Dissociation Without Detonation: A DFT Analysis of the Thermally Induced Fragmentation of Binary Sulfur–Nitrogen Rings and Cages

Potential thermal dissociation pathways available to binary sulfur nitrides have been explored by density functional theory methods, and the results interpreted in terms of their known thermochemical behavior. The cyclic cation/anion pair S₃N₃^± both undergo concerted (4 + 2) cycloreversions to afford the 3-membered thiadiazirine ring <i>c</i>-NSN and, respectively, the SNS^± cation/anions. A similar pathway has been identified for S₄N₂, leading to S₃ and <i>c</i>-NSN. More complex multistep routes for the elimination of <i>c</i>-NSN have been identified for the bicyclic cation/anion pair S₄N₅^±, as well as for the neutral cage structures S₄N₄ and S₅N₆. For the S₄N₅⁺ cation a transition state for competing skeletal scrambling via a 1,3-nitrogen σ-bond shift has been located. For the multiply charged cations S₃N₂²⁺ and S₄N₄²⁺, dissociation mechanisms are driven by charge repulsion effects, affording SNS⁺/SN⁺ and S₃N₃⁺/SN⁺ respectively. Channels leading to loss of NS⁺ from the cyclic cation species S₄N₃⁺ and S₅N₅⁺ have also been examined, and the possible role of open-chain and cyclic S₃N₂-based intermediates in the formation of S₂N₂ during the thermal cracking of S₄N₄ over silver wool is explored.