Density Functional Theory Investigations On The Structure,Energy And Stability Of Novel Pentazoles

Pentazole compounds,as a new type of high energy density materials,are one of the research hotspots in the field of energetic materials.It is of theoretical and practical significance to study their structures and properties.In this thesis,the molecular structure,crystal structure,and the effects of pressure on the structure and properties of pentazoles were studied using the density functional theory（DFT）method.The main contents are as follows:（1）The stability and pyrolysis mechanism of covalent pentazoles substituted with the electron-donating group（R-N₅ or N₅-R-N₅,R=alkyl,hydroxyl,or amino group）were studied.When R was the hydroxyl or amino group,the side chain and the N₅ ring were easy to break;when R was alkyl,the N₅ ring cracking energy barrier（E_a）was relatively larger.The C-C bond will break before the C-N bond,and the cleavage of the C-C bond may significantly reduces the bond decomposition energy(E_BD)of the C-N bond,while little affects the stability of the N₅ ring,which will be beneficial to obtain the N₅^-fragment.（2）The stability and its influencing factors of covalent pentazoles substituted with the electron-withdrawing group(N₅（CH₂）_x-1R,R=-NO₂,-CF₃,-CN,-CHO,-COOH,x=1,2,3,4)were studied.The effects of substituent,related bond length,bond order,electron density at the critical point,charge on N₅ ring,and oxygen balance on E_BD and E_a were analyzed and discussed.It was found that E_a increased with the increase of bond strength and the decrease of oxygen balance.（3）The Compass force field was used to predict the crystal structure of N₅CH₃,N₅（CH₂）₃NO₂,N₅（CH₂）₃CHO,and N₅（CH₂）₃COOH among the seven most likely space groups.The best molecular packing modes of these compounds were found to be P2₁/c,P2₁/c,P2₁,and P2₁2₁2₁,respectively.The crystal structures and properties were predicted by using the periodic DFT method.The density,infrared spectrum,and frontier orbital energy obtained by quantum chemistry and molecular mechanics were compared and analyzed.It was found that although the numerical values obtained by the two methods are different,the change rules are the same.（4）The effects of pressure on the crystal structure,electronic structure,and optical absorption properties of N₅CH₃,N₅（CH₂）₃NO₂,N₅（CH₂）₃CHO,and N₅（CH₂）₃COOH were studied.With the increase in pressure,the crystal parameters and band gaps of N₅CH₃,N₅（CH₂）₃NO₂,N₅（CH₂）₃CHO,and N₅（CH₂）₃COOH decreased gradually and the optical absorption activity increased gradually.When external pressure was applied,the band gap of N₅（CH₂）₃NO₂ decreased most significantly,and it was the only one that decreased at first and then increased slightly.