Theoretical Studies On The Relationships Between Noncovalent Interactions And Properties For Aromatic,Nitramine,and Azide Energetic Materials

Noncovalent interactions,especially hydrogen bond andπ-πinteractions,are able to significantly affect the energy level and safety performance of energetic materials.In the present thesis,density functional theory（DFT）,molecular dynamics（MD）,and transition state theory（TST）were used.The variation in the geometry structure,electronic characters and intermolecular interactions of 2,2’,4,4’,6,6’-hexanitrostilbene（HNS）-based cocrystals with the change of non-energetic components was studied.The influences of different types of noncovalent interactions on the impact sensitivity of the cocrystal explosives were explored;The distribution of different hydrogen bonds inα-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine（α-HMX）crystals under high pressures were investigated separately by cluster analysis method,while the effects of intermolecular interactions on the macroscopic properties were verified;The impact of emerging halogen-bond interactions on the packing pattern of1,3,5-trinitrobenzene（TNB）crystals and a supplement to the structure-sensitivity structure-activity relationship at the crystal level were studied;The variations of intralayer/interlayer intermolecular interactions when different guest molecules were embedded in the molecular cavity of 2,4,6-triamino-5-nitropyrimidine-1,3-dioxide（ICM-102）were analyzed and a scaling parameter that is able to take different kinds of non-covalent interactions into account quantitatively was established to predict the impact sensitivity of corresponding host-guest complex energetic crystals;The reaction kinetic model of poly（3,3′-bis-azidomethyl oxetane）-tetrahydrofuran（PBT）/toluene diisocyanate（TDI）curing system with different components was constructed and the strength of hydrogen bonding among the cured products from each reaction pathway was compared.The contents of the dissertation are as follows:1.Effects of noncovalent interactions on the impact sensitivity of HNS-based cocrystalsThe crystal packing,electronic structure,Hirshfeld surface,Bader’s atoms in molecules（AIM）,and independent gradient model（IGM）for HNS and HNS-based cocrystals（HNS/4,4’-Bipyridine（BP）,HNS/trans-1,2-Bis（4-pyridyl）ethylene（BPE）,and HNS/1,2-Bis（4-pyridyl）ethane（BPA））were investigated to understand how noncovalent interactions affect their impact sensitivity.The results indicate that there are strong C-H···N interactions andπ-stacking in the three cocrystals.Among all related cocrystals,HNS/BP has most strong intralayer hydrogen bonds andπ-πinteractions,which can be responsible for its lowest impact sensitivity.Among the emergingπ-πinteractions,the face-to-face interactions lying between components is the most important one.Therefore,the strong intralayer hydrogen bonds and face-to-faceπ-πinteractions are important factors determining the cocrystal explosives to be low impact sensitivity.2.Possible pre-phase transition ofα-HMX crystal observed by the variation of hydrogen-bonding network under high pressuresDFT calculations were performed to investigate the pressure-induced variation of noncovalent interactions inα-HMX crystal by geometrical evolution,electronic structure,Hirshfeld surface,mechanical property,and vibration spectrum.The results show that the hydrogen bond network predominates in the intermolecular interactions in the crystal and the crystal becomes more sensitive in low pressure region（below 10 GPa）.Compared to the relatively strong C-H···O interactions related to vertical nitro groups,the weak hydrogen bonds around horizontal N-NO₂ are more complicated and may govern the physicochemical property of the metastable a-HMX crystal under compression.The abrupt variation of the hydrogen bond network in the crystal at 4 GPa that attributes to the alteration of relative position of adjacent molecules indicates an emergence of possible pre-phase transition before the transformation fromβ-toδ-HMX.3.Influence of halo-bond interactions on the relationship between crystal packing and impact sensitivity for aromatic energetic materialsThe geometric structures,electrostatic potential surface,crystal packing,electronic structure,and intermolecular interactions of TNB,1,3,5-tribromo-2,4,6-trinitrobenzene（TBTNB）,1,3,5-trichloro-2,4,6-trinitrobenzene（TCTNB）,1,3,5-triiodo-2,4,6-trinitrobenzene（TITNB）,and 1-azido-2,4,6-triiodo-3,5-dinitrobenzene（ATIDNB）were studied through DFT-D calculations.The mix-tape packing style of TNB stems from the different configuration of energetic molecules in the crystals,which also mainly contribute to its high sensitivity.Both TCTNB and TBTNB possess strong intralayer intermolecular interactions,while the interlayerπ-stacking in the latter distributes in a wider range.The remarkable increase in the impact sensitivity of TITNB attributes to the enhancement of interlayer I···O interactions that hinder the potential sliding between adjacent molecular layers,and the subsequent introduction of azide groups further weakened theπ-πinteractions between benzene rings,leading to a continuous decline in the safety performance of corresponding energetic crystals.The analysis of extended charge decomposition（ECDA）demonstrates that the nature of halo-bond in the explosives is electrostatic interaction rather than charge transfer.4.Regulating the safety performance of ICM-102 based host-guest inclusion energetic crystals through intermolecular interactionsPeriodic DFT and MD were employed to investigate the geometry evolution,molecular cavity,free space,intermolecular interactions,radial distribution function（RDF）and mechanical properties of a series of 2,4,6-triamino-5-nitropyrimidine-1,3-dioxide（ICM-102）-based host-guest inclusion energetic crystals.Compared to the neutral guest molecules H₂O and H₂O₂,the insertion of acid oxidant molecules HCl O₄ and HNO₃ into voids have more apparent effect on the planarity of aromatic molecules.The introduction of extremely strong charged hydrogen bond had changed the crystal packing from face-to-face to wavelike and sandwich-like,respectively.The scale parameter constructed through the volume of IGM isosurface is able to predict the impact sensitivity of relevant host-guest inclusion energetic crystals effectively,consistent with the experimental results.Under decomposition temperature,the stability of the average noncovalent interactions and the proportion of newly emerging steric hindrance can significantly affect the safety performance of corresponding energetic material.5.Influence of typical components on the curing mechanisms of azide solid propellantThe effects of several typical ingredients on the thermal curing reactions of PBT adhesive with curing agent TDI were studied using DFT.The results show that the most energetic favorable reaction route in curing system points to the formation of cured product with 2urethane segments and its rate-determining step is the nucleophilic addition reaction of TDI with triethanolamine（TEA）.Compared to the reaction of TDI with PBT,the reaction involving trimethylolpropane（TMP）has a relatively lower activation free energy mainly due to the weakening of N-H bond in corresponding transition states（TS）.After the pre-modification of TDI by diethylene glycol（DEG）,the reactivity of remaining isocyanate group is significantly decreased,which is supported by the less torsion angle in the active region of TS,where the electron density in the N-H and C-O bond areas is locally concentrated.Meanwhile,the strong hydrogen bonds in the cured products containing ammonium and urea groups have proved that the introduction of TEA and H₂O indeed enhances the mechanical property of energetic adhesive system.