Structural Design And Stability Of Non-classical Energetic Molecules

High-energy-density materials（HEDMs）are compounds or mixtures that contain explosive groups,oxidants and combustibles,and can release a large amount of heat under certain external conditions,accompanied by a large number of gases and heat.It is the motive and power of modern weapons and precision strike,with the characteristics of high temperature,high pressure,high speed and instantaneous.Thus,HEDMs play a very important role in the field of military and civil development.In order to satisfy requirements of HEDMs in military and civil fields（high energy density,well detonation performance,lower sensitivity,good chemical stability,and higher exothermic property）,continuous design and synthesis of new energetic materials have been the main research objectives for chemists in this field.Typical energetic materials include azines,azoles and other nitrogen heterocycle compounds with rich-nitrogen.The extraordinarily large bond energies are from the N-N,N-C,N-O,C-O bonds.And these rich-nitrogen compounds have the common characteristic of high nitrogen content,and the products are mainly nitrogen and less smoke.Therefore,it is called"green energetic material".However,due to the instability caused by the tension of its rings,it is extremely challenging in the design and synthesis of high-energy fields,and it is also a research hotspot in this field.In particular,there are two major breakthroughs in this field in China,1)Hexanitrohexaazaisowurtzitane,also called CL-20.In 1980s,CL-20 was developed by Yong-zhong Yu group of Beijing Institute of Technology,primarily to be used in propellants.This is the fourth generation of explosive after nitroglycerin,Trinitrotoluene（TNT）and 1,3,5-Trinitro-1,3,5-triazinane（RDX）,and it is also the largest and most powerful non-nuclear monomer explosive known for practical application.2)The research team（He-ming Xiao,Wei-hua Zhu,Bing-cheng Hu and Ming Lu）from Nanjing University of Science&Technology has been engaged in the theoretical and experimental researches of high-energy materials.In 2017,the third stable nitrogen skeleton compound---cyclo-N₅^-was successfully synthesized by this research team.These high-nitrogen energetic compounds（amino,nitro and azide groups）have broad application prospects in the field of high-energetic materials due to their characteristics of large heat release,low smoke,low residue and low pollution.Unfortunately,the two difficulties seem to be encountered,which may be sensitive and low kinetics stability.Therefore,by using the computational techniques and making use of the available building blocks that are kinetically stable,we can design a series of new energy-rich compounds through the desired stabilization strategies such as assembly and coordination.In such a strategy,the inherent large exothermicity and good kinetics stability of the parent HEDM molecule can be inherited to the new HEDMs by suitable isovalent or analogous skeleton/ligand modifications.Besides,we can design some non-traditional energetic molecular with good kinetics stability.The overall aim is to provide more effective basic data for the experimental synthesis.This project should be very important to reduce the synthetic cost,difficulty and operation risk in dealing with HEDMs.The main results are summarized as follows:1)Bottom-up design of high-energy-density molecules（N₂CO）_n（n=2-8）.Seeking high-energy-density materials（HEDMs）with balanced huge energy release and good stability has remained a quite tough task to both experimentalists and theoreticians.The nowadays HEDM design mostly concentrates on the chemical modification of either the skeletons or ligands.To increase the number of HEDM candidates,novel designing strategy is highly appealed.In this thesis,we computationally proposed a bottom-up strategy,i.e.,a suitable HEDM seed（e.g.,cyc-N₂CO）can breed novel HEDMs while retaining the good stability and good performance.Starting from the experimentally known diazirinone（cyc-N₂CO）as a―seed‖and by considering various bond-addition channels（2+2/2+3/3+3 cyclo-addition at the N=N/C=O/C-N bonds）,we found that the cyc-N₂CO dimer isomer 1（i.e.,（N₂CO）₂ containing a COCO ring with an exocyclic side-N₂ at each C-atom）possess the rate-determining barrier of 29.9kcal/mol and exothermicity of 168.7 kcal/mol into 2N₂+2CO at the composite CBS-QB3 level.Moreover,the trimer and tetramer of cyc-N₂CO each possess high rate-determining barrier of 25.8 and 30.3 kcal/mol,respectively,at the CBS-QB3level.Even higher oligomers with n=5-8 have the rate-determining barriers around 25and 34 kcal/mol.The spiral skeletons were shown to have a contribution to their good inherent destruction stability.By comparing the detonation properties with some known HEDM compounds,the oligomers of cyc-N₂CO may well deserve future synthetic trial for novel HEDMs.Our designed（N₂CO）_n with all the untouched N=N bonds differ sharply from the recently reported high-pressure polymerized forms,in which all the double bonds have been transformed into single bonds.The present bottom-up strategy from an HEDM seed（i.e.,cyc-N₂CO）to novel oligomeric HEDMs confirmed by the CBS-QB3 calculations seem to be quite promising and may open a new designing way in the HEDM realm.2)Structurally uneasy but destruction stable nitrogens in 1,3-disubstituted cyclotetrazenes:viable high-energy-density materials.In designing the polynitrogen and nitrogen-rich high-energy-density materials（HEDMs）one continuously challenged issue is to solve the severe conflict between―stability and detonation‖.Most known nitrogen-containing and non-saltlike HEDMs involve the normally bonded skeletal nitrogens.Here,we computationally proposed a non-traditional class of nitrogen-rich compounds with structurally uneasy nitrogens,i.e.,1,3-disubstituted cyclotetrazenes N₄R₂（I）,which bear the moderate 6π-aromaticity and slight singlet diradical character.The target N₄R₂（I）can preferably balance the―stability and detonation‖necessity of HEDM,and can be fabricated into diversified compounds ranging from simple to nano-sized high-energy porous frameworks.The robustness of N₄R₂（I）should much expand our perceiving limitation in the nitrogen-rich HEDM chemistry.3)Unprecedented cyclic isomer of triazenes:a computational identification and analysis of bond strength and bond energy of NN ylide bond.First,isomerism is very important in chemistry.Over the past 65 years,the energy-rich N₃R₃ family has received considerable attention both experimentally and computationally.Up to now,four isomeric types of N₃R₃ have been identified,i.e.,triazenes I,triimides II,iso-triazenes III,and cyclo-triazanes IV.In this thesis,via the composite CBS-QB3study on the isomers and transition states of N₃H₃?the simplest N₃R₃ system,we unexpectedly found a new structural type V,which contains a N₃ three-membered ring with one nitrogen-nitrogen dative bond in the ring（i.e.,cyclic ammonia-nitrene interaction）.Of all the N₃H₃ isomers,V lies the highest in energy（78.7 kcal/mol above the global triazene）and possesses a relatively low conversion barrier 8.9kcal/mol.Yet quite promisingly,suitably choosing substituents can tune the rate-determining barrier of V to reach around 20 kcal/mol.Thus,synthesis of the newly found triazene isomer V is highly probable.Second,we report our quantum chemical study on the fourth homoatomic nitrogen-nitrogen（NN）interation,i.e.,NN ylidic bonding,which has been never considered as potential in HEDMs though their derivatives have been known for a long time.In this thesis,the N-N bond dissociation as well as the 1,2-shift and 1,2-elimination of substituents were analyzed in detail.By comparing the energetics of different processes,the rate-determining step of each substituted form were then determined.Further,based on the explosive calculations,we evaluated their possibility as HEDMs.