Theoretical design of new bridge-ring insensitive high energy compounds by selected normal Diels-Alder reactions between NH2-substituted oxazoles and NO2/NF2/NHNO2-substituted ethylenes/acetylenes

Qiong Wu ,Qin-nn Hu ,Ming-qun Li ,Ze-wu Zhng ,Wei-hu Zhu

a Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology,School of Materials Science and Engineering,Nanjing Institute of Technology,1 Hongjing Road,Nanjing,211167,China

b Institute for Computation in Molecular and Materials Science and Department of Chemistry,Nanjing University of Science and Technology,Nanjing,210094,China

Keywords: Diels-Alder Energetic compounds Oxazole High energy Insensitive

ABSTRACT In this work,NH2-substituted oxazoles and NO2/NF2/NHNO2-substituted ethylenes/acetylenes were designed and used as dienes and dienophiles,respectively,in order to develop new bridge-ring insensitive high energy compounds through the Diels-Alder reaction between them.The reaction type,reaction feasibility and performance of reaction products were investigated in detail theoretically.The results showed that dienes most possibly react with dienophiles through the HOMO-diene controlled normal Diels-Alder reaction at relatively low energy barrier.Tetranitroethylene could react with the designed dienes much more easily than other dienophiles,and was employed to further design 29 new bridge-ring energetic compounds.Due to high heat of formation,density and oxygen balance,all designed bridge-ring energetic compounds have outstanding detonation performance,16 of them have higher energy than HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocine) and 2 others even possess comparative energy with the representative of high energy compounds CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane).The predicted average h50 value of these bridge-ring energetic compounds is 83 cm,showing their low impact sensitivity.The NH2 groups could obviously impel the proceeding of Diels-Alder reactions,but would slightly decrease the energy and sensitivity performance.In all,the new designed bridge-ring compounds have both high energy and low sensitivity,and may be produced through Diels-Alder reactions at relatively low energy barrier.This paper may be helpful for the theoretical design and experiment synthesis of new advanced insensitive high energy compounds.

1.Introduction

The Diels-Alder reaction [1] is a classical,simple,fast but very powerful reaction,which has been applied to develop different kinds of cyclic and bridge-ring compounds used in many fields[2-5] like organic electronic devices [2] and antibody-drugs [3].Lately,the Diels-Alder reaction was also used to design energetic compounds [6,7].Compared to common compounds for civilian use,energetic compounds are one kind of dangerous but essential special energy materials.There are tremendous energy contained in the structure,and these energy could be released in a short period of time under external stimulus,which would lead to combustion,deflagration or detonation.Thus,it is important to develop new advanced energetic compounds with better and better performance to improve the civil industry and national defense level.Traditional widely used and famous organic energetic compounds like TNT (trinitrotoluene),RDX (1,3,5-trinitro-1,3,5-triazine) and HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocine) are simple cyclic compounds generally.However,their energy level has reached the limit almost due to their simple structures mainly.Thus,many studies have been done to search for energetic compounds with more unique and complicated structures like the bridge-ring [6,7],cage [8-10] and metal-organic frameworks[11-13].He [6] designed several new bridge-ring energetic compounds using a series of fused ring compounds and tetranitroethylene as dienes and dienophiles,respectively.Yang [7] also reported a novel energetic compound trans-3,3,4,4,7,7,8,8-octanitro-9,10-dioxatricyclo [4.2.1.12,5]-decane with ideal zero oxygen balance(OB)through the Diels-Alder reaction,based on 1,3,4-oxadiazole and tetranitroethylene.These new compounds were found to have better detonation performance than RDX and HMX,showing the further investigate and implication value of bridgering energetic compounds,therefore,more studies are needed.

Heat of formation (HOF),OB and density are three key parameters which greatly related with the detonation performance of energetic compounds.In general,higher HOF,OB(ideal value:zero)and density value will lead to better detonation performance.Oxazole and its derivatives like isoxazole and oxadiazole are conjugate rings with good stability.Comparing to the parent ring of TNT,RDX and HMX,they have better HOF,OB and density,making them be good parent rings used to further find high energy compounds,and many good representatives [14-16] have been synthesized successfully lately.Thus,first of all,the oxazole,isoxazole,1,2,3-oxadiazole,1,2,4-oxadiazole,1,2,5-oxadiazole and 1,3,4-oxadiazole were selected as the diene in this work.In addition,the normal Diels-Alder reaction,featuring an electron rich diene reacting with an electron poor dienophile,which means that introduce electron-donating groups like the -NH2group to the diene may favor the reaction.Furthermore,the NH2group[17,18]is a popular and effective energy group used to combine with the NO2group to obtain energetic compounds with both high energy and low sensitivity like the 1,1-diamino-2,2-dintroethylene [17].Thus,six series of NH2-substituted dienes used as the reagent were designed and showed in Fig.1 (series RA-F).Then,NO2,NF2and NHNO2are three common but powerful energy groups.We combined these three strong electron-withdrawing groups with two dienophiles ethylene and acetylene,six electron-poor compounds which are named as tetranitroethylene (TNE),tetradifluoraminoethylene (TFAE),tetranitraminoethylene (TNAE),dinitroacetylene(DNA),difluoraminoacetylene(DFAA)and dinitroacetylene(DNAA)were then designed and used as dienophiles.

In all,based on the designed six series of dienes and six dienophiles,the reaction energy direction,energy barrier and feasibility of the Diels-Alder reaction between them were studied theoretically,and compared with each other to select the optimal dienophile.Then,using the optimal dienophile to react with six series of dienes to design many new bridge-ring energetic compounds.Finally,the structure and properties like HOF,OB,density,detonation and safety performance of designed new bridge-ring energetic compounds were predicted by the density functional theory(DFT)method mainly,and some good representatives which may be used as potential insensitive high energy compound would be picked out.The effects of the amount and introduced positions of NH2groups on the Diels-Alder reaction,the performance of bridge-ring energetic compounds were investigated also.

2.Computational methods

Most of the calculations were carried out at the B3LYP/6-31G(d)level,while the calculation of transition states(TS)were done at the M062X/6-31G(d) level,performed with the Gaussian 09 software[19].Vibration frequency analysis showed that all geometries are located at the minimum point of the potential energy surface.The optimized structure of one diene (RC3) was found not stable because the ring was observed broken,thus,RC3 would not be discussed in this work.The gas-phase HOF (HOFg) at 298 K was predicted by the atomization method (Eq.(1)).Then,based on the gas-phase HOF,the solid-phase HOF (HOFs) was calculated by the Hess’s law (Eq.(2)) [20,21].The solid phase density (ρ) was estimated by the popular electrostatic potential (ESP) Politzer’s method (Eq.(3)) [22].The detonation velocity (D) and detonation pressure(P)were calculated by popular and famous Kamlet-Jacobs equations (Eq.(4) and (5)) [23].The impact sensitivity (h50) was estimated also by the Politzer’s method (equation (6) for N-NO2derivatives,equation(7)for C-NO2derivatives)[24].RDX and one famous cage high energy compound CL-20(2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane)were selected as the reference compounds,their properties were calculated using the abovementioned methods also.The results showed that the calculated values are close to the corresponding experimental results.For instance,the predicted values of HOFs(102 kJ/mol for RDX,462 kJ/mol for CL-20),density(1.80 g/cm3for RDX,1.96 g/cm3for CL-20),D(8.80 km/s for RDX,9.41 km/s for CL-20) andh50(33 cm for RDX,10 cm for CL-20)are close to the experimental values[HOFs(93 kJ/mol for RDX,460 kJ/mol for CL-20),density (1.82 g/cm3for RDX,1.96 g/cm3for CL-20),D(8.75 km/s for RDX,9.38 km/s for CL-20),h50(30 cm for RDX,14 cm for CL-20)].The electronic chemical potential (μ),global electrophilicity index(ω),and global nucleophilicity index (N) were calculated by equations (8)-(10) (TCE is tetracyanoethylene) [25,26].

Fig.1.The framework of designed NH2-substituted dienes.

3.Results and discussion

3.1.Key features of the Diels-Alder reaction

3.1.1.Reaction orientation

Fig.2.The electronic chemical potential of dienes and dienophiles.

First of all,we need to check which Diels-Alder type dominates the reaction between NH2-substituted oxazoles (diene) and NO2/NF2/NHNO2-substituted ethylenes/acetylenes (dienophile) in fact,whether it is just the normal Diels-Alder reaction.Fig.2 and Table S1 (as seen in supporting information) display the chemical potential(μ)of designed dienes and dienophiles,it can be seen that most of dienes have higher chemical potential value than dienophiles obviously.In a same series,the compound with more NH2groups will have higher chemical potential generally,due to the electron donor effect.This shows that charge transfer would probably tend to take place from the electron-rich diene to the electron-poor dienophile,and the introduction of NH2groups can make this process be more favor.Furthermore,as seen from Table S2(as seen in supporting information),theEHOMO,dieneis close toELUMO,dienophile,showing that the charge is possible to transfer from the HOMO of dienes to the LUMO of dienophiles,that is the HOMO-diene controlled normal Diels-Alder reaction.Plus,from Table S2,it can be found that theEHOMO,dieneandELUMO,dieneare obviously not similar to theEHOMO,dienophileandELUMO,dienophile,respectively,showing that the neutral Diels-Alder reaction is not possible to happen between the designed dienes and dienophiles.The designed dienophiles all are electron-poor,which is difficult to induce the inverse Diels-Alder reaction with the electron-rich dienes.The chemical potential value increases with the order of series E,series C/D,series F,series B and series A,indicating that the charge transfer will become more and more favourable.Fig.3 and Fig.4 shows the normal Diels-Alder reactions of dienes with NO2/NF2/NHNO2-substituted ethylenes (TNE,TFAE,TNAE) and acetylenes (DNA,DFAA,DNAA),respectively.

3.1.2.Selection of dienophile

Fig.3.Diels-Alder reactions between NH2-substituted dienes and NO2/NF2/NHNO2-substituted ethylenes.

Fig.4.Diels-Alder reactions between NH2-substituted dienes and NO2/NF2/NHNO2-substituted acetylenes.

Fig.5.The global nucleophilicity index of dienes.

Since the Diels-Alder reaction is the normal one,we turn to investigate the nucleophilicity of dienes and electrophilicity of dienophiles.Fig.5 and Table S1 display the calculated global nucleophilicity index(N)of designed 29 dienes,first,it is found that there are 15 dienes (RA1-RA7,RB2-RB7,RD3,RF2) have global nucleophilicity index higher than 3.0 eV,indicating that these compounds are strong electrophiles [25].While 9 other dienes(RA0,RB1,RC1,RC2,RD1,RD2,RE1,RE2,RF1) are moderate nucleophiles [25],as their global nucleophilicity index values are ranged from 2.0 to 3.0 eV.These show that most of the designed dienes have good nucleophilicity property,setting a good basis for the proceeding of Diels-Alder reactions.Series A have higher global nucleophilicity index values than series B than series F than series C/D than series E,the nucleophilicity property would decrease with the same order.While for the compounds in a same series,the global nucleophilicity index increases with the increment of number of NH2groups generally,suggesting that NH2groups improves the nucleophilicity of dienes.The position of NH2groups also affects the nucleophilicity of dienes to some degree.Fig.6 and Table S1 list the global electrophilicity index(ω)of six dienophiles,as seen that all of them have values higher than 1.5 eV,showing that they all are strong electrophiles[26],making them be easily to accept electron from dienes,thus,laying another good foundation for the Diels-Alder reaction with dienes.TNE has stronger electrophilicity than DNA/TFAE than TNAE than DFAA/DNAA,because that their global electrophilicity index values decrease with this sequence.

3.1.3.Reaction feasibility

As found that the studied dienes and dienophiles have strong nucleophilicity and electrophilicity,respectively,the Diels-Alder reaction between them may be easy to happen.The electron is transferred from the HOMOdieneto LUMOdienophile,the energy barriervalues be tween all dienes and dienoph iles have been calculatedto investigate the reaction feasibility,as displayed in Fig.7.It can be observed that the|ΔEH-L|between dienes and six dienophiles decreases with the order of DNAA/DFAA,TNAE,TFAE,DNA and TNE,showing that the energy barrier between designed dienes and TNE is the lowest one.That is to say,TNE is probably the dienophile that reacts with designed dienes most easily.For the |ΔEH-L| between dienes and TNE,15 of them(RA1-RA7,RB2-B7,RD3 and RF2)have values lower than 1.1 eV (the |ΔEH-L| of the Diels-Alder reaction between butadiene with ethylene).This shows that TNE can proceed the Diels-Alder reaction with most of the dienes with low barrier,and 20 dienes (Order:RA4,RA7,RA6,RA5,RB7,RA3,RA2,RB6,RB5,RA1,RF2,RB4,RB3,RB2,RD3,RC1,RB1,RC2,RE2,RF1) have the lowest value when reacts with TNE.We further calculated the TS and the corresponding activation energy (Ea) between dienes and TNE,using dienes of series F as the representative.Fig.8 displays the predicted optimized structures of RF1,RF2,TNE,TS-RF1(TS of RF1 reacts with TNE),TS-RF2 (TS of RF2 reacts with TNE),and theEavalues of two corresponding TS.TheEavalues of TS-RF2(14.4 kcal/mol) and TS-RF1 (16.4 kcal/mol) are obviously lower than that of typical reference Diels-Alder reaction of butadiene with ethylene(27.5 kcal/mol),showing that RF1 and RF2 are sufficiently reactive to participate in [4+2] cycloaddition with TNE to form the final bridge-ring compounds PF1 and PF2.Furthermore,these two transitions(TS-RF1 and TS-RF2)associated with increasing of Gibbs free energy of reaction system about 18 kcal/mol,suggesting that reactions to form them should be considered as allowed from kinetic point of view.The HOMO and LUMO of RF1/RF2 were found could match with those of TNE,respectively,indicating that both interactions are symmetry-allowed,as seen in Fig.9.Plus,theEavalue of TS-RF2 is lower than that of TS-RF1 than that of TS-RF0(TS of RF0 reacted with TNE,24.8 kcal/mol) [7],which is consistent with the change order of the|ΔEH-L|(0.71 eV→1.54 eV→2.88 eV).The first NH2group can decrease theEavalue at 8.4 kcal/mol and the second NH2group further decreases it at 2.0 kcal/mol,showing that the introduction of NH2groups into the diene can obviously favor the Diels-Alder reaction and will make the synthesis of final bridge-ring compounds be more easily,and this positive effect could be further strengthened with the increased amount of NH2groups.This information above may be helpful for the experimental synthesis.

Fig.6.The global electrophilicity index of dienophiles.

Fig.7.The energy barrier of the electron transferred from the HOMOdiene to LUMOdienophile.

Fig.8.The transition states and activation energy of RF1/RF2 reacted with TNE.

Fig.9.The HOMO and LUMO diagrams of RF1,RF2 and TNE.

In all,the designed oxazole-based dienes are most possible to react with NO2/NF2/NHNO2-substituted ethylenes and acetylenes through HOMO-diene controlled normal Diels-Alder reaction.The introduction of NH2groups into the oxazole-based dienes can favor this reaction obviously.Among all six dienophiles,the energy barrier of TNE reacted with designed dienes is lower than that of other dienophiles obviously.Thus,TNE is selected as the optimal dienophile employed to further design new bridge-ring energetic compounds,that is the reaction products of TNE and six series designed dienes.The performance of these products will be predicted and judged in the following section.

3.2.Performance of designed bridge-ring energetic compounds

3.2.1.Detonation performance

Fig.10 displays the six series of products (PA-F) of Diels-Alder reactions between TNE and oxazole-based dienes,the optimized structures of these products can be seen in Fig.S1 (as seen in supporting information).For advanced energetic compounds,high energy is needed particularly.Thus,we predicted and judged the detonation performance of these designed new bridge-ring compounds in detail.First of all,three key parameters (HOF,OB and density) that can make great influence on the energy were estimated,as depicted in Fig.11 and Table S3 (as seen in supporting information).For the HOF in the solid-phase,all compounds have positive values,and 21 compounds have higher HOFs than HMX(105 kJ/mol)[27]while 3 other compounds have value close to CL-20 (460 kJ/mol,one of most powerful energetic compounds) [28].This shows that the designed bridge-ring compunds have good HOF property and will be helpful for achieving high energy,due to the bridge-ring structure which contains extra strain energy than the tradition simple chain or cyclic energetic compounds.But it should be notable that this extra strain energy may do harm to the structure stability of bridge-ring compounds sometimes,thus some of designed products may not that stable on certain conditions,and more studies on the stabilization of them will be done in the further work.In the series A,some NH2-substituted compounds(PA2,PA4,PA6 and PA8) have lower HOF than PA0,while some others (PA3 and PA5) have higher HOF than PA0,similar phenomenon can be observed in other series.This indicates that the introduction of NH2groups can either decrease or increase the HOF,the amount and position also affects the HOF,and the effects of them are coupled with each other.Series E have higher HOF values than series C than series D/B than series F than series A,which is consistent with the change order of HOF of their corresponding parent rings.For the OB,the ideal OB value is around 0%,because this can lead to relatively complete combustion and release the maximum energy.For example,CL-20 and ONC(octanitrocubane)both possess extremely high energy,while their OB values are-10.5%and 0%,respectively.The OB of designed bridge-ring compounds can be seen in Fig.11,we found that 13 compounds have better OB than CL-20,3 other compounds(PC0,PD0,PE0 and PF0)even just have the most ideal value (0%),and the rest compounds all have higher OB than HMX and RDX.This shows that excellent OB values of designed compounds and is another good basis for obtaining outstanding detonation performance.Due to the two hydrogen atoms which will react with one oxygen atom to form H2O,the introduction of NH2groups into the structure decreases the OB slightly,but the OB values of designed compounds are still in the acceptable range(-20%-0%).For density,due to the bridge-ring structure and combination of NO2and NH2groups,almost all of designed bridgering compounds have higher values than 1.90 g/cm3,15 of them have better density than HMX.This indicates that these compounds also have outstanding density property,which is the third good foundation for high energy.It can be found that the NH2groups can increase the density sometimes,when the position and amount are appropriate for the formation of hydrogen bonds between NH2and NO2groups,which will be favourable for the crystal packing.But the effects of position and amount on the density are coupled with each other also.Due to the higher nitrogen content,series C/D/E/F have higher density values than series A/B.

Fig.10.The framework of designed bridge-ring energetic compounds.

Fig.11.The HOF,OB and density of designed bridge-ring energetic compounds.

Fig.12.The detonation performance of designed bridge-ring energetic compounds.

Based on the above-mentioned analysis,the designed bridgering compounds have good HOF,OB and density properties at the same time.Thus,it may be inferred that they would have good detonation performance.The detonation velocity (D) and detonation pressure(P)were predicted,as displayed in Fig.12.For the detonation velocity,it is found that series E have higher values than series C than series D than series F than series B than series A,which is in accord with the changing order of HOF,OB and density in general.All of the bridge-ring compounds have detonation velocity values higher than 8.90 km/s,there are 9 compounds (PF2,PB0,PB2,PB1,PB3,PB6,PB4,PB5,PB7)have better values than HMX(9.10 km/s),while 12 other compounds (PE0,PE1,PC0,PE2,PC2,PC1,PD0,PD2,PD1,PF0,PD3,PF1)even possess higher values than CL-20 (9.38 km/s) [27,28].PE0 has the highest detonation velocity(9.75 km/s).This shows the excellent detonation velocity of designed compounds.In each series,the detonation velocity decreases gradually with the introduction of NH2into the system,the maximum decrease ratios for series A to F are 1.5%,1.0%,0.6%(lowest),0.9%,1.6% and 1.6% (highest),respectively.This indicates the slight negative effect of NH2groups on detonation velocity,and this effect is various to different system.For the detonation pressure (P),all new designed bridge-ring compounds have higher values than 36.2 GPa,14 of them(PC0,PE2,PC1,PC2,PD0,PD1,PD2,PF0,PD3,PF1,PF2,PB0,PB1,PB2) have better detonation pressure than HMX (39.0 GPa) and 2 others (PE0,PE1) have comparable value with CL-20 (44.6 GPa).This shows the high detonation pressure of designed compounds.Similar to detonation velocity,the introduction of NH2into the system also decreases the detonation pressure of A-F serious to different degree,and this negative effect is even more obvious than that on detonation velocity.The maximum decrease ratios for series A to F are 3.1%,2.0%,1.5%(lowest),1.5% (lowest),3.7% and 3.9% (highest),respectively.In all,when detonation velocity and detonation pressure are considered together,all designed compounds are belonged to high energy compounds.All 29 compounds have better detonation performance than RDX,16 compounds(PC0,PE2,PC1,PC2,PD0,PD1,PD2,PF0,PD3,PF1,PF2,PB0,PB1,PB2) have higher energy than HMX,and 2 compounds(PE0,PE1)have comparative energy with CL-20.In all,due to the unfavorable effects on HOF and OB,the introduction of the NH2group into the system could decrease the energy slightly,and this negative effect on energy to different series is different with each,it decreases the energy of series C and F least and most obviously,respectively.

3.2.2.Sensitivity performance

Fig.13.The impact sensitivity of designed bridge-ring energetic compounds.

For the advanced insensitive high energy compounds,both high energy and low sensitivity are necessary.Since there are no obvious weak bonds like the single N-NO2or sensitive groups like the azido group(N3)in the structure of designed bridge-ring compounds,the sensitivity of them may be not high.Fig.13 displays the predicted impact sensitivity (h50) values of designed compounds,as it is the most important and widely used parameter employed to judge the safety performance of energetic compounds.Generally,the higher theh50is,the lower the sensitivity,the better the safety performance.First of all,it can be seen that all compounds haveh50values higher than 49 cm,which are obviously better than RDX/HMX(28-30 cm)and CL-20(14 cm)[29,30].The averageh50value for all compounds is 83 cm,which is close to that of one famous insensitive compound TNT (98 cm) [29],suggesting the low sensitivity feature of designed bridge-compounds.20 compounds(order:PD0,PF0,PE0,PC0,PA0,PB0,PD2,PF1,PC2,PA3,PA1,PE1,PB3,PD1,PA2,PF2,PC1,PB2,PB1,PA5) are the most insensitive compounds.The introduction of NH2groups into the system deceases theh50value,and the amount and position both could affect this and their effects are coupled with each.It was observed that the introduction of NH2groups into the system can lengthen the maximum length of two relatively weak bonds C-NO2and endocyclic bonds to some degree,which may be one reason why NH2groups make negative effects on the sensitivity performance.To further and deeper understand why NH2groups do harm to the sensitivity performance of designed bridge-ring compounds,we calculated the ESP area distribution diagram (Fig.14) and map (Fig.15) by using the Multiwfn software [31-33].Fig.14 compares the area percent in each electrostatic potential range of PF0,PF1 and PF2.The sensitive energetic compounds usually have wider positive region and stronger intensity than the insensitive compounds [31,32].From this Figure,it can be seen that the area in the most positive region(>45 kcal/mol) increases gradually with the order of PF0,PF1 and PF2,showing that the sensitivity increases with the same order,this is consistent with the results fromh50.Fig.15 displays the ESPmapped vdW surface of PF0,PF1 and PF2,it can be clearly seen that the most positive value is just located on the region of NH2groups in PF2 and PF1 mainly.In all,the introduction of NH2groups into the designed bridge-ring compounds could lengthen the maximum length of weak C-NO2and endocyclic bonds,and enlarge the value and area of positive ESP,as a result,leading to worse impact sensitivity performance.

Fig.14.The area percent in each electrostatic potential range of PF0,PF1 and PF2.

Fig.15.ESP-mapped molecular vdW surface of PF0,PF1 and PF2.

In addition,the friction sensitivity was predicted using the model proposed by Huang [34],and the calculated molar activity index(F)values of designed compounds can be seen in Table S4(as seen in supporting information).Generally,one energetic compound has high friction sensitivity will also has highFvalue.For example,theFvalue of RDX (6.08) is higher than TNT (5.12) than 1,3,5-triamino-2,4,6-trinitrobenzene (TATB,4.52),while the friction sensitivity of them decreases with the same order[34].As seen from Table S4,it can be seen that all designed compounds haveFvalues lower than 5.6,which are obviously lower than RDX also.The averageFvalue for all compounds is 5.13,which is comparative with TNT,showing the low friction sensitivity feature of designed bridge-compounds.The deflagration temperature(DT)was estimated to learn the thermal sensitivity preliminarily using the model proposed by Keshavarz [35],and the results have been listed in Table S4.The predicted DT of designed compounds are ranged from 474 K to 487 K,and the average value is 481 K,which is higher than pentaerythritol tetranitrate (PETN) (467 K),and comparable with RDX(479 K)and CL-20(481 K)[35].This suggests that the thermal sensitivity of designed compounds may be close to RDX or CL-20 and better than PETN.

4.Conclusions

In the present work,from the point of experimental synthesis,we designed and selected out ten new bridge-ring energetic compounds with both high energy and low sensitivity through the Diels-Alder reaction between NH2-substituted oxazoles and NO2/NF2/NHNO2-substituted ethylenes/acetylenes.First,most of the designed dienes and dienophiles are strong nucleophilicity and electrophilicity,respectively.They proceed the HOMO-diene controlled normal Diels-Alder reaction most probably,and the energy barrier is low,while the introduction of NH2groups into the oxazole-based dienes could further favor the reaction obviously.Among all six dienophiles,tetranitroethylene may be the optimal dienophile because that it could react with the designed dienes significantly easily than other dienophiles,thus,it was employed to further design new energetic compounds.Then,six new series of bridge-ring energetic compounds were generated,which are the one-step Diels-Alder reaction products of tetranitroethylene and all dienes.All 29 designed compounds have higher energy than RDX,16 compounds (PC0,PE2,PC1,PC2,PD0,PD1,PD2,PF0,PD3,PF1,PF2,PB0,PB1,PB2)have better detonation performance than HMX,and 2 other even compounds (PE0,PE1) have comparative energy with CL-20.The highest,lowest and averageh50values are 114,49 and 83 cm,indicating that the designed bridge-ring energetic compounds also have low sensitivity,all of them are less sensitive than RDX,HMX and CL-20 obviously.The introduction of NH2groups into the system slightly against the energy and sensitivity by decrease the HOF/density/OB and increase the positive ESP value and C-NO2/endocyclic bond length to some degree,respectively.Finally,when low energy barrier,high detonation performance and low sensitivity performance are considered together,C2,C1,F2,F1,B3,D1,E2,B2,B1,B5 may be the ten optimal compounds,which may be used as the new potential insensitive high energy compounds.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgments

The present work was supported by the Natural Science Foundation of Jiangsu (BK20170761),Natural Science Foundation of Nanjing Institute of Technology (JCYJ201806),Science Innovation Project for Undergraduates of Nanjing Institute of Technology(TB202002005).Outstanding Scientific and Technological Innovation Team in Colleges and Universities of Jiangsu Province,and Jiangsu Overseas Visiting Scholar Program for University Prominent Young &Middle-aged Teachers and Presidents.

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.dt.2020.09.016.