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硝基类炸药作为主要的含能材料在极端条件下引爆可释放巨大能量, 深入研究硝基类炸药光激发后的载流子动力学过程, 将有助于揭示含能材料复杂的超快爆轰物理机制. 本文利用含时密度泛函理论和分子动力学计算方法, 研究了典型的硝基类炸药, 包括硝基甲烷(NM)、环三亚甲基三硝胺(RDX)和三氨基三硝基苯(TATB)分子的光激发解离过程, 观察了含能分子瞬时的结构变化和分子能级随时间的演化过程. 结果显示, 含能分子在200 fs范围内发生解离, 激发载流子的能量通过电声耦合以热能的形式传输给原子, 从而导致C—H, N—H或N—N化学键的断裂, 从甲基、亚甲基或氨基中解离出的氢原子会与近邻的硝基形成新的基团. 在这一过程中, 激发电子和空穴附近的能级也随着时间演化发生明显变化, 表明电子耦合也在解离过程中发挥了作用. 我们的研究加深了含能材料激发态结构弛豫和激发能级演化的认识, 并对光诱导反应及含能材料激光点火初始步骤提供了新的理解.Nitro explosive is a main type of energetic material which can release a large amount of energy when detonated under extreme conditions. Further study of the excited state dynamics of photo-induced nitro explosive can provide an effective method to understand the complex process of ultrafast detonation physics. In this paper, the initial step of photodissociation at the first excited electron state of some typical nitro explosives including nitromethane (NM), cyclotrimethylenetrinitramine (RDX) and triaminotrinitrobenzene (TATB) is studied using the time-dependent density functional theory and the molecular dynamic method. The transient structures of energetic molecules and time evolutions of excited energy levels are observed. It is found that the structural relaxation of energetic molecules occurs immediately after the electronic excitation, and the entire photoexcitation process comes into being within a range of 200 fs. At the same time, the positions of molecular energy levels change to various degrees with the oscillations of different frequencies, such as the overlap between HOMO and LUMO, which is related to the obvious change of molecular configuration, indicating that the energy of excited carriers transfers to atoms in the form of heat through electron-phonon coupling, and the energy is redistributed through vibration relaxation in the initial stage of photodissociation which causes the chemical bonds of C—H, N—N and N—N to rupture, and the hydrogen atoms dissociated from methyl, methylene or amino groups, and the nearest nitro group to form some new intermediate states. In this process, the energy levels near the excited electron and hole energy also change significantly with time, suggesting that the coupling between electron and electron also plays a role in the dissociation process. Comparing with NM and RDX, the evolution of the excited energy level of TATB has obvious lower-frequency (phonon frequency) oscillations, showing that the coupling between electronic state and phonon of TATB is weak and thus makes it more difficult to dissociate. Our study can deepen the understanding of the structural relaxation of excited states and the time evolution of excitation energy levels in energetic materials, and provide a new understanding of the photoinduced reaction and the initial steps of laser ignition in energetic materials.
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Keywords:
- nitro explosives /
- time-dependent density functional /
- excited carriers /
- photo dissocation
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图 1 基态含能分子结构示意图 (a) 硝基甲烷(NM); (b) 环三亚甲基三硝胺(RDX); (c) 三氨基三硝基苯(TATB); 其中蓝色为N原子, 红色为O原子, 棕色为C原子, 白色为H原子
Fig. 1. Structure diagrams of (a) nitromethane (NM); (b) cyclotrimethylenetrinitramine (RDX); (c) triaminotrinitrobenzene (TATB) at ground state. Blue ball is N atom, red ball is O atom, brown ball is C atom, and white ball is H atom.
图 2 三种含能分子的基态轨道能级排布及最高占据态轨道(HOMO)与最低非占据态轨道(LUMO)的电荷密度分布图 (a) 硝基甲烷(NM); (b) 环三亚甲基三硝胺(RDX); (c) 三氨基三硝基苯(TATB)
Fig. 2. The ground state molecular orbital (MO) energy levels and charge density of the highest occupied state orbitals and the lowest unoccupied state orbitals of (a) nitromethane (NM), (b) cyclotrimethylenetrinitramine (RDX), (c) triaminotrinitrobenzene (TATB).
图 3 (a) 硝基甲烷(NM); (b) 环三亚甲基三硝胺(RDX); (c) 三氨基三硝基苯(TATB)在300 K时光致激发后分子结构随时间演化示意图
Fig. 3. Time evolution of molecular structure at 300 K for (a) nitromethane (NM); (b) cyclotrimethylenetrinitramine (RDX); (c) triaminotrinitrobenzene (TATB).
图 4 分子部分键长随时间的演化 (a) 硝基甲烷(NM); (b) 环三亚甲基三硝胺(RDX); (c) 三氨基三硝基苯(TATB)
Fig. 4. Time evolution of bond lengths: (a) Nitromethane (NM); (b) cyclotrimethylenetrinitramine (RDX); (c) triaminotrinitrobenzene (TATB).
图 5 分子轨道能级随时间的演化图 (a) 硝基甲烷(NM)分子HOMO, LUMO能级演化; (b), (c) 环三亚甲基三硝胺(RDX)分子HOMO, LUMO及其附近能级演化; (d) 三氨基三硝基苯(TATB)分子HOMO, LUMO能级演化. 绿色实线为最高占据态轨道, 对应激发空穴所在能级, 红色实线为最低非占据态轨道, 对应激发电子所在能级, 灰色实线为附近的其他能级
Fig. 5. Time evolution of excited energy level: (a) HOMO and LUMO of Nitromethane (NM); (b), (c) HOMO, LUMO and nearby orbitals of cyclotrimethylenetrinitramine (RDX); (d) HOMO and LUMO of Triaminotrinitrobenzene (TATB). Green solid line denotes the highest occupied molecular orbit corresponding to the excited hole, red solid line denotes the lowest unoccupied molecular orbit corresponding to the excited electron, gray solid lines denote other molecular orbit.
表 1 三种含能分子基态的键长信息
Table 1. Bond lengths of energetic molecules at ground state
NM C—N C—H1 C—H2 C—H3 N—O1 N—O2 Length/Å This work 1.547 1.096 1.099 1.099 1.148 1.157 Exp.[40] 1.481 1.093 1.092 1.092 1.223 1.224 RDX C—N N1—N2 N3—N4 N5—N6 C1—H1 C1—H2 Length/Å This work 1.443 1.443 1.471 1.472 1.182 1.183 Exp.[41] 1.464 1.351 1.352 1.398 1.085 1.087 TATB C—C C—Nnitro C—Namino N—O N—H O···H Length/Å This work 1.425 1.436 1.318 1.216 1.021 1.646 Exp.[42] 1.441 1.442 1.316 1.243 0.925 1.780 
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表 2 300 K与0 K下RDX和TATB的键角对比
Table 2. Bond angles of RDX and TATB under 300 K and compared with 0 K
RDX Angle/(°) TATB Angle/(°) 0 K 300 K Change 0 K 300 K Change α1 113.7 112.1 –1.6 θ 118.6 119.2 +0.6 α2 114.8 109.8 –5.0 γ1 179.9 173.3 –6.6 δ 140.6 135.1 –5.5 γ2 179.1 174.4 –4.7
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[1] Field J E 1992 Acc. Chem. Res. 25 489
Google Scholar
[2] Zhang S Q, Wang Y Q, Zheng X M 2006 Acta Phys-Chim Sin. 22 1489
Google Scholar
[3] Bhattacharya A, Guo Y, Bernstein E R 2010 Acc. Chem. Res. 43 1476
Google Scholar
[4] Fang X, McLuckie W G 2015 J. Hazard. Mater. 285 375
Google Scholar
[5] Gruzdkov Y A, Gupta Y M 1998 J. Phys. Chem. A 102 8325
Google Scholar
[6] Aduev B P, Nurmukhametov D R, Belokurov G M, Nelyubina N V, Kalenskii A V, Aluker N L 2017 Russ. J. Phys. Chem. B 11 460
Google Scholar
[7] Jordan M J T, Kable S H 2012 Science 335 1054
Google Scholar
[8] Spighi G, Gaveau MA, Mestdagh JM, Poisson L, Soep B 2014 Physi. Chem. Chem. Phys. 16 9610
Google Scholar
[9] Parada G A, Markle T F, Glover S D, Hammarstrom L, Ott S, Zietz B 2015 Chem. Eur. J 21 6362
Google Scholar
[10] Zhang W, Sang J, Cheng J, Ge S, Yuan S, Lo G V, Dou Y 2018 Molecules 23 1593
Google Scholar
[11] Rehwoldt M C, Wang H, Kline D J, Wu T, Eckman N, Wang P, Agrawal N R, Zachariah M R 2020 Combust. Flame 211 260
Google Scholar
[12] Cabalo J, Sausa R 2005 Appl. Optics 44 1084
Google Scholar
[13] Mattos E C, Diniz M F, Nakamura N M, Dutra R d C L 2009 J. Aerosp. Technol. Manag. 1 167
Google Scholar
[14] Rom N, Zybin S V, van Duin A C T, Goddard W A, III, Zeiri Y, Katz G, Kosloff R 2011 J. Phys. Chem. A 115 10181
Google Scholar
[15] Blais N C, Engelke R, Sheffield S A 1997 J. Phys. Chem. A 101 8285
Google Scholar
[16] Citroni M, Bini R, Pagliai M, Cardini G, Schettino V 2010 J. Phys. Chem. B 114 9420
Google Scholar
[17] Kuklja M M, Aduev B P, Aluker E D, Krasheninin V I, Krechetov A G, Mitrofanov A Y 2001 J. Appl. Phys. 89 4156
Google Scholar
[18] Guo Y Q, Greenfield M, Bhattacharya A, Bernstein E R 2007 J. Chem. Phys. 127 154301
Google Scholar
[19] Owens F J, Sharma J 1980 J. Appl. Phys. 51 1494
Google Scholar
[20] Gares K L, Bykov S V, Brinzer T, Asher S A 2015 Appl. Spectrosc. 69 545
Google Scholar
[21] Tang T B, Chaudhri M M, Rees C S, Mullock S J 1987 J. Mater. Sci. 22 1037
Google Scholar
[22] Williams D L, Timmons J C, Woodyard J D, Rainwater K A, Lightfoot J M, Richardson B R, Burgess C E, Heh J L 2003 J. Phys. Chem. A 107 9491
Google Scholar
[23] Firsich D W 1984 J. Hazard. Mater. 9 133
Google Scholar
[24] Britt A D, Moniz W B, Chingas G C, Moore D W, Heller C A, Ko C L 1981 Propell. Explos. 6 94
Google Scholar
[25] Glascoe E A, Zaug J M, Armstrong M R, Crowhurst J C, Grant C D, Fried L E 2009 J. Phys. Chem. A 113 5881
Google Scholar
[26] Runge E, Gross E K U 1984 Phys. Rev. Lett. 52 997
Google Scholar
[27] Theilhaber J 1992 Phys. Rev. B 46 12990
Google Scholar
[28] Castro A, Appel H, Oliveira M, Rozzi C A, Andrade X, Lorenzen F, Marques M A L, Gross E K U, Rubio A 2006 Phys. Status Solidi. B 243 2465
Google Scholar
[29] Friend R H, Gymer R W, Holmes A B, Burroughes J H, Marks R N, Taliani C, Bradley D D C, Dos Santos D A, Bredas J L, Logdlund M, Salaneck W R 1999 Nature 397 121
Google Scholar
[30] Polyak I, Hutton L, Crespo-Otero R, Barbatt M, Knowles P J 2019 J. Chem. theory Comput. 15 3929
Google Scholar
[31] Kolesov G, Granas O, Hoyt R, Vinichenko D, Kaxiras E 2016 J. Chem. theory Comput. 12 466
Google Scholar
[32] Nelson T, Fernandez-Alberti S, Chernyak V, Roitberg A E, Tretiak S 2011 J. Phys. Chem. B 115 5402
Google Scholar
[33] Ghosh J, Gajapathy H, Konar A, Narasimhaiah G M, Bhattacharya A 2017 J. Chem. Phys. 147 204302
Google Scholar
[34] Myers T W, Bjorgaard J A, Brown K E, Chavez D E, Hanson S K, Scharff R J, Tretiak S, Veauthier J M 2016 J. Am. Chem. Soc. 138 4685
Google Scholar
[35] Soler J M, Artacho E, Gale J D, Garcia A, Junquera J, Ordejon P, Sanchez-Portal D 2002 J. Phys-Condens. Mat. 14 2745
Google Scholar
[36] Sugino O, Miyamoto Y 1999 Phys. Rev. B 59 2579
Google Scholar
[37] Perdew J P, Burke K, Ernzerhof M 1996 Phys. Rev. Lett. 77 3865
Google Scholar
[38] Grimme S 2006 J. Comput. Chem. 27 1787
Google Scholar
[39] Troullier N, Martins J L 1991 Phys. Rev. B 43 8861
Google Scholar
[40] Gorse D, Cavagnat D, Pesquer M, Lapouge C 1993 J. Phys. Chem. 97 4262
Google Scholar
[41] Choi C S, Mapes J E, Prince E 1972 Acta Crystallogr. B 28 1357
Google Scholar
[42] Cady H H, Larson A C 1965 Acta Crystallogr. 18 485
Google Scholar
[43] Zhong M, Liu Q-J, Qin H, Jiao Z, Zhao F, Shang H-L, Liu F-S, Liu Z-T 2017 Eur. Phys. J. B 90 115
Google Scholar
[44] Fan J, Su Y, Zheng Z, Zhang Q, Zhao J 2019 J. Raman Spectrosc. 50 889
Google Scholar
[45] Su Y, Fan J, Zheng Z, Zhao J, Song H 2018 Chin. Phys. B 27 056401
Google Scholar
[46] Flicker W M, Mosher O A, Kuppermann A 1979 Chem. Phys. Lett. 60 518
Google Scholar
[47] Whitley V H 2006 AIP Conf. Proc. 845 1357
Google Scholar
[48] Kakar S, Nelson A J, Treusch R, Heske C, van Buuren T, Jimenez I, Pagoria P, Terminello L J 2000 Phys. Rev. B 62 15666
Google Scholar
[49] Zhang W, Shen R, Ye Y, Wu L, Hu Y, Zhu P 2014 Spectrosc. Lett. 47 611
Google Scholar
[50] Nelson T, Bjorgaard J, Greenfield M, Bolme C, Brown K, McGrane S, Scharff R J, Tretiak S 2016 J. Phys. Chem. A 120 519
Google Scholar
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