Quantum Chemical Studies On The Structures And Stabilization Of Molecular Systems Containing N₅

Energetic materials are a kind of compounds or mixtures containing explosive groups,oxidants or combustibles and so on,which can independently undergo chemical reactions and produce energy.Energetic materials have extensive and important applications in civil,military,space transportation,space exploration and aviation rescue.Nitrogen-rich energetic compounds usually refer to the new energetic materials with nitrogen content higher than 50%.Compared with traditional energetic materials,nitrogen-rich energetic compounds have the advantages of high density,high formation enthalpy,high energy and clean products.However,the thermodynamically unstable nitrogen-rich energetic compounds can bring considerable challenges to their chemical synthesis.Pentazole is the last member of the azole family,which is the all-nitrogen isoelectronic analogue of the well–known and highly stable cyclopentadiene.Due to its high nitrogen content and high kinetic stability,pentazole compounds have always been a research hotspot in the field of nitrogen-rich energetic compounds.In 2017,the solid phase cyclo-N₅^–anion(Pentazole anion)was successfully synthesized,which led to a new research upsurge on cyclo-N₅^–anion.Along with the rapid development of theoretical chemistry and computer technology,the theoretical prediction about the intrinsic properties of nitrogen rich energetic compounds is much more accurate and efficient.Theoretical prediction will provide more valuable theoretical data for experimental synthesis,and has important practical significance to reduce the cost,difficulty and risk of synthesis.Therefore,this thesis studies the following aspects:(1)The global potential energy surface search for three–dimensional fully single bond N₅H isomers with kinetic stabilityNitrogen-rich energetic compounds have been facing a huge difficulty,that is,an ideal nitrogen-rich energetic compound must satisfy two seemingly contradictory conditions,namely,thermodynamic instability and kinetic stability.Such harsh condition poses a great challenge to the rational design and successful synthesis of nitrogen-rich energetic compounds.Until now,few energetic skeletons meet the conditions.Therefore,the synthesis and theoretical calculation of new nitrogen-rich energetic compounds are of great value.In the 3^rd chapter of this thesis,we use the commercial software GRRM and the locally developed GPESS program to search the global potential energy surface of N₅H,the last member of the classic azole family.We found 9 new isomers and 104 new transition states.Fortunately,we found a three–dimensional total nitrogen single bond molecule N₅H with stable kinetics,and its energy barrier can reach 20.18 kcal/mol after substitution.It is close to the stability barrier of arylpentazoles.Therefore,the predicted three–dimensional total nitrogen single bond compounds are very potential to be synthesized experimentally.(2)Theoretical prediction of a new kind of kinetically stable co-stabilized arylpentazole derivativeArylpentazole derivatives are a kind of special nitrogen-rich compounds in the field of high energy density materials(HEDMS),which have a long history of research.In 1956,the first arylpentazole derivative was successfully synthesized,but it can only be preserved at low temperature.Arylpentazole contains N₅^–ring,which has been considered as the precursor of all-nitrogen pentazole anion(cyclo-N₅^–)for a long time.Therein,the all-nitrogen pentazole anionic compounds have been paid close attention by chemists.Although arylpentazole derivatives as precursors have achieved great success in the synthesis of cyclo-N₅^–,the application of arylpentazole derivatives in the fields of inorganic chemistry,coordination chemistry and HEDMS is severely restricted by the harsh storage conditions.In order to make arylpentazole derivatives more widely applied,it is urgent to propose a new stabilization strategy.In the 4^thchapter of this thesis,we introduced one or two Lewis acid group(–BR₂)onto the ortho carbon of benzene to stabilize arylpentazoles.The calculation results showed that the kinetic stability of arylpentazole derivatives can be improved to 25–45 kcal/mol at the composite method of CBS–QB3,which is the most kinetically stable pentazole derivative so far.(3)C–sp3–attached pentazole derivatives as precursors of new generation of pentazole anionsDue to the advantages of high energy,high density and no pollution of explosion products,the all-nitrogen energetic compounds have become a typical representative of the new generation of energetic materials.Cyclo-N₅^–,as a well–known all-nitrogen isoelectronic analogue of the highly stable cyclopentadiene,has been a research hotspot in the field of all-nitrogen energetic compounds due to its high nitrogen content and high kinetic stability.At present,chemists have successfully synthesized cyclo-N₅^–using arylpentazoles as precursors.However,the yield of cyclo-N₅^–synthesized via the oxidative cleavage is only 19%,which has greatly limited the wide application of cyclo-N₅^–in the energetic fields.The strategy for improving the yield of cyclo-N₅^–has become a new research hotspot.Compared with the oxidative cutting mechanism,the reductive cutting mechanism is relatively simple.In the 5^th chapter of this thesis,we explored the possibility to improve the yield of cyclo-N₅^–via reductive cutting.The reason for the low yield of the produced cyclo-N₅^–via reductive cutting method can be attributed to the problems in finding precursors by this method:(1)there exists a high energy barrier for N₅^–removal,(2)the kinetic competition of N₅^–removal is lower than that of N₂ removal,(3)the resulting R reacts with cyclo-N₅^–in situ to form RN₅^–.To solve these problems,we began to explore a new precursor.It was found that C-sp³-attached pentazole derivatives are more suitable as precursors of cyclo-N₅^–than traditional aromatic X-sp²-attached pentazole derivatives.After electron implantation,the C–sp³–attached pentazole derivatives have low or almost no cyclo-N₅^–barrier.The competitive kinetic barrier of cyclo-N₅^–-removal is higher than that of N₂-removal,and the binding of R radical to cyclo-N₅^–is also inhibited.Thus,using C-sp³-attached pentazole derivatives as precursors is expected to increase the yield of cyclo-N₅^–.In addition,we proposed for the first time that the R-N bond energy of BDE^–(R-N)anion can be used as an effective and simple description factor for evaluating the precursor of cyclo-N₅^–.(4)Hydration stabilization of pentazole radicalDue to the unique structure and good stability,pentazole clusters have always been a research hotspot in high nitrogen energetic realms.Recently,the successful synthesis of the cyclo-N₅^–salt has made pentazole clusters the star molecules.However,among the pentazole family,whether the N₅ radical can exist or not has remained a mystery.Previous theoretical studies have shown a very easy decomposition for the N₅ radical.In the 6^th chapter,we reported a new method to stabilize N₅ radical.We found that the charge transfer from the lone pair electrons of N₅ radicals to H₂O can weaken the"Jahn-teller"distortion caused by electron deficiency,thus enhancing the kinetic stability of N₅ radical.Thus,hydration-stabilized N₅ radical can be facilely synthesized,which upon generation would react with other compounds to form novel high nitrogen energetic compounds.