.