Molecular dynamics simulation of the solid-state topochemical polymerization of S2N2.

Molecular dynamics simulations of the solid-state topochemical polymerization of four-membered S2N2 rings to (SN)x have been presented by involving DFT methods and periodic functions. Isotropic pressure compression and a slightly elevated temperature have been applied to lower the activation barriers and to increase the rate of the reaction to be within the framework of MD simulations. The polymer formation is initiated by the cleavage of one bond in one S2N2 ring with a virtually instantaneous attack of the fragment thus formed on the neighboring ring. The energetically most-favored reaction then quickly propagates along a axis throughout the lattice. The structures of the polymer chains are in good agreement with that observed experimentally in the crystal structure determination, but there is less long-range order between the neighboring chains. Upon polymerization the packing of the molecules changes from the herringbone structure of the S2N2 lattice to a layered structure in the (SN)x lattice. While not the same, the simulated and experimental packing changes bear a qualitative similarity. The simulated polymerization was also observed to propagate along c axis in addition to a axis, but these side effects generally disappear toward the end of the simulations. In some cases, the polymers propagating simultaneously in both a and c axis directions persist at the end of the simulation resulting in a complicated network of sulfur-nitrogen chains. This finds experimental support in the observation of several polymorphs (SN)x with severe disorder in the lattice.