Theoretical Study On A Series Of New Polynitrogen And Nitrogen-rich Compounds

Polynitrogen and nitrogen-rich compounds as high energy density materials,(HEDM) play important roles in the fields of military project and aerospace, andhave been always a hot topic in these fields. Here, we predicted a series of stablepolynitrogen and nitrogen-rich compounds and investigated the origin of the stabilityin these compounds by two density functional methods, B3LYP and B3PW91, andMP2method. The main contents are described as follows:(1) A series of polynitrogen molecules composed by multiple cyclic-N3and/orcyclic-N5were systematically investigated. The results indicated that the most stableN6and N10structures could appear with a corner to corner connection between twocyclic-N3and two cyclic-N5systems, respectively. In these molecules, interactionbetween two N3-rings and two N5-rings would decrease the stability of cyclic-N3andincrease the stability of cyclic-N5as compared to the correspondingcyclic-N3andcyclic-N5radicals. For the stable structure N8consisted of cyclic-N3and cyclic-N5, itwas demonstrated that the charge was transferred from cyclic-N3to cyclic-N5,facilitating the stability of N5, and destroying the stability of N3. Furthermore, theattacked site of the N8structure by the cyclic-N3or cyclic-N5radical is analyzed bycondensed Fukui function. Based on these results, the structures N3(N5)3and N5(N5)5were predicted, and the origin of their stability could be，attributed to the chargetransfer and the coordination number of N3-ring and N5-ring.(2) Thirteen isomers of the formula CN8were predicted by using DFT and MP2methods. It was found that the six-membered heterocyclic isomer and the planarchain-like isomer owned similar energies, and were the most stable structures amongall isomers. The stability of these two structures may be attributed to their planarstructures and thus electron delocalization running across the whole molecule.Furthermore, the formation energies of the most stable isomers were calculated,which clearly show that these two isomers could have similar thermodynamicstability to the synthesized CN12.Investigation on the transition states of theseisomers decomposition into a N2molecule demonstrated that the six-memberedheterocyclic isomer had a higher barrier and better kinetic stability as compared to the planar chain-like isomer Thus, it is possible to use the six-membered heterocyclicisomer as a good candidate of HEDM.(3) We explored stability of a series of the nitrogen-rich compounds N6XHm,N8XHmand N10XHm(X=B, Al, Ga, m=1and X=C, Si, Ge, m=2) consisted of smallN3,N5systems and group13or14elements. It was found that the structures withN3-chain (or N5-ring) are more stable than those containing N3-ring (or N5-chain) inall the nitrogen-rich compounds. This result is well explained by means of intrinsicstructure of the N3and N5systems and their charge distribution in nitrogen-richcompounds. By comparing the dissociation energies of the above structures, it wasfurther found that the B-doped and C-doped structures were the most stable amongthe structures with three elements of group13and14, respectively. Furthermore,energy decomposition analysis of B-doped and C-doped nitrogen-rich compoundsand the component analysis of C-N and B-N bonds demonstrated that the boronatom is more suitable than carbon atom to design nitrogen-rich compounds.(4) The adsorption structures and electronic properties of N5-rings on C60weresystematically investigated. It was found that in three isomers consisting of N5adsorbed on the C60or encapsulated in the C60, the isomer consisted of N5-ring andC60linked by a C-N single bond, was the most stable. This could be attributed to thefive N-N bonds equalized in the N5-ring fragment with respect to the isolated N5-ringand the decrease of the strain in the C60fragment relative to the isolated C60Furthermore, the optimal adsorption site of the second N5ring on C60wasdetermined by calculating condensed Fukui function of the above structure, that isthe adsorption of two N5-rings on the C60is completed through the addition across a6,6-juction. This is in consistent with the most stable structure among all isomerscontaining two N5-rings optimized by DFT method. Based on the structure consistedof two N5-rings and C60, the adsorption structure containing six or ten N5-rings wasobtained by calculating condensed Fukui function. In addition, we studied thethermal and chemical stability of the aforementioned structures by means of theadsorption energy and the gap between the highest occupied molecular orbital andthe lowest unoccupied molecular orbital.