Structure-Property Relationship Of Novel Cyclo-N₅^- Energetic Materials

Cyclo-N₅^-energetic materials（EMs）have stimulated extensive attention due to their high-energy release（over 3.0 times of TNT equivalent）and more environmentally friendly products.However,the design and synthesis of such novel EMs have been a challenge,because cyclo-N₅^-is highly susceptible to decompose into N₂ and N₃^-.After a century,until 2017,the first room-temperature stable material in the world:（N₅）₆（H₃O）₃（NH₄）₄Cl,PHAC,was successfully synthesized by Chinese scientists,which was rated as opening a new situation for the research of all-nitrogen EMs.Different from the traditional CHON EMs and azide EMs,the interaction between ionic groups of cyclo-N₅^--containing EMs is hydrogen bond（HB）,neither covalent bond nor ionic bond.Cyclo-N₅containing materials show the unique structural characteristics and represent a novel type of EMs’ structure.Although several new EMs have been synthesized in recent years,there are still urgent problems need to be solved,such as,synthesis mechanism,stabilization mechanism and novel structure design.Focusing on cyclo-N₅^-EMs,this thesis systematically investigated a set of complete scientific research proposal about synthesis,structure-property relationship,and new structures design,based on the first principle method.The findings will provide a theoretical basis for the design and synthesis of such novel cyclo-N₅^-EMs with excellent properties.The main contents and innovations of the thesis include:1)A general synthesis path from non-metallic to metallic cyclo-N₅EMs was proposed.The feasibility and rationality of the experimental preparation were verified.The synthesis mechanism of metallic cyclo-N₅EMs was revealed.Taking（N₅）₆（H₃O）₃（NH₄）₄Cl（PHAC）as reactant,the synthesis paths of four metal cyclo-N₅^-hydrate EMs were designed:[Na（H₂O）（N₅）]·2H₂O（Na-cyclo-N₅^-）,[M（H₂O）₄（N₅）₂]·4H₂O（M=Mn,Fe,Co）.Both ΔE<0 of all reactions and the low reaction barrier（in the range of 7.39～14.94 kcal/mol）proved that the reactions are easy to occur and released energy.At the same time,the rationality of the experimental preparation was verified.The cyclo-N₅^-anion in PHAC was vulnerable to the electrophilic attack of metal cations,which made the hydrogen bond broken,and finally formed ionic bonds or coordination bonds between the metal cations and cyclo-N₅^-,resulting in electrophilic substitution reaction.Due to the solvent effect,the energy of ionic bonds was weakened,making the reaction easier.2)The high-precision theoretical models of two kinds of metal energetic materials were established.Based on the crystal level,the structure-property relationship between structure and stability was systematically analyzed.The unique structure and stability mechanism of such cyclo-N₅^-EMs were revealed.Both binding/lattice energy calculations and density of states analysis showed that[Mg（H₂O）₆（N₅）₂]·4H₂O（Mg-cyclo-N₅^-）was more stable than Na-cyclo-N₅^-.Hydrogen-bond network and π-π stacking interactions were the main stabilization mechanisms of both crystals and cyclo-N₅^-rings.Two types of hydrogen bonds,O-H…O and O-H…N,were clarified,which constructed 3D hydrogen-bond network in Mg-cyclo-N₅^-,while intralayer 2D hydrogen-bond network in Na-cyclo-N₅^-.Moreover,non-uniform stress could lead to the distortion of cyclo-N₅^-.3)Protonated hydrogen bonds have been found and defined in nonmetallic cyclo-N₅^-EMs for the first time.In such protoned hydrogen bond,the proton competition between energetic groups（R）and cyclo-N₅^-anions was revealed,and the controversy of whether cyclo-N₅^-is protonated or not was answered.The nature of such special hydrogen bond system was deeply analyzed,which would provide a theoretical basis for the research on structure-property relationship.Different from the traditional EMs,such four EMs:PHAC,N₂H₅N₅,NH₃OHN₅ and NH₄N₅,were mianly stabilized by 3D hydrogen-bond（HB）network.Face-to-face π-π stacking interactions also played a certain role in promoting stability.One new type of hydrogen bond,protonated HB（pH,R-H…N₅^-）,was defined and discovered to be a key stabilizing factor for cyclo-N₅^-.In the p-H HBs,it was found that,through the Crystal Orbital Hamiltonian population analysis,there existed proton competition between R and cyclo-N₅^-.Proton H+possessed high atomic activity resulting p-H HBs are stronger than np-H HBs.In p-H HBs,H+tended to combine with energetic molecules（R）to form cations（R+）rather than to combine with cyclo-N₅^-to form HN₅.This answered the controversial issue about whether cyclo-N₅^-is protonated.The more symmetrical and homogeneous of HBs between cationic groups and cyclo-N₅^-,the less the distortion of cyclo-N₅^-ring,and the better the stability.4)Based on the experimental impact sensitivity（IS）data,the structure-property relationship between the structure and IS of three nonmetallic cyclo-N₅^-EMs was explored and revealed.Then some models of low-sensitivity cyclo-N₅^-EMs were designed from a theoretical point of view,which can effectively reduce the number of experiments and the resear-ch&development（R&D）cost.Based on N₂H₅N₅,NH₃OHN₅ and NH₄N₅,we found that the band gap is not suitable for the theoretical prediction of IS of this kind of cyclo-N₅EMs with special structure.By comparing the experimental data of their impact sensitivity,it was found that strong charged hydrogen bonds and ππ stacking interactions were the two important factors affecting the impact sensitivity of such non-metallic cyclo-N₅^-EMs.Based on this finding,nine cluster models of four kinds of guanidine cyclo-N₅^-EMs,C（NH₂）₃⁺,C（NH₂）₂（N₂H₂）+,CNH₂（N₂H₂）₂⁺,C（NH₂）₂NHC（NH₂NH）⁺,and there crystal models of alkali metal-N₅ EMs were designed.