Theoretical Study On The Structure And Stability Of Pentanitrogen Cation

As a new generation of green High-energy-density materials(HEDMs),all-nitrogen-based energetic compounds have attracted much attention because they are all composed of nitrogen atoms,and the explosive product is clean energy N₂,which is green and pollution-free.The development of all-nitrogen energetic materials is related to the national security development strategy.Pentanitrogen cation(N₅⁺)is the third important member of the all-nitrogen family after relaying N₂ and N₃^–,and plays an important role in promoting the development of energetic materials.Based on this,the structures and stabilities of pentanitrogen cation(N₅⁺)were systematically studied.Firstly,the kinetic stability of the only experimentally characterized chain-like pentanitrogen cation(N₅⁺)was revised,resulting in a new N₂-extrusion transition state;secondly,the kinetic stability of a high-energy spiro-N₅⁺,i.e.,all-nitrogen spiropentadiene was predicted for the first time and revealed that it has a barrier to be used for spectroscopic detection;then,the minimum structures of three pentazole cations(cyclo-N₅⁺)were obtained,revealing the specific reasons for their instabilities;Immediately,a series of covalently pentazole cations were designed,so that cyclo-N₅⁺can exist in a covalently stable form;finally,a comprehensive potential energy surface search was carried out on the pentanitrogen cations and obtained several new unreported isomers and transition states.The specific research contents are as follows:(1)Redetermining the kinetic stability of pentanitrogen cation(N₅⁺).For the 22-year-old pentanitrogen cation N₅⁺(01),we surprisingly found that the previously reported transition states(TSs)do not correspond to the N₂-extrusion.We located the real N₂-extrusion TS,which can well reconcile the hitherto remained inconsistency between gas-phase and salt-like forms of 01 both in structures and energetics.(2)Study on the kinetic stability of All-nitrogen spiropentadiene—N₅⁺(Spiro-N₅⁺).Of the pentanitrogen cation(N₅⁺)family,the only experimentally known isomer is the V-shaped structure 01.Here we showed that a super high-energy(?100 kcal/mol above01)all-nitrogen spiropentadiene 02 with considerable?-delocalization deserves pursuit as the first spirocyclic all-nitrogen molecule at least spectroscopical.(3)Obtained the minimum structure of Pentazole cation(cyclo-N₅⁺).The 6paromatic pentazole anion(cyclo-N₅^-)has a rather long research history of more than half a century,and has now evolved into a star species for its recent solid-phase synthesis.By sharp contrast,its two-electron-less counterpart,the“pentazole cation”(cyclo-N₅⁺),has remained to be in no scientific attention possibly due to the generally conceived 4pantiaromaticity.In this work,by investigating the structure and bonding of cyclo-N₅⁺for the first time with the aid of various electronic structure calculation and analysis techniques,we revealed a quite different bonding picture for the naked?-N₅⁺,i.e.,with 6p(rather than 4p)-electron system and 2s(rather than 2p)-electron less than cyclo-N₅^-.The unique electronic structure of cyclo-N₅⁺results in the poor electron delocalization,with which cyclo-N₅⁺can undergo the easy ring destruction in the free and salt forms,and can be kinetically stabilized in the covalently attached form,i.e.,pentazole derivatives(RN₅)with large electronegativity R,in which the electron deficiency of cyclo-N₅⁺can be effectively made up.(4)The pentazole cation can exist in a covalently stabilized form.Contrasting the well-known pentazole anion(cyclo-N₅^-),the pentazole cation(cyclo-N₅⁺)has been almost neglected.By using a series of sophisticated computational methods,we for the first time showed that cyclo-N₅⁺could be accessible in the covalently stabilized forms,e.g.,the free and co-stabilized fluoropentazoles.(5)Global potential energy surface survey of pentanitrogen cation.Despite the great potential in high-energy density material applications,synthesis and characterization of nitrogen-rich compounds have to face significant challenges due to their risks of explosion.Thus,detailed knowledge of the thermodynamic and kinetic information(i.e.,isomerization and decomposition)obtained in advance would be very advantageous to help experimentalists judge whether to give up or direct the future trial of an energetic species.In this work,we reported hitherto the most extensive isomeric,interconversion and decomposition landscape of the pentanitrogen cation(N₅⁺)at the CCSD(T)?CBS//MP2/cc-p VTZ,CBS-QB3 and CCSD(T)?CBS//CCSD/cc-p VTZ levels.A mamimum number of eleven isomers and fourteen transition states were located,among which three and eight were reported for the first time.At the CCSD(T)?CBS//CCSD/cc-p VTZ level,only the V-shaped isomer 01 can have the good barrier heights of larger than 20 kcal/mol against isomerization and decomposition,whereas the other N₅⁺isomers bear rather low barriers of less than 3 kcal/mol.For the relevant N₅⁺isomers,the counterion stabilization effect by the model BF₄^–as well as the solvation effect in anhydrous HF solvent at different temepratures were investigated.Notably,although 01 is rather well known,a new type of N₂-extrusion process for the01 salt(with BF₄^–counterion)was identified.The computed structural,spectroscopic and barrier height parameters should decide the fingerprints of the N₅⁺isomers,and would provide a useful guide for spectroscopic characterization of non-01 N₅⁺isomers(at low temperatures)as well as for study of the hetero-doped N₅⁺species in future.