Theoretical Study On The Structural Stability And Aromaticity Of Sulfurnitrogen Cyclic Compounds

Binary sulfur-nitrogen ring heterocycles, which can be synthesized by evaporating the superconductive material polymeric material polythiazyl (SN)_x or reacting the atomic nitrogen with sulfur compound in gase-phase and solid state, have intrigued experimental and theoretical chemists since Gregory's discovery of S₄N₄ in 1835. Nitrogen-rich compounds will release amount energy when they dissociate into piece, these materials with high energy-density (HEDM) and also environmental friendly can be used as energy storage and explosive substance. Sulfur-nitrogen ring conpounds possess abundance geometrical characters and stabilities, which can be used as the researching objects, and indeed, they make an excellent annotation on"predicting molecules, more realism please!"proposed by Hoffmann,Schleyer and Schaefer recently. Planar ring structures show the aromaticity character for their abundanceÏ€electrons, which will enlarge the scope of aromaticity. On the basis of considerations mentioned above, we investigate the geometric and electronic properties, stability based on both thermodynamically and kinetically, and the aromaticity of several sulfur-nitrogen cyclic compounds with density functional theory (DFT-B3LYP) and ab initio (MP2) methods. The main contents are presented as the following:A theoretical study of the geometries, energies, dissociation pathways, and aromaticity of the isomeric sulfur-nitrogen S₂N₃⁺ rings reveals that the experimentally known 1,2-isomer is only stable kinetically. A rather high barrier inhibits its dissociation into the slightly more stable N2 and NSS+ fragments via a stepwise mechanism. Its second possible dissociation mode, into NNS and NS+ via a concerted [3+2] mechanism, is endothermic. Instead, the reverse cycloaddition reaction has a low barrier and offers an exothermic route for the formation of cyclic 1,2-S₂N₃⁺. Despite being thermodynamically more stable, the 1,3-isomer has only fleeting existence: its facile exothermic [3+2] cycloreversion into N2 and SNS+ fragments precludes observation. Nucleus-independent chemical shift (NICS) analysis reveals considerable sixÏ€electron aromaticity for both cyclic S₂N₃⁺ isomers.Five-membered rings N₅^-,SN₄ and S₃N₂²⁺, the angulous with S₂N₃⁺, have been studied on the geometries, isomers, possible dissociation channels on the potential energy surface (PES) and also aromaticity. The experimental detected N₅^- possesses high dissociation barrier. However, it is not suitable for the application of HEDM for the low energy released. In contrast, the experimental absent SN₄ is also not suitable for the using of HEDM for the lower kinetics viable, although, there is abundance energy released during its decomposition. The 1,2-S₃N₂²⁺ (given for the first time) is 16.2 kcal/mol lower in energy than the experimental detected 1,2-S₃N₂²⁺. The dissociation barrier of 1,2-S₃N₂²⁺ is also lower than 1,3-S₃N₂²⁺, which has shown that the stability both theormdynamic and kinetic is higher for 1,3-S₃N₂²⁺ than its 1,2-S₃N₂²⁺ isomer. Nucleus- independent chemical shift (NICS) analysis reveals considerable sixÏ€electron aromaticity for all five-membered rings N₅^-,SN₄ and S₃N₂²⁺.No S₂N₄ species, neutral or charged, are known among the many members of the binary S-N family. Is kinetic and/or energetic instability, perhaps due to 8Ï€electron antiaromaticity, responsible? A DFT investigation of the S₂N₄ potential energy surface reveals ten minima, five of which have low energies. The six-membered ring with the two sulfur atoms located at 1 and 4 positions is the global minimum, which is determined by the single point energy calculations at CCSD(T)/6-311+G(3df) level. To be the global minimum energetically, the cyclic C2v 1,4-S₂N₄ boat-like structure is not viable, since dissociation channels with low (6.4 and 7.6 kcal/mol) barriers lead to two N₂S as well as to N₂ + SNNS fragments. All but the highest energetic four-membered ring N(S₂N₂)N have lower barriers towards dissociations and isomerizations,which has explained well on the question above. Like planar D4h cyclooctatetraene, the planar D2h 1,4-S₂N₄ transition state is destabilized (by 11.0 kcal/mol, relative to the C2v boat form) both by angle strain and by its 8Ï€electron antiaromaticity. CMO-NICS(0)Ï€zz analysis reveals the large paratropicity of both the D2h 1,4-S₂N₄ HOMO (sigma) and HOMO-1 (Ï€). The bending of planar structure into boat decreases this paratropicity greatly and increases the theormdynamic stability. A series related species with D2h 1,4-S₂N₄ transition state having differentÏ€electrons have been studied. The geometric characters, number of imaginary frequencies and the gap of HOMO and LUMO were discussed by electronic structure analysis. No charged S₂N₄ can be detected in experiment for the low stability.