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)_<i>x</i> 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 <i>a</i> 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)_<i>x</i> lattice. While not the same, the simulated and experimental packing changes bear a qualitative similarity. The simulated polymerization was also observed to propagate along <i>c</i> axis in addition to <i>a</i> axis, but these side effects generally disappear toward the end of the simulations. In some cases, the polymers propagating simultaneously in both <i>a</i> and <i>c</i> 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)_<i>x</i> with severe disorder in the lattice

Theoretical and Synthetic Study on the Existence, Structures, and Bonding of the Halide-Bridged [B₂X₇]⁻ (X = F, Cl, Br, I) Anions

While hydrogen bridging is very common in boron chemistry, halogen bridging is rather rare. The simplest halogen-bridged boron compounds are the [B₂X₇]⁻ anions (X = F, Cl, Br, I), of which only [B₂F₇]⁻ has been reported to exist experimentally. In this paper a detailed theoretical and synthetic study on the [B₂X₇]⁻ anions is presented. The structures of [B₂X₇]⁻ anions have been calculated at the MP2/def2-TZVPP level of theory, and their local minima have been shown to be of <i>C</i>₂ symmetry in all cases. The bonding situation varies significantly between the different anions. While in [B₂F₇]⁻ the bonding is mainly governed by electrostatics, the charge is almost equally distributed over all atoms in [B₂I₇]⁻ and additional weak iodine···iodine interactions are observed. This was shown by an atoms in molecules (AIM) analysis. The thermodynamic stability of the [B₂X₇]⁻ anions was estimated in all phases (gas, solution, and solid state) based on quantum-chemical calculations and estimations of the lattice enthalpies using a volume-based approach. In the gas phase the formation of [B₂X₇]⁻ anions from [BX₄]⁻ and BX₃ is favored in accord with the high Lewis acidity of the BX₃ molecules. In solution and in the solid state only [B₂F₇]⁻ is stable against dissociation. The other three anions are borderline cases, which might be detectable under favorable conditions. However, experimental attempts to identify [B₂X₇]⁻ (X = Cl, Br, I) anions in solution by ¹¹B NMR spectroscopy and to prepare stable [PNP]­[B₂X₇] salts failed