劉英哲, 康 瑩, 來蔚鵬, 尉 濤, 葛忠學(xué)

(西安近代化學(xué)研究所, 陜西 西安 710065)

1 引 言

基于碳納米材料摻雜的復(fù)合含能材料能夠改善傳統(tǒng)含能材料的某些性能,如在HMX中添加C60、石墨烯(Graphene, GRA)氧化物等碳納米材料能夠有效降低HMX的感度,并增加了熱解反應(yīng)的活化能,從而提高了熱分解穩(wěn)定性[1-2]。理論計算[3-4]預(yù)測通過碳納米管封裝新型高能量密度物質(zhì)N8等,能夠提高其室溫常壓下的穩(wěn)定性。除了新型高能量密度材料,典型含能化合物RDX(hexahydro-1,3,5-trinitro-striazine)、HMX(octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine)、FOX-7(1,1-diamino-2,2-dinitroethylene)等封裝在碳納米管空腔、雙層GRA間隙時,其穩(wěn)定性也能得到一定程度的提高[5]。此外,GRA及其氧化物還能催化硝基甲烷(Nitromethane, NM)的燃燒[6-7]。從微觀角度來說,碳納米材料可能改變了含能材料的結(jié)構(gòu)、能量及反應(yīng)活化能等,使含能材料的感度、穩(wěn)定性及反應(yīng)速率等性質(zhì)發(fā)生了變化,因此研究含能材料在碳納米材料摻雜時的反應(yīng)機理及相應(yīng)的結(jié)構(gòu)與能量變化具有重要的意義。

硝基甲烷(NM)是最簡單的含能化合物,常作為模型廣泛用于理論研究中,包括結(jié)構(gòu)性質(zhì)[8-13]、融化與結(jié)晶[14-15]、熱分解[16-19]等方面。然而,NM的反應(yīng)機理仍十分復(fù)雜,在反應(yīng)過程中會出現(xiàn)許多基元反應(yīng)以及新的物質(zhì)。因此,對于含能材料反應(yīng)機理的研究通常先集中在初始反應(yīng)上。NM初始反應(yīng)包括硝基甲烷-亞硝酸甲酯(methyl nitrite, MN)重排反應(yīng)、氫遷移重排反應(yīng)及C—N鍵均裂反應(yīng),反應(yīng)式分別如下[19]:

CH3NO2→ CH3ONO

(1)

CH3NO2→ CH2NOOH

(2)

CH3NO2→·CH3+·NO2

(3)

盡管碳納米材料可作為鈍感劑、催化劑、添加劑應(yīng)用在含能材料領(lǐng)域,并且表現(xiàn)出了良好的效果[20],但是碳納米材料對含能化合物反應(yīng)機理的影響仍研究不夠完全。因此,本研究選取GRA與NM作為碳納米材料與含能化合物的模型,采用組合量子化學(xué)ONIOM方法探索GRA對NM初始反應(yīng)、結(jié)構(gòu)與能量的影響。

2 計算方法

2.1 分子模型

單層GRA初始結(jié)構(gòu)由VMD軟件[21]中Carbon Nanostructure Builder模塊構(gòu)建,尺寸為20?× 20?,共含有180個碳原子,其邊緣的懸鍵用38個氫原子進(jìn)行飽和。NM有交錯式和重疊式兩種異構(gòu)體,H—C—N—O二面角最小值分別為30°和0°,初始結(jié)構(gòu)由VMD軟件[21]中Molefacture模塊構(gòu)建。

圖1NM在B3LYP/6-31+G**理論水平下的優(yōu)化結(jié)構(gòu)(括號內(nèi)數(shù)據(jù)為實驗值[22],鍵長單位為?)

Fig.1The optimized geometries of NM at B3LYP/6-31+G**level (data in parentheses are the experimental values[22], the bond length unit is given in ?)

2.2 量化計算

考慮到研究體系共含有225個原子,難以完全用高精度的量子化學(xué)方法進(jìn)行計算,故采用組合量子化學(xué)ONIOM(our Own N-layer Integrated molecular Orbital and molecular Mechanics)方法[23]將體系分為不同計算精度的兩層進(jìn)行處理: 內(nèi)層為NM,采用高精度的密度泛函理論B3LYP/6-31+G**[24-25]進(jìn)行計算; 外層為GRA,采用分子力學(xué)方法UFF力場[26]進(jìn)行計算。原子電荷由Qeq方法[27]計算,利用電子內(nèi)嵌技術(shù)[23]描述內(nèi)層與外層之間的靜電相互作用。為了確認(rèn)最穩(wěn)定的幾何構(gòu)型與反應(yīng)過渡態(tài),在ONIOM (B3LYP/6-31+G**:UFF)理論水平下對振動頻率進(jìn)行求解。利用內(nèi)稟反應(yīng)坐標(biāo)法[28]進(jìn)一步確認(rèn)反應(yīng)路徑。作為對比,采用B3LYP/6-31+G**對孤立NM進(jìn)行同樣的計算。所有理論計算均采用Gaussian09[29]軟件完成。

3 結(jié)果與討論

3.1 結(jié)構(gòu)分析

采用B3LYP/6-31+G**理論水平計算的NM最優(yōu)結(jié)構(gòu)與實驗結(jié)果[22]吻合較好,僅C—N鍵比實驗值長0.01?(見圖1)。重疊式與交錯式NM之間的能量差小于0.05 kJ·mol-1,表明—CH3與—NO2基團(tuán)幾乎能夠繞著C—N鍵自由地旋轉(zhuǎn),這與文獻(xiàn)[30]理論計算結(jié)果是一致的。然而,NM在GRA表面的最優(yōu)結(jié)構(gòu)基本保持交錯式的構(gòu)型,不論NM初始結(jié)構(gòu)為重疊式還是交錯式,此時NM能夠最大程度地與GRA平面結(jié)構(gòu)相互作用(見圖2)。這與NM在另一種碳納米材料——碳納米管中的結(jié)構(gòu)有所不同,量子化學(xué)計算表明NM在(5,5)碳納米管空腔內(nèi)的最優(yōu)結(jié)構(gòu)接近于重疊式構(gòu)型[31],表明碳納米材料的幾何形狀對NM的最優(yōu)結(jié)構(gòu)有著直接影響。與孤立交錯式結(jié)構(gòu)相比,NM在GRA表面的主要結(jié)構(gòu)變化為H—C—N—O二面角從29.5°(見圖1)增加到30.9°(見圖2)。另外,NM距GRA表面約3?,處于合理的非鍵作用區(qū)域。

圖2NM@GRA在ONIOM (B3LYP/6-31+G**:UFF)理論水平下的優(yōu)化結(jié)構(gòu)

Fig.2The optimized geometries of NM@GRA at ONIOM (B3LYP/6-31+G**:UFF) level

3.2 NM-MN重排反應(yīng)

對于NM和NM@GRA,分別在B3LYP/6-31+G**和ONIOM (B3LYP/6-31+G**: UFF)理論水平下計算了NM-MN重排反應(yīng)中的過渡態(tài),記為TS1和TS1@GRA。圖3為NM與NM@GRA在NM-MN重排反應(yīng)中過渡態(tài)及反應(yīng)產(chǎn)物的優(yōu)化結(jié)構(gòu)。表1為NM與NM@GRA體系相關(guān)結(jié)構(gòu)的總能量、相對能量及振動頻率。由圖3可以看出,TS1結(jié)構(gòu)中C—N鍵長為1.977?, C—O鍵長為2.029?,與文獻(xiàn)[32]的理論計算結(jié)果C—N鍵長1.9?和C—O鍵長2.0?相似。相比之下,GRA對TS1結(jié)構(gòu)的影響主要體現(xiàn)為: C…O距離由原來的2.029?增加到2.055?,同時虛頻減小了27.1 cm-1(見表1)。另外,MN在GRA表面構(gòu)型發(fā)生了翻轉(zhuǎn),—CH3基團(tuán)圍繞C—O鍵旋轉(zhuǎn)了約30°,即從重疊式轉(zhuǎn)為交錯式。究其原因,是由于GRA平面結(jié)構(gòu)的特點所致,MN為交錯式時,能夠最大程度地與GRA作用。

圖3NM與NM@GRA在NM-MN重排反應(yīng)中過渡態(tài)及反應(yīng)產(chǎn)物的優(yōu)化結(jié)構(gòu)

Fig.3Optimized structures of the transition states and products for the NM-MN rearrangement reaction of NM and NM@GRA

表1NM與NM@GRA體系的總能量、相對能量及振動頻率

Table1Total energies, relative energies and vibrational frequencies of NM and NM@GRA systems

compoundtotalenergy/a.u.ZPE/kJ·mol-1relativeenergy/kJ·mol-1imaginaryfrequency/cm-1NM-245.02873131.0 0.01)MN-245.02285126.410.91)CH2NOOH-244.99423126.485.81)(·CH3+·NO2)- -254.81)TS1-244.92111118.8270.71)-820.34TS2-244.92446116.3259.01)-2111.78NM@GRA-243.893644926.2 0.02)MN@GRA-243.886634921.613.82)CH2NOOH@GRA-243.873954923.749.42)(·CH3+·NO2)@GRA- -260.22)TS1@GRA-243.791024914.5257.32)-793.24TS2@GRA-243.787794911.2262.82)-2136.62

Note: 1) Relative to the NM calculated at the B3LYP/6-31+G**level of theory, 2) Relative to the NM@GRA calculated at the ONIOM(B3LYP/6-31+G**:UFF) level of theory.

由表1可以看出,TS1相對于NM的能量為270.7 kJ·mol-1,這與B3LYP/6-311++G**理論水平計算的271.5 kJ·mol-1 [31]及G2MP2理論水平計算的270.3 kJ·mol-1 [33]十分接近,說明B3LYP/6-31+G**方法是較為可靠的。TS1@GRA相對于NM@GRA的能量為257.3 kJ·mol-1,比TS1降低了約13.4 kJ·mol-1,說明NM-MN重排反應(yīng)在GRA表面更容易發(fā)生。另一方面,MN比NM能量高10.9 kJ·mol-1,MN@GRA比NM@GRA能量高13.8 kJ·mol-1,NM-MN重排的正、逆反應(yīng)活化能相差不大,表明NM-MN重排反應(yīng)也能夠可逆地發(fā)生。

3.3 氫遷移重排反應(yīng)

氫遷移重排反應(yīng)是NM-MN重排反應(yīng)的一個競爭反應(yīng),由NM分子內(nèi)C原子上的一個H原子遷移到O原子上,從而生成CH2NOOH。采用與NM-MN重排反應(yīng)相同的理論水平,計算NM及NM@GRA氫遷移重排反應(yīng)的過渡態(tài),記為TS2和TS2@GRA。如圖4所示,TS2為四元環(huán)過渡態(tài),其中,C…H距離為1.478?,O…H距離為1.245?。當(dāng)NM在GRA表面發(fā)生氫遷移重排反應(yīng)時,TS2@GRA 的結(jié)構(gòu)與TS2相比無明顯變化。相反,GRA對產(chǎn)物CH2NOOH的結(jié)構(gòu)有著較大影響,GRA的平面結(jié)構(gòu)誘導(dǎo)CH2NOOH分子也呈平面結(jié)構(gòu),從而更有利于非鍵相互作用。在孤立CH2NOOH分子內(nèi),與O原子相鄰的H原子并不處于其他原子所形成的平面內(nèi),H—O—N—C二面角為38.4°,而在CH2NOOH@GRA結(jié)構(gòu)中,該二面角為178.4°。此外,在GRA作用下,CH2NOOH分子內(nèi)N—O鍵長增加了0.022?,N—OH鍵長縮短了0.023?。

圖4NM與NM@GRA在氫遷移重排反應(yīng)中過渡態(tài)及反應(yīng)產(chǎn)物的優(yōu)化結(jié)構(gòu)

Fig.4Optimized structures of the transition states and products for the H-migration rearrangement reaction of NM and NM@GRA

TS2相對于NM的能量為259.0 kJ·mol-1(見表1),分別比B3LYP/6-311++G**及G2MP2理論水平計算的結(jié)果少0.8 kJ·mol-1 [31]和8.8 kJ·mol-1 [33]。TS2@GRA相對于NM@GRA的能量為262.8 kJ·mol-1,與TS2相差不到4 kJ·mol-1,說明NM在GRA表面發(fā)生氫遷移重排反應(yīng)時反應(yīng)能壘變化不大。但是,反應(yīng)產(chǎn)物CH2NOOH在GRA表面上更加穩(wěn)定,其相對能量比孤立狀態(tài)時下降了36.4 kJ·mol-1,這源于CH2NOOH分子在GRA誘導(dǎo)下呈平面結(jié)構(gòu)所致。

3.4 C—N鍵均裂反應(yīng)

C—N鍵均裂反應(yīng)被認(rèn)為是NM熱解反應(yīng)的主要過程,該反應(yīng)生成一個甲基與一個硝基的自由基。分別在UB3LYP/6-31+G**和ONIOM (UB3LYP/6-31+G**:UFF)理論水平下掃描了NM與NM@GRA隨C—N鍵變化的勢能曲線。為了更好地研究GRA對C—N鍵均裂反應(yīng)的影響,在掃描過程中,同時對NM結(jié)構(gòu)進(jìn)行優(yōu)化。如圖5所示,隨著C—N鍵長的增加,NM與NM@GRA兩個體系的能量均增加,并且兩個體系在能量上只有微小差距。另外,在C—N鍵斷裂過程中,沒有發(fā)現(xiàn)過渡態(tài)的存在,這與NM在(5,5)碳納米管內(nèi)發(fā)生C—N鍵均裂反應(yīng)時不同[31]: NM在(5,5)碳納米管內(nèi)時,由于管內(nèi)空腔限制,C—N鍵均裂存在反應(yīng)過渡態(tài),且反應(yīng)能壘明顯降低。由勢能曲線可以得出NM與NM@GRA發(fā)生C—N鍵均裂反應(yīng)的能壘分別為254.8 kJ·mol-1與260.2 kJ·mol-1,說明NM在GRA表面發(fā)生該反應(yīng)時反應(yīng)能壘略有增加。

圖5NM及NM@GRA發(fā)生C—N鍵均裂反應(yīng)的勢能曲線

Fig.5Potential energy curves of the C—N homolytic cleavage reaction forNM and NM@GRA

3.5 反應(yīng)勢能面

圖6為NM與NM@GRA兩個體系初始反應(yīng)的勢能面。對于NM,三個初始反應(yīng)的活化能順序為C—N鍵均裂反應(yīng)<氫遷移重排反應(yīng)

圖6NM(實線)與NM@GRA(虛線)初始反應(yīng)的勢能面曲線(單位為kJ·mol-1)

Fig.6Potential energy surface of the initial reaction pathways for NM(solid line) and NM@GRA(dotted line)(relative energies are given in kJ·mol-1)

4 結(jié) 論

采用組合量子化學(xué)ONIOM方法研究了NM在GRA表面的初始反應(yīng),包括NM-MN重排反應(yīng)、氫遷移重排反應(yīng)及C—N鍵均裂反應(yīng)。計算結(jié)果表明,GRA對反應(yīng)過渡態(tài)及產(chǎn)物的結(jié)構(gòu)、能量均有不同程度的影響。GRA表面使NM三種初始反應(yīng)的活化能依次降低了13.4 kJ·mol-1、增加了3.8 kJ·mol-1及增加了5.4 kJ·mol-1,活化能的順序由C—N鍵均裂反應(yīng)<氫遷移重排反應(yīng)

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