1,2,4,5-四嗪的四唑類衍生物高能量密度材料的分子設(shè)計(jì)

陳 沫, 宋紀(jì)蓉, 馬海霞

(1. 西北大學(xué)化工學(xué)院, 陜西 西安 710069; 2. 北京故宮博物院文保科技部, 北京 100080)

1 引 言

四嗪類高氮化合物是近年發(fā)展起來(lái)的一類具有良好應(yīng)用前景的新型含能材料,其分子結(jié)構(gòu)中含有較多的N—N和C—N鍵,四嗪環(huán)含氮量高達(dá)68.3%,具有高能量、熱穩(wěn)定性好、低特征信號(hào)、燃燒無(wú)殘?jiān)盁o(wú)污染等優(yōu)點(diǎn),廣泛應(yīng)用于推進(jìn)劑、新型高能鈍感炸藥和煙火劑等含能材料領(lǐng)域[1-14]。目前的研究主要是發(fā)現(xiàn)和合成可以取代環(huán)三亞甲基三硝胺(RDX)和1,3,5,7-四硝基-1,3,5,7-四氮雜環(huán)辛烷(HMX)的四嗪化合物,實(shí)驗(yàn)中研究較多的四嗪類化合物有: 3,6-二氨基-1,2,4,5-四嗪-1,4-二氧化物(LAX-112)[15]、3,3′-偶氮-(6-氨基-1,2,4,5-四嗪)(DAAT)[16]、3,6-二肼基-1,2,4,5-四嗪(DHTz)[17]、3,6-雙(1-氫-1,2,3,4-四唑-5-氨基)-1,2,4,5-四嗪(BTATz)[18]、3,6-雙硝基胍基-1,2,4,5-四嗪(DNGTz)[19]、3,6-二(3,5-二甲基吡唑-1-基)-1,2,4,5-四嗪(BT)[20]等。

四唑環(huán)是一種五元氮雜環(huán),是唑類化合物中研究較多的一類,由于H原子位置的變動(dòng),可以分為三種同分異構(gòu)體,即1H-四唑、2H-四唑和5H-四唑,而1H-四唑是目前研究較多的一類。四唑環(huán)含氮量高(80%),具有高正生成焓,可以用于研究光敏材料及燃料電池,也可以制備起爆藥和固體推進(jìn)劑等含能化合物,還可以作為多種反應(yīng)中的催化劑,例如Suzuki、Heck等偶聯(lián)反應(yīng),應(yīng)用較為廣泛[21-25]。四唑環(huán)上的所有原子共平面,具有芳香性,穩(wěn)定性好,因此是開發(fā)新型含能化合物的理想基團(tuán)[26,27]。四唑環(huán)可以通過(guò)各種反應(yīng)取代環(huán)上的H原子來(lái)接多種含能基團(tuán)以改善物質(zhì)的性能,如硝基(—NO2)、偶氮基(—NN—)和氰基(—CN)等基團(tuán),可以很好地增加分子內(nèi)氮含量及能量密度。

本研究選擇能量較高的1,2,4,5-四嗪環(huán)為基本骨架,結(jié)合四唑環(huán)設(shè)計(jì)了30種1,2,4,5-四嗪衍生物,篩選出wB97/6-31+G**作為研究該類化合物的方法和基組,在該水平下對(duì)這些化合物進(jìn)行理論計(jì)算研究,采用原子化方案獲得其標(biāo)準(zhǔn)摩爾生成焓,并在生成焓和密度基礎(chǔ)上預(yù)測(cè)爆轟性能,以對(duì)新型四嗪含能物的設(shè)計(jì)和性能研究提供參考。

2 計(jì)算方法

運(yùn)用DFT方法,在B3LYP/(6-31G*,6-311G*,6-31+G*,6-31G**,6-311G**,6-31+G**,6-311++G**,cc-pVDZ,cc-pVTZ)水平下計(jì)算分析1,2,4,5-四嗪、3,6-二氨基-1,2,4,5-四嗪、3,6-二肼基-1,2,4,5-四嗪和3,6-二疊氮基-1,2,4,5-四嗪4種化合物生成焓,通過(guò)與實(shí)驗(yàn)值進(jìn)行對(duì)比擬合,在B3LYP/6-31+G**水平下計(jì)算的結(jié)果與實(shí)驗(yàn)值線性相關(guān)性最好,達(dá)到0.9863,所以選擇基組6-31+G**和不同方法(B3PW91,M05,M05-2X,M06,M06-2X,wB97)組合計(jì)算上述4種化合物生成焓,通過(guò)與實(shí)驗(yàn)值進(jìn)行對(duì)比擬合,在wB97/6-31+G**水平下計(jì)算的結(jié)果與實(shí)驗(yàn)值線性相關(guān)性最好,達(dá)到0.9896。在wB97/6-31+G**水平下對(duì)所設(shè)計(jì)的30種1,2,4,5-四嗪衍生物的幾何結(jié)構(gòu)進(jìn)行全優(yōu)化,經(jīng)振動(dòng)頻率分析表明優(yōu)化構(gòu)型為勢(shì)能面上極小點(diǎn)(無(wú)虛頻),得到的熱力學(xué)數(shù)據(jù)采用原子化方案(atomization scheme)[28-32]預(yù)測(cè)目標(biāo)化合物的標(biāo)準(zhǔn)生成焓。首先將分子分解為原子:

CaHbOcNd(g)→aC(g)+bH(g) +cO(g) +dN(g)

(1)

則該反應(yīng)在298 K時(shí)的標(biāo)準(zhǔn)反應(yīng)焓ΔH298由(2)式計(jì)算:

ΔH298=ΣΔHf,p-ΣΔHf,R=

aΔHf,C+bΔHf,H+cΔHf,O+dΔHf,N-ΔHf,CaHbOcNd

(2)

式中,ΔHf,R和ΔHf,p分別表示反應(yīng)物和生成物在298 K的標(biāo)準(zhǔn)生成焓; ΔHf,C、ΔHf,H、ΔHf,O和ΔHf,N分別為原子C、H、O和N在298 K的標(biāo)準(zhǔn)生成焓,可從手冊(cè)中查得; ΔHf,CaHbOcNd為分子CaHbOcNd在298 K的標(biāo)準(zhǔn)生成焓,為待求項(xiàng)。同時(shí)存在下列關(guān)系式:

ΔH298=ΔE298+Δ(pV) =ΔE0+ΔEZPE+ΔET+ΔnRT

=E0,C+E0,H+E0,O+E0,N-E0,CaHbOcNd-

EZPE,CaHbOcNd-ΔET,CaHbOcNd+ΔnRT

(3)

式中,E0,C、E0,H、E0,O、E0,N和E0,CaHbOcNd分別為wB97/6-31+G**水平下計(jì)算得到的原子C、H、O、N和分子CaHbOcNd在0 K的總能量;EZPE,CaHbOcNd和ΔET,CaHbOcNd分別為分子CaHbOcNd的零點(diǎn)能和熱校正值,可從振動(dòng)分析獲得的熱力學(xué)數(shù)據(jù)得到,對(duì)原子而言,EZPE和ΔET項(xiàng)均為0; Δn表示氣體產(chǎn)物和反應(yīng)物的物質(zhì)的量之差,R是氣體常數(shù),8.314 J·mol-1·K-1;T表示絕對(duì)溫度,K。綜合上式,化合物CaHbOcNd在298 K的標(biāo)準(zhǔn)生成焓ΔHf,CaHbOcNd即可求得。

運(yùn)用Kamlet和Jacobs于1968年提出的半經(jīng)驗(yàn)K-J方程[30-36]估算其爆速(D)、爆壓(p)值:

D=Φ0.5(1.011+1.312ρ)

(4)

(5)

p=1.558Φρ2

(6)

化學(xué)鍵強(qiáng)度可用鍵離解能(EBD)來(lái)衡量,它對(duì)理解化學(xué)反應(yīng)過(guò)程十分重要[37]。在UwB97/6-31+G**水平下計(jì)算標(biāo)題化合物的EBD,對(duì)比化合物鍵的強(qiáng)度及熱穩(wěn)定性。在298 K和1個(gè)標(biāo)準(zhǔn)大氣壓壓下,化學(xué)鍵A—B均裂所需能量等于反應(yīng)A—B(g)→A·(g)+B·(g)的反應(yīng)焓,該反應(yīng)焓可定義為A—B的鍵離解焓[38]。文獻(xiàn)報(bào)道許多有機(jī)化合物的鍵離解能和鍵離解焓是相等的[39-42]。因此,標(biāo)題化合物的鍵離解能可依據(jù)方程式(7)進(jìn)行計(jì)算[39]:

EBD(A-B)=H(A·)+H(B·)-H(A-B)

(7)

本研究所有化合物密度均采用摩爾體積法(ρ=M/Vm)計(jì)算得到,其中M為化合物的摩爾質(zhì)量,Vm為化合物的摩爾體積,是在穩(wěn)定構(gòu)型下,基于0.001e·bohr-3等電子密度面所包圍的體積空間,用Monte-Carlo方法求得。所有計(jì)算使用Gaussian09[43]量子化學(xué)軟件包完成。

3 結(jié)果討論

3.1 幾何結(jié)構(gòu)

選擇能量較高的1,2,4,5-四嗪環(huán)作為母體,結(jié)合四唑環(huán)、偶氮基(—NN—)以及硝基(—NO2)設(shè)計(jì)了30種四唑類1,2,4,5-四嗪衍生物,分子結(jié)構(gòu)如圖1所示。在wB97/6-31+G**水平下對(duì)30種1,2,4,5-四嗪衍生物進(jìn)行幾何全優(yōu)化,表1列出了標(biāo)題化合物中部分對(duì)稱雙取代化合物部分幾何參數(shù)(鍵長(zhǎng)、鍵角和二面角),計(jì)算結(jié)果表明,1,2,4,5-四嗪中H原子被四唑環(huán)取代后,四嗪環(huán)上各鍵(C—N、N—N)鍵長(zhǎng)均受到影響,化合物iii1～iv3中與四唑環(huán)相鄰的四嗪環(huán)上C—N鍵長(zhǎng)依次為0.1333/0.1334,0.1329/0.1330,0.1333,0.1333/0.1336,0.1332/0.1335和0.1333/0.1335 nm,略小于未被取代的1,2,4,5-四嗪中C—N鍵長(zhǎng)0.1336 nm(實(shí)驗(yàn)值0.1338 nm),減幅為0～0.0007 nm之間; 連接兩環(huán)間偶氮基(—NN—)的引入使四嗪環(huán)上與取代基相鄰的C—N鍵長(zhǎng)略增加; 四唑環(huán)上引入硝基—NO2和偶氮硝基—NN—NO2使環(huán)上C—N鍵長(zhǎng)略減小,N—N鍵長(zhǎng)略增加。四嗪環(huán)上所有C—N鍵的鍵長(zhǎng)在0.1329～0.1336 nm之間,N—N鍵長(zhǎng)約為0.1303～0.1310 nm,鍵長(zhǎng)趨于均勻化,C—N鍵鍵長(zhǎng)短于標(biāo)準(zhǔn)C—N單鍵(0.1470 nm)而長(zhǎng)于標(biāo)準(zhǔn)CN雙鍵(0.1280 nm)的鍵長(zhǎng); 同樣,N—N鍵也短于標(biāo)準(zhǔn)N—N單鍵(0.1450 nm)長(zhǎng)于標(biāo)準(zhǔn)NN雙鍵(0.1250 nm),四嗪環(huán)中六個(gè)p電子形成大π鍵,而非三個(gè)首尾相接的雙鍵,表明1,2,4,5-四嗪衍生物的四嗪環(huán)具有一定的芳香性。

與未取代的1,2,4,5-四嗪比較,化合物iii1～iv3的四嗪環(huán)內(nèi)鍵角N(2)C(3)N(4)和N(5)C(6)N(1)增大,其余鍵角減小,且環(huán)內(nèi)鍵長(zhǎng)和鍵角差異都較小,這是因?yàn)榛衔锞哂休^好的對(duì)稱性和共軛性。化合物iii1,iii2和iii3的二面角N(1)C(6)N(7)N(9)和N(5)C(6)N(7)C(12)分別為172.96°和173.57°,115.85°和122.71°, 8.66°和24.97°,iii1的接近180°,所以iii1的四嗪環(huán)和四唑環(huán)具有較好的共面性?；衔飅v1,iv2和iv3的二面角N(5)C(6)N(7)N(9)和N(2)C(3)N(8)N(10)分別為-141.63°和-140.63°,-40.92°和-139.79°,40.64°和142.29°,二面角C(6)N(7)N(9)N(11)和C(3)N(8)N(10)N(12)分別為-178.49°和2.00°,164.82°和14.39°,0.46°和11.32°,表明四嗪環(huán)與四唑環(huán)不共面,連接橋—NN—的引入使四嗪環(huán)與四唑環(huán)共面性更差。

圖11,2,4,5-四嗪衍生物的分子結(jié)構(gòu)

Fig.1Molecular structures of 1,2,4,5-tetrazine derivatives

表1部分標(biāo)題化合物的鍵長(zhǎng)、鍵角和二面角

Table1Calculated bond lengths, bond angles and dihedral angles of part of the title compounds

parametersatomiii1iii2iii3iv1iv2iv3s-tetrazinebondlength/nmR(1,2)0.13060.13080.13090.13070.13070.13070.1313R(2,3)0.13330.13290.13290.13350.13350.13350.1336R(3,4)0.13340.13300.13300.13350.13330.13330.1336R(4,5)0.13060.13080.13030.13090.13100.13100.1313R(5,6)0.13330.13290.13330.13330.13320.13330.1336R(6,1)0.13340.13300.13330.13360.13350.13350.1336R(6,7)0.14030.14190.14000.14230.14240.1425R(7,9)0.13560.13470.13640.12380.12350.1234R(7,12)0.13590.13490.1359R(9,10)0.12750.12840.1269R(10,11)0.13640.13530.1368R(11,12)0.13040.12980.1299R(9,11)0.13660.13770.1368R(11,13)0.13490.13470.1354R(11,16)0.13620.13580.1359R(13,14)0.12730.12750.1274R(14,15)0.13720.13680.1368R(15,16)0.13020.12930.1299bondangle/(°)A(1,2,3)116.13115.97116.67116.71116.55116.69116.98A(2,3,4)126.37127.54126.90126.34126.73126.60126.04A(3,4,5)117.48116.49116.31116.78116.59116.58116.98A(4,5,6)116.13115.97116.69116.69116.53116.70116.98A(5,6,1)126.37127.54126.72126.41126.77126.59126.04A(6,1,2)117.48116.49116.07116.78116.61116.62116.98

(Table1Continued)

parametersatomiii1iii2iii3iv1iv2iv3s-tetrazinebondangle/(°)A(8,3,2)118.10116.37117.48114.54114.37114.32A(8,3,4)115.53116.08115.62118.95118.75118.92A(5,6,7)118.10116.37114.96114.44118.93118.93A(1,6,7)115.53116.08118.32119.01114.17114.35A(6,7,9)122.85121.14123.20111.59111.90110.82A(6,7,12)129.09131.91128.40A(7,9,10)106.09106.67106.52A(9,10,11)111.73111.56112.06A(10,11,12)106.04105.08105.13A(11,12,7)108.09109.93109.51A(12,7,9)108.05106.76106.73A(9,11,16)132.92134.26123.74A(9,11,13)118.32118.42128.32A(11,13,14)106.00106.59105.72A(13,14,15)111.51111.61112.35A(14,15,16)106.29105.20105.12A(15,16,11)107.43109.36108.89A(16,11,13)108.76107.24107.88dihedralangle/(°)D(1,6,7,9)172.96115.858.6642.40142.92-143.17D(5,6,7,12)173.57122.7124.97D(2,3,8,16)173.57-122.71132.20D(4,3,8,13)172.96-115.85125.00D(1,6,7,12)-5.95-58.46-154.61D(5,6,7,9)-7.52-62.99-171.76-141.63-40.9240.64D(2,3,8,10)-140.63-139.79142.29D(6,7,9,11)-178.45177.30-178.64D(3,8,10,12)-177.93-177.37178.82D(7,9,11,13)-178.49164.82 0.46D(8,10,12,17) 2.0014.3911.32

Note: These data in the brackets are atom number.

3.2 生成焓

生成焓是化合物的基本熱力學(xué)性質(zhì),是其能量高低的標(biāo)志,也是衡量高能量密度材料(HEDM)爆轟性能的重要參數(shù),因此精確計(jì)算生成焓對(duì)設(shè)計(jì)、篩選以及合成新型品優(yōu)HEDM具有重要意義。本研究在wB97/6-31+G**水平下采用原子化方案估算1,2,4,5-四嗪衍生物及傳統(tǒng)含能材料RDX和HMX的生成焓,表2列出了各化合物的總能量(E0)、零點(diǎn)能(EZPE)、溫度校正值(ET)、含氮量(N%)及生成焓(ΔHf)。計(jì)算結(jié)果表明,所有1,2,4,5-四嗪衍生物生成焓均大于傳統(tǒng)含能材料RDX和HMX,具有高正生成焓,其中最高的生成焓值為2610.45 kJ·mol-1。四唑環(huán)有利于提高四嗪衍生物的ΔHf,如化合物i1和iii1,且四唑環(huán)上引入—NO2和—NN—NO2能夠進(jìn)一步改善四嗪衍生物的生成焓,比其它衍生物(如i1和iii1)生成焓高,且基團(tuán)數(shù)量越多,生成焓越高,說(shuō)明四唑環(huán)上引入—NO2和—NN—NO2能夠有效地改善四嗪衍生物的ΔHf。含氮量越高,生成焓不一定越大,如化合物iv1的含氮量(81%)最高,生成焓為1920.40 kJ·mol-1。

圖2對(duì)比不同取代基對(duì)四嗪衍生物生成焓的影響,結(jié)果表明對(duì)稱雙取代化合物的生成焓明顯高于相應(yīng)單取代的,且四唑環(huán)上引入—NN—NO2能夠顯著提高四嗪衍生物的生成焓。不同系列衍生物與氮原子數(shù)的關(guān)系如圖3所示,從圖3可以看出,同系列化合物的生成焓隨氮原子數(shù)量的增加而逐漸增大,且相同氮原子數(shù)量的四嗪衍生物生成焓不同但相近。綜上所述,四唑環(huán)及在其上引入—NO2和—NN—NO2在增加1,2,4,5-四嗪衍生物的生成焓方面起了非常重要的作用,且同系列1,2,4,5-四嗪衍生物的生成焓隨氮原子數(shù)的增加而逐漸增大,含氮量越高,生成焓不一定越大。

3.3 前線軌道能量

基于分子軌道理論,化合物的最高占據(jù)軌道能量(EHOMO)越低,最低空軌道能量(ELUMO)越高,即分子軌道能級(jí)差ΔE=ELUMO-EHOMO越大,化合物越穩(wěn)定。

表21,2,4,5-四嗪衍生物及RDX和HMX的總能量(E0)、零點(diǎn)能(EZPE)、溫度校正值(ET)和生成焓(ΔHf)計(jì)算值

Table2Calculated total energies (E0), zero-point energies (EZPE), thermal corrections (ET) and enthalpies of formation(ΔHf)of 1,2,4,5-tetrazine derivatives together with RDX and HMX

compd.formulaE0/HartreeEZPE/HartreeET/HartreeN/%ΔHf/kJ·mol-1i1C3H2N8 -553.31980.08030.008574.7 920.46i2C3N10O4 -962.23970.08550.013658.3 1045.69i3C3N14O4 -1181.09830.10390.017766.2 1580.41i4C3N12O4 -1071.66620.09460.015662.7 1319.86i5C3N12O4 -1071.67220.09480.015762.7 1305.09i6C3HN9O2 -757.77790.08320.011064.6 988.48i7C3HN9O2 -757.78350.08260.011164.6 972.74i8C3HN11O2 -867.20390.09240.012969.1 1264.36i9C3HN11O2 -867.21540.09210.013269.1 1233.96ii1C3H2N10 -662.73770.09020.010278.7 1219.16ii2C3N12O4 -1071.65490.09490.015662.7 1350.39ii3C3N16O4 -1290.51180.11300.019669.1 1888.81ii4C3N14O4 -1181.08440.10410.017566.2 1616.96ii5C3N14O4 -1181.08700.10420.017766.2 1610.79ii6C3HN11O2 -867.19200.09270.012969.1 1296.36ii7C3HN11O2 -867.20160.09250.012969.1 1270.36ii8C3HN13O2 -976.62650.10200.014972.5 1550.48ii9C3HN13O2 -976.63350.10170.015072.5 1531.42iii1C4H2N12 -810.35050.10800.012077.1 1322.77iii2C4N14O4 -1219.26400.11330.017163.6 1465.14iii3C4N18O4 -1438.12230.13170.021069.2 2000.01iii4C4N16O4 -1328.69070.12250.019166.7 1738.84iii5C4HN13O2 -1014.80820.11070.014669.2 1391.53iii6C4HN15O2 -1124.23340.11980.016572.2 1669.24iv1C4H2N16 -1029.18570.12730.015681.8 1920.40iv2C4N18O4 -1438.09390.13220.021169.2 2075.90iv3C4N22O4 -1656.95230.15020.025173.3 2610.45iv4C4N20O4 -1547.51930.14120.022971.4 2352.78iv5C4HN17O2 -1233.64110.12980.018374.7 1995.03iv6C4HN19O2 -1343.06580.13870.020376.7 2273.37RDXC3H6N6O6-897.35320.14700.013437.8244.21HMXC4H8N8O8-1196.47500.19720.017837.8317.19

圖2均四嗪衍生物生成焓的比較

Fig.2Compared the ΔHfof s-tetrazine derivatives

本研究在幾何優(yōu)化基礎(chǔ)上,獲得1,2,4,5-四嗪衍生物EHOMO及ELUMO,進(jìn)一步計(jì)算得到分子軌道能級(jí)差ΔE,列于表3。通過(guò)比較1,2,4,5-四嗪衍生物i1(0.33645a.u.)和i2(0.34241a.u.)、iii1(0.33575a.u.)和iii2(0.34138a.u.)能級(jí)差可以看出,當(dāng)四唑環(huán)上H被—NO2取代后,EHOMO和ELUMO均減小,能級(jí)差增大,說(shuō)明—NO2有利于提高1,2,4,5-四嗪衍生物的穩(wěn)定性。通過(guò)i1(0.33645a.u.)和ii1(0.33331a.u.)、iii1(0.33575a.u.)和iv1(0.32594a.u.)能級(jí)差可以看出,連接橋偶氮基—NN—使1,2,4,5-四嗪衍生物EHOMO和ELUMO增大,能級(jí)差減小,且iv3偶氮基最多,能級(jí)差最小(0.31803a.u.),說(shuō)明—NN—不利于1,2,4,5-四嗪衍生物穩(wěn)定性的提高。i2(0.34241a.u.)和iii2(0.34138a.u.)能級(jí)差在所有體系中較高,使電子躍遷幾率降低,預(yù)示其反應(yīng)活性最低,最穩(wěn)定,而iv3和iv4(0.31803a.u.)能級(jí)差相同且最小,故最不穩(wěn)定的化合物可能是iv3和iv4。

圖3生成焓與氮原子數(shù)的關(guān)系

Fig.3The relationship of the ΔHfand N atom numbers

表31,2,4,5-四嗪衍生物的EHOMO,ELUMO和ELUMO-HOMO

Table 3 Calculated HOMO and LUMO energies and energy gap (ΔELUMO-HOMO) of 1,2,4,5-tetrazine derivatives a.u.

3.4 爆轟性能

爆速和爆壓是衡量含能材料爆轟性能的重要參數(shù)。本研究在wB97/6-31+G**水平下預(yù)測(cè)1,2,4,5-四嗪衍生物及RDX和HMX的爆速(D)和爆壓(p),見表4。由表4可知,RDX和HMX的理論預(yù)測(cè)值與實(shí)驗(yàn)值對(duì)比較為接近,說(shuō)明運(yùn)用該方法得到的結(jié)果用于預(yù)測(cè)標(biāo)題化合物的爆轟性能是相對(duì)可靠的。結(jié)果表明,帶有不同取代基的1,2,4,5-四嗪衍生物密度不同,最大為1.84 g·cm-3,最小為1.60 g·cm-3,這也因此說(shuō)明了目標(biāo)化合物可能具備不同的D和p。通過(guò)比較i1和i2、ii1和ii2、iii1和iii2、iv1和iv2的ρ、D、p可以看出,四唑環(huán)上引入—NO2有利于增加1,2,4,5-四嗪衍生物的ρ、D、p; 比較i1和ii1,iii1和iv1,i2和i3的ρ、D、p可以看出,四唑環(huán)上引入—NN—使四嗪衍生物ρ減小,而D和p顯著增加; 比較i1和iii1的ρ、D、p可以看出,四唑骨架能顯著增加1,2,4,5-四嗪衍生物的ρ、D、p。取代基相同但位置不同的1,2,4,5-四嗪衍生物(i4和i5、i6和i7、i8和i9、ii4和ii5、ii6和ii7、ii8和ii9)的ρ、D、p計(jì)算結(jié)果不同但相近,說(shuō)明取代基的位置不同對(duì)均四嗪衍生物的爆轟性能影響較小。

1,2,4,5-四嗪衍生物與RDX和HMX的ρ、D和p的比較如圖4所示。從圖4可以看出,有13種標(biāo)題化合物的ρ接近RDX[44](1.82 g·cm-3),最高達(dá)到1.84 g·cm-3。D和p的趨勢(shì)一致,有16種1,2,4,5-四嗪衍生物的D和p比RDX[44](8.75 km·s-1和34.00 GPa)高,且有12種化合物有較理想的爆轟性能,ρ、D和p分別接近于1.90 g·cm-3、9.00 km·s-1和40.00 GPa,其D超過(guò)HMX(9.10 km·s-1[44]),p接近于HMX(39 GPa[44]),尤其是iv3,D和p達(dá)到9.307 km·s-1,38.673 GPa,具有很好的爆轟性能,因此可以作為潛在的HEDM候選物。

圖4目標(biāo)化合物及RDX和HMX的ρ、D、p

Fig.4Calculatedρ,D, andpvalues of the title compounds together with RDX and HMX

表41,2,4,5-四嗪衍生物及RDX和HMX的平均摩爾體積(V)、摩爾質(zhì)量(M)、理論密度(ρ)、爆熱(Q)、爆速(D)和爆壓(p)

Table4Predicted average molar volumes(V), molar mass(M), theoretical densities (ρ), heats of detonation (Q), detonation velocities (D) and detonation pressures (p) of 1,2,4,5-tetrazine derivatives together with RDX and HMX

compd.V/cm3·mol-1M/g·mol-1ρ/g·cm-3Q/J·g-1D/km·s-1p/GPai193.498150.0401.601466.2067.6924.45i2130.365240.0101.841825.0049.1437.55i3163.096296.0231.821911.4019.2738.32i4147.310268.0171.821878.7869.1937.77i5148.941268.0171.801865.6159.1136.82i6110.860195.0251.761721.2378.5431.91i7110.695195.0251.761701.9488.5131.73i8129.114223.0311.731800.7348.6332.27i9128.026223.0311.741768.1588.6332.35ii1109.196178.0461.631636.5368.1127.49ii2147.447268.0171.821906.0149.2237.96ii3183.323324.0291.771973.6699.2237.30ii4163.766296.0231.811940.9089.2938.40ii5163.353296.0231.811935.9279.2838.36ii6128.340223.0311.741835.0258.7132.95ii7127.936223.0311.741807.1648.6732.70ii8147.365251.0381.701872.2358.6932.34ii9144.572251.0381.741854.0898.8133.71iii1128.015218.0531.701449.8437.9727.22iii2170.421308.0231.811747.4968.8634.91iii3198.446364.0351.831829.7769.1637.66iii4183.604336.0291.831796.5209.0636.76iii5147.494263.0381.781642.3988.4831.69iii6164.581291.0441.771712.4078.6132.59iv1162.980274.0651.681674.6978.3529.68iv2200.528364.0351.821879.6009.1537.38iv3230.986420.0471.821933.1109.3138.67iv4216.398392.0411.811914.1229.2237.85iv5183.788319.0501.741806.1428.6932.83iv6200.288347.0561.731852.0738.7633.27RDX129.53222.041.71(1.82[44])1679.208.76(8.75[44])33.05(34.00[44])HMX162.87296.051.82(1.91[44])1672.409.11(9.10[44])37.08(39.00[44])

3.5 熱穩(wěn)定性

鍵級(jí)(BO)是衡量物質(zhì)化學(xué)鍵強(qiáng)度的重要電子結(jié)構(gòu)參數(shù)。鍵級(jí)越大,其間電子云密度重疊較多,表明該鍵越強(qiáng),不易斷裂。鍵離解能(EBD)為判斷含能化合物的穩(wěn)定性提供了非常有用的信息。一般來(lái)說(shuō),斷開一個(gè)鍵所需能量越小,鍵越弱,越易成為引發(fā)鍵,也就是說(shuō),化合物靈敏度較高,不穩(wěn)定。因此,鍵離解能常被用來(lái)衡量含能化合物熱穩(wěn)定性的相對(duì)順序。表5列出了標(biāo)題化合物幾個(gè)相對(duì)較弱鍵的Wiberg鍵級(jí)及其鍵離解能。

通過(guò)比較化合物1,2,4,5-四嗪(473.93 kJ·mol-1)、i1(463.57 kJ·mol-1)和iii1(462.98kJ·mol-1)最弱鍵的EBD可以看出,四唑環(huán)的引入使1,2,4,5-四嗪衍生物的穩(wěn)定性降低,但影響較小。通過(guò)比較化合物i1(463.57 kJ·mol-1)和ii1(220.42 kJ·mol-1)及化合物iii1(462.98 kJ·mol-1)和iv1(216.45 kJ·mol-1)最弱鍵的EBD可以看出,連接橋—NN—的加入使EBD降低約240 kJ·mol-1。通過(guò)比較化合物i系列中i1(463.57 kJ·mol-1)、i7(256.89 kJ·mol-1)和i2(252.03 kJ·mol-1)及iii系列中iii1(462.98 kJ·mol-1)、iii5(273.10 kJ·mol-1)和iii2(268.45 kJ·mol-1)最弱鍵的EBD可以看出,四唑環(huán)上引入—NO2使BDE降低約189～207 kJ·mol-1,但化合物i7和i2及iii5和iii2的EBD非常接近,相差約4 kJ·mol-1,表明隨著—NO2數(shù)目的增多,四嗪衍生物的穩(wěn)定性并無(wú)較大的變化。綜上所述,四唑環(huán)與四嗪環(huán)直接連接使1,2,4,5-四嗪衍生物穩(wěn)定性降低,但影響較小,而通過(guò)—NN—連接兩者則明顯地降低了其穩(wěn)定性; 四唑環(huán)上引入—NO2或—NN—NO2使1,2,4,5-四嗪衍生物穩(wěn)定性降低,且增加取代基數(shù)量對(duì)其穩(wěn)定性影響較小; Rg1—Rg2、Rg1—R、Rg2—R或Rg2—NN(Rg1代表四嗪環(huán),Rg2代表四唑環(huán),R代表取代基)均有可能成為1,2,4,5-四嗪衍生物的熱引發(fā)鍵。

表5目標(biāo)化合物相對(duì)較弱鍵的Wiberg鍵級(jí)(BO, a.u.)和鍵離解能(EBD, kJ·mol-1)

Table5BO andEBDof the relatively weak bonds of the title compounds

compd.Rg1—Rg2BOEBDRg1—RBOEBDRg1—NNBOEBDRg2—RBOEBDRg2—NNBOEBDN—NO2(Rg1)BOEBDN—NO2(Rg2)BOEBDi10.99463.570.89482.680.89514.55i20.93453.340.86252.030.90269.12i30.98429.511.0024.261.0225.670.8448.430.7735.02i40.99427.790.86254.411.0225.340.7735.05i50.93456.201.0023.030.90270.600.8448.05i60.92460.100.89482.750.90273.02i71.01461.730.86256.890.89515.58i80.97432.850.89483.421.0227.530.7735.23i91.00462.781.0026.060.89515.240.8449.30ii10.90480.601.01317.090.89513.331.07220.42ii20.86253.941.01313.000.88260.231.0619.87ii31.0024.581.02315.601.0219.871.07175.970.8448.780.7426.85ii40.86266.041.02325.191.0224.051.08186.650.7637.55ii51.0023.931.01314.860.88267.501.05205.370.8348.48ii60.89481.471.00316.580.88262.621.04207.78ii70.86255.901.02316.350.89513.781.09220.07ii80.89506.031.01341.231.0438.901.05202.290.8351.47ii91.0025.231.02317.540.89520.381.09220.180.8449.21iii11.00462.980.89515.37iii20.93453.250.90268.45iii30.95443.551.0441.370.8251.67iii40.92428.181.0225.140.7735.30iii51.01458.770.90273.10iii61.00429.691.0225.780.7733.97iv11.02311.790.89516.491.08216.45iv21.01314.540.88260.911.05205.32iv31.02315.551.0220.051.06176.080.7427.09iv41.01294.461.0214.421.07177.070.7426.59iv51.01316.260.88268.351.05206.38iv61.02294.771.0220.401.07176.670.7427.26

Note: Rg1, Rg2and R represent the tetrazine ring, tetrazole ring and substituents,respectively.

圖5比較了1,2,4,5-四嗪衍生物最弱鍵的BO和EBD的變化趨勢(shì)。從圖5可以看出,EBD和BO的變化趨勢(shì)并不是完全吻合的,有的鍵級(jí)較小的化合物鍵離解能反而大,如化合物i2、i6、i7、iii2和iii5。所以,衡量一個(gè)物質(zhì)的穩(wěn)定性不但要看其BO,還要考慮其EBD的大小。Chung[45]等提出,高能量密度化合物最弱鍵的EBD高于80 kJ·mol-1,從圖中可以看出,低于80 kJ·mol-1的1,2,4,5-四嗪衍生物EBD很小,在14～42 kJ·mol-1之間,穩(wěn)定性不好,有14種1,2,4,5-四嗪衍生物的EBD高于80 kJ·mol-1,綜合生成焓和爆轟性能的結(jié)果,化合物i2、ii2和iv2可以作為潛在的HEDM候選物。

圖5目標(biāo)化合物較弱鍵的BO與EBD的比較

Fig.5The comparison ofBOandEBDof the relatively weak bonds of the title compounds

3.6 熱力學(xué)性質(zhì)

基于統(tǒng)計(jì)熱力學(xué),運(yùn)用Gaussian09和自編程序,求得部分1,2,4,5-四嗪衍生物在200～800 K溫度范圍的熱力學(xué)性質(zhì),即標(biāo)準(zhǔn)摩爾熱容(cp)、標(biāo)準(zhǔn)摩爾熵(Sm)和標(biāo)準(zhǔn)摩爾焓(Hm)。通過(guò)線性擬合求得各目標(biāo)化合物在200～800 K溫度范圍的cp、Sm和Hm與溫度間關(guān)系如圖6所示。由圖6可見,cp及Hm均隨溫度的升高而增加,這主要是因?yàn)樵谳^低溫度時(shí),分子的平動(dòng)和轉(zhuǎn)動(dòng)對(duì)熱力學(xué)函數(shù)的貢獻(xiàn)大; 但是溫度升高后,分子的振動(dòng)增強(qiáng),對(duì)熱力學(xué)函數(shù)貢獻(xiàn)大,從而導(dǎo)致熱力學(xué)函數(shù)值增加。計(jì)算結(jié)果表明,在同一溫度下,—NO2、—NN—及四唑骨架均有利于cp、Sm和Hm的增加,且對(duì)稱雙取代1,2,4,5-四嗪衍生物的比相應(yīng)單取代的高。由圖6還可以看出,標(biāo)題化合物的cp和Sm的增幅均隨溫度的升高而逐步減小,而Hm的增幅均隨溫度的升高而逐步增大。通過(guò)iv2和iv3的熱力學(xué)性質(zhì)與溫度間的函數(shù)關(guān)系式可以看出,曲線二次方項(xiàng)的系數(shù)非常小,近似為直線,化合物iv2和iv3的三個(gè)熱力學(xué)函數(shù)值隨溫度的升高基本呈線性遞增。其余化合物的熱力學(xué)性質(zhì)與溫度間也存在類似的線性關(guān)系。

a.b.c.

d.e.f.

圖6目標(biāo)化合物cp、Sm和Hm與T的關(guān)系

Fig.6The relationships betweenTandcp、SmandHmof the title compounds

4 結(jié) 論

利用wB97/6-31+G**方法對(duì)四唑類1,2,4,5-四嗪衍生物的幾何構(gòu)型、前線軌道能量、生成焓、熱穩(wěn)定性、爆轟性能及熱力學(xué)性質(zhì)進(jìn)行計(jì)算研究。計(jì)算結(jié)果表明,對(duì)稱雙取代對(duì)于提高1,2,4,5-四嗪的生成焓具有顯著效果,同時(shí)含能骨架四唑環(huán)及在其上引入—NO2和—NN—NO2能夠顯著增加四嗪衍生物的生成焓; 爆轟性能結(jié)果表明,在四唑環(huán)上引入—NO2和—NN—能夠顯著提高1,2,4,5-四嗪衍生物的爆轟性能,且對(duì)稱雙取代對(duì)于提高衍生物的爆轟性能具有很好的效果; 通過(guò)分析標(biāo)題化合物最弱鍵的鍵離解能,有14種化合物的EBD高于80 kJ·mol-1,具有較理想的熱穩(wěn)定性,在四唑環(huán)上引入—NO2和—NN—不利于提高衍生物的穩(wěn)定性; 熱容cp、熵Sm及焓Hm均隨溫度的升高而增加,且所有取代基均使cp、Sm、Hm值增大,cp和Sm的增幅隨溫度升高而逐步減小,而Hm的增幅則隨溫度的升高而逐步增大。綜合分析,化合物i2、ii2和iv2從能量、爆轟性能和熱穩(wěn)定性上可以作為備選的高能量密度材料。

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