Unusual reversibility in molecular break-up of PAHs: the case of pentacene dehydrogenation on Ir(111)

In this work, we characterize the adsorption of pentacene molecules on Ir(111) and their behaviour as a function of temperature. While room temperature adsorption preserves the molecular structure of the five benzene rings and the bonds between carbon and hydrogen atoms, we find that complete C–H molecular break up takes place between 450 K and 550 K, eventually resulting in the formation of small graphene islands at temperatures larger than 800 K. Most importantly a reversible temperature-induced dehydrogenation process is found when the system is annealed/cooled in a hydrogen atmosphere with a pressure higher than 5 × 10⁻⁷ mbar. This novel process could have interesting implications for the synthesis of larger acenes and for the manipulation of graphene nanoribbon properties.

In this work, we characterise the adsorption of pentacene molecules on Ir(111) and their dissociation behaviour as a function of temperature.     

Studies on the properties of EPDM–CSE blend containing HTPB for case‐bonded solid rocket motor insulation

Elastomeric blends based on ethylene propylene diene (EPDM) rubber as a primary polymer have been investigated for the thermal insulation of case‐bonded solid rocket motors (SRMs) cast with composite propellant containing hydroxy terminated polybutadiene (HTPB) as a polymeric binder. EPDM rubber found as an attractive candidate for the thermal insulation of case‐bonded SRM due to the advantages such as low specific gravity, improved ageing properties, and longer shelf life. In spite of these advantages, EPDM, a non‐polar rubber, lacks sufficient bonding with the propellant matrix. Bonding properties are found to improve when EPDM is blended with other polar rubbers like polychloroprene, chlorosulphonated polyethylene (CSE), etc. This type of polar polymer when blended with EPDM rubber enhances the insulator‐to‐propellant interface bonding. In the present work, an attempt has been made to study the properties of EPDM–CSE based insulator by incorporating HTPB, a polar polymer as well as a polymeric binder, as an additive to the EPDM–CSE blend by varying the HTPB concentration. Blends prepared were cured and characterized for rheological, mechanical, interface, and thermal properties to study the effect of HTPB addition. This paper reports the preliminary investigation of the properties of EPDM–CSE blend containing HTPB, as a novel and futuristic elastomeric insulation for case‐bonded SRM containing HTPB as propellant binder. Copyright © 2007 John Wiley & Sons, Ltd.     

端羟基聚丁二烯/增塑剂共混物相容性的分子动力学模拟

固体推进剂和炸药的力学性能在很大程度上依赖于配方中高分子粘结剂与增塑剂的相容性. 本文对相容和非相容两种体系进行了分子动力学(MD)模拟, 以考察分子模拟方法的实用性. 为预测固体推进剂中端羟基聚丁二烯(HTPB)与增塑剂癸二酸二辛酯(DOS)、硝化甘油(NG)的相容性, 采用MD模拟方法在COMPASS力场下, 对HTPB、DOS、NG和共混物HTPB/DOS、HTPB/NG的密度、内聚能密度及溶度参数等进行了模拟计算. 通过比较溶度参数差值(△δ)的大小、分子间径向分布函数和模拟前后体系密度变化情况均可以预测HTPB/DOS属于相容体系,而HTPB/NG属于不相容体系, 与实验结果一致. 径向分布函数分析同时揭示了HTPB/增塑剂组分之间的相互作用及本质. 本文的模拟方法可以作为预测聚合物与增塑剂相容性的有利工具, 也可以为固体推进剂和炸药的配方设计提供理论指导.     

理论研究丁羟粘合剂化学键解离及其对力学性能的影响

端羟基聚丁烯(HTPB)是推进剂中的常用的粘合剂,老化是其贮存和使用中的重要问题。通过量子化学计算HTPB 与甲基二异腈酸酯(TDI)形成的网络模型简化结构中化学键的均裂解离能(BDE),分析了键能与老化分解的关系。键能计算结果证明可靠且可用于比较分析。与CH₂ 基团相连的C-O 键的BDE值最小,推测该键最弱并且在热老化过程会发生断裂分解,降解产物主要是CO₂。HTPB 中的烯丙基伯羟基与TDI 形成的聚氨酯中α-C-H 属于最弱的X-H(X=C, N)键,推测其容易受到自由基的进攻发生氢转移反应。对容易断裂分解的C-O 键,提出了可能的老化机理。计算的C-O 键断裂活化能与其解离能近似相等,热老化过程中断键生成自由基并通过无势垒过程释放出CO₂。整个过程的热老化半衰期是温度的指数衰减函数,表明随着温度的提高HTPB-TDI 聚氨酯老化加速。热力学计算证明老化形成的氨基自由基和烷基自由基能够重新结合。采用分子动力学动态分析方法,分析了HTPB-TDI 聚氨酯网络老化前后的结构变化及其对弹性力性质的影响,发现释放CO₂ 的过程伴随着体系密度降低。对假定的CO₂ 扩散聚集以及CO₂ 从体系中扩散消失的模型进行了模拟,发现拉伸模量和剪切模量在这两种情形下会增加。     

Ab initio molecular dynamics simulations study on initial decompositions of β‐HMX at high temperature coupled with high pressures

We performed ab initio molecular dynamics simulations to investigate initial decomposition mechanisms and subsequent chemical processes of β‐HMX (cyclotetramethylene tetranitramine) (octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine) crystals at high temperature coupled with high pressures. It was found that the initial decomposition step is the simultaneous C–H and N–NO₂ bond cleavage at 3,500 K. When the pressure (1–10 GPa) is applied, the first reaction steps are primarily the C–N and C–H bond fission at 3,500 K. The C–H bond cleavage is a triggering decomposition step of the HMX crystals at 3,500 K coupled with 16 GPa. This indicates that the C–H bonds are much easier to be broken and the hydrogen radicals are much more active. The applied pressures (1–10 GPa) accelerate the decompositions of HMX at 3,500 K. The decomposition pathways and time evolution of the main chemical species demonstrate that the temperature is the foremost factor that affects the decomposition at high pressures (1–10 GPa). However, the decomposition of HMX is dependent on both the temperature (3,500 K) and the pressure (16 GPa). This work will enrich the knowledge of the decompositions of condensed energetic materials under extreme conditions.     

Towards the understanding of the spectroscopic behaviour of the C–H oscillator in C–HO hydrogen bonds: the effect of solvent polarity

The effect of solvent polarity versus specific C–HO contacts on the vibrational νC–H mode is studied using CHCl₃ as a model system. Ab initio SCI–PCM calculations show that the overall shift of the νC–H band, sometimes ascribed to the C–HO hydrogen bonding, can in fact be explained by the electrostatic interaction with a dielectric environment. The presence of a new νC–H band – assigned to the C–HO bonded forms – remains as the most reliable evidence of C–HO hydrogen bonding.     

Pot life extension of hydroxyl terminated polybutadiene based solid propellant binder system by tailoring the binder polymer microstructure

A possibility of extending the pot life of the HTPB-TDI based propellant binder system without adverse modification of mechanical properties is explored in the present study. It is proposed that by tailoring functionality distribution of the base HTPB polymer and changing the binder composition concurrently, the pot life of the binder system can be extended while the mechanical characteristics are kept within the acceptability window. Using an existing empirical relationship between fraction of high molecular weight (Fh) in HTPB and r-value corresponding to a more optimized set of mechanical properties, the r-values were calculated for different HTPB resins. HTPB resins with widely varying fractions of high molecular weight (Fh) were chosen and binder networks were prepared at different r values. Viscosity build up and chemical kinetics were studied for different formulations. From the studies, it is shown that the extension of pot-life is achievable by about 150 minutes without sacrificing the mechanical characteristics.     

定常冲击波作用下六硝基六氮杂异伍兹烷(CL-20)/奥克托今(HMX)含能共晶初始分解机理研究

采用ReaxFF分子动力学方法同时结合多尺度冲击技术(MSST)模拟了4–10 km×s~(-1)定常冲击波加载下含能共晶CL-20/HMX沿不同晶格矢量的初始物理化学响应。获得了系统温度、压力、密度以及粒子速度的时间演化路径,以及初始分解路径,最终稳定反应产物和冲击雨贡纽等。研究结果表明:冲击波入射至含能共晶后,物理上依次经历诱导期、快压缩、慢压缩以及膨胀过程。快压缩和慢压缩过程分别对应反应物的快分解和慢分解。采用指数函数对反应物的衰减曲线进行拟合,并比较了共晶中反应物的衰减速率。整体上,随着冲击波速度的增加,反应物响应的时间逐渐提前,并且,冲击波沿各晶格矢量入射后,共晶中CL-20分子分解的响应时间均早于HMX。CL-20快分解阶段的衰减速率最高,HMX快分解的衰减速率居其次。相对于快分解阶段,慢分解阶段各反应物的衰减速率差异较小。含能共晶的初始反应路径是CL-20聚合形成二聚体,而冲击诱导共晶分解的初始反应路径是CL-20中N-NO_2键断裂形成NO_2。随后产生N_2O,NO,HONO,OH,H等中间小分子。最终稳定产物是N_2,H_2O,CO_2,CO和H_2。晶格矢量b,c方向冲击感度相同,低于晶格矢量a方向的感度。冲击诱导共晶中CL-20和HMX分解的最小冲击波速度(us)分别为6 km×s~(-1)和7 km×s~(-1)。采用冲击雨贡纽关系计算得到沿晶格矢量a,b,c冲击诱导CL-20/HMX共晶起爆的压力分别为16.52 GPa,17.41 GPa和17.41 GPa。爆轰压力范围介于36.75 GPa–47.43 GPa。     

Initial chemical events in shocked octahydro-1,3,5,7-tetranitro-1,3,5,7- tetrazocine: a new initiation decomposition mechanism

We have performed ab initio molecular dynamics simulations in conjunction with the multiscale shock technique to study the initial chemical processes of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) under shock wave loading. The results show that the initial decomposition of shocked HMX is triggered by the N-O bond breaking and the ring opening. This will initiate many decomposition reactions and lead to the production of many small radicals at a moment. As the shock compression continues, these small radicals recombine to produce many large radicals and further form ring-shaped radicals. Then, these radicals begin to further decompose. It is also found that the system transiently produces a large number of metallic states under the shock compression. Our simulations thus suggest a new mechanism for the initial chemical processes of shocked HMX and provide fundamental insight into the initial mechanism at the atomistic level, which is of important implication for understanding and development of energetic materials.     

HMX和HMX/HTPB PBX的晶体缺陷理论研究

建立空位和掺杂点缺陷模型, 用分子动力学(MD)方法, 研究晶体缺陷对β-环四亚甲基硝胺(HMX)和β-HMX/HTPB(端羟基聚丁二烯)高聚物粘结炸药(PBX)的力学性能和爆炸性能的影响. 结果表明, 相对于HMX“完美”晶体(1)考察缺陷晶体(2和3), 以及相对于HMX完美晶体基PBX(1)考察缺陷PBX 2和PBX 3, 均发现弹性系数和(拉伸、体积、剪切)模量下降, 导致体系刚性减弱, 延展性和韧性增强. 这与在基炸药HMX晶体(1, 2和3)中分别加入HTPB高聚物粘结剂形成PBX 1, PBX 2和PBX 3呈现类似的相应的变化趋势和效果. 此外, 研究表明, 爆炸性质也依赖于体系的组成和结构. 因加入的是低能高聚物, 故PBX(1), PBX(2)和PBX(3)的爆热、爆速和爆压均比相应的基炸药(1, 2和3)低, 即晶体(1)＞PBX(1), 晶体(2)＞PBX(2), 晶体(3)＞PBX(3). PBX(1), PBX(2), PBX(3)与对应基炸药(1, 2, 3)的爆速和爆压取相同变化次序, 亦即PBX(1)＞PBX(2)＞PBX(3)对应于晶体(1)＞晶体(2)＞晶体(3). 这些计算结果和规律对PBX配方设计显然具有指导作用.     

EDOT-based conjugated polymers accessed via C–H direct arylation for efficient photocatalytic hydrogen production

3,4-Ethylene dioxythiophene (EDOT), as a monomer of commercial conductive poly(3,4-ethylene dioxythiophene) (PEDOT), has been facilely incorporated into a series of new π-conjugated polymer-based photocatalysts, i.e., BSO₂–EDOT, DBT–EDOT, Py–EDOT and DFB–EDOT, through atom-economic C–H direct arylation polymerization (DArP). The photocatalytic hydrogen production (PHP) test shows that donor–acceptor (D–A)-type BSO₂–EDOT renders the highest hydrogen evolution rate (HER) among the linear conjugated polymers (CPs) ever reported. A HER up to 0.95 mmol h⁻¹/6 mg under visible light irradiation and an unprecedented apparent quantum yield of 13.6% at 550 nm are successfully achieved. Note that the photocatalytic activities of the C–H/C–Br coupling-derived EDOT-based CPs are superior to those of their counterparts derived from the classical C–Sn/C–Br Stille coupling, demonstrating that EDOT is a promising electron-rich building block which can be facilely integrated into CP-based photocatalysts. Systematic studies reveal that the enhanced water wettability by the integration of polar BSO₂ with hydrophilic EDOT, the increased electron-donating ability by O–C p–π conjugation, the improved electron transfer by D–A architecture, broad light harvesting, and the nano-sized colloidal character in a H₂O/NMP mixed solvent rendered BSO₂–EDOT as one of the best CP photocatalysts toward PHP.

The excellent reactivity toward C–H direct arylation, water wettability and O–C p–π conjugation endow EDOT to be an attractive electron donor unit for CP photocatalysts, yielding an unprecedented hydrogen evolution rate up to 0.95 mmol h⁻¹/6 mg catalyst.     

HTPB/增塑剂玻璃化转变温度及力学性能的分子动力学模拟

为了预测高分子粘结剂端羟基聚丁二烯(HTPB)与增塑剂癸二酸二辛酯(DOS)、硝化甘油(NG)的相容性及HTPB/增塑剂共混物的玻璃化转变温度(Tg)和力学性能,在COMPASS力场条件下采用分子动力学(MD)模拟方法对相容体系(HTPB-DOS)和不相容体系(HTPB-NG)进行了研究.结果表明,通过比较溶度参数差值(Δδ)的大小可以预测HTPB与增塑剂的相容性,即HTPB与DOS属于相容体系,而HTPB与NG不相容.通过温度-比容曲线可以得到HTPB、HTPB/DOS与HTPB/NG的Tg分别为197.54,176.30和200.03K.力学性能分析结果表明,添加DOS增塑剂后使HTPB的弹性模量(E),体积模量(K)和剪切模量(G)下降,材料刚性减弱,柔性增强,力学性能得到改善.本模拟方法可以作为预测聚合物/增塑剂共混物性能的有利工具,也可以为固体推进剂和高聚物粘结炸药的配方设计提供理论指导.     

The formyloxyl radical: electrophilicity,C–H bond activation and anti-Markovnikov selectivity in the oxidation of aliphatic alkenes

In the past the formyloxyl radical, HC(O)O˙, had only been rarely experimentally observed, and those studies were theoretical-spectroscopic in the context of electronic structure. The absence of a convenient method for the preparation of the formyloxyl radical has precluded investigations into its reactivity towards organic substrates. Very recently, we discovered that HC(O)O˙ is formed in the anodic electrochemical oxidation of formic acid/lithium formate. Using a [Co^IIIW₁₂O₄₀]⁵⁻ polyanion catalyst, this led to the formation of phenyl formate from benzene. Here, we present our studies into the reactivity of electrochemically in situ generated HC(O)O˙ with organic substrates. Reactions with benzene and a selection of substituted derivatives showed that HC(O)O˙ is mildly electrophilic according to both experimentally and computationally derived Hammett linear free energy relationships. The reactions of HC(O)O˙ with terminal alkenes significantly favor anti-Markovnikov oxidations yielding the corresponding aldehyde as the major product as well as further oxidation products. Analysis of plausible reaction pathways using 1-hexene as a representative substrate favored the likelihood of hydrogen abstraction from the allylic C–H bond forming a hexallyl radical followed by strongly preferred further attack of a second HC(O)O˙ radical at the C1 position. Further oxidation products are surmised to be mostly a result of two consecutive addition reactions of HC(O)O˙ to the C C double bond. An outer-sphere electron transfer between the formyloxyl radical donor and the [Co^IIIW₁₂O₄₀]⁵⁻ polyanion acceptor forming a donor–acceptor [D⁺–A⁻] complex is proposed to induce the observed anti-Markovnikov selectivity. Finally, the overall reactivity of HC(O)O˙ towards hydrogen abstraction was evaluated using additional substrates. Alkanes were only slightly reactive, while the reactions of alkylarenes showed that aromatic substitution on the ring competes with C–H bond activation at the benzylic position. C–H bonds with bond dissociation energies (BDE) ≤ 85 kcal mol⁻¹ are easily attacked by HC(O)O˙ and reactivity appears to be significant for C–H bonds with a BDE of up to 90 kcal mol⁻¹. In summary, this research identifies the reactivity of HC(O)O˙ towards radical electrophilic substitution of arenes, anti-Markovnikov type oxidation of terminal alkenes, and indirectly defines the activity of HC(O)O˙ towards C–H bond activation.

The formyloxyl radical, formed electrochemically, is electrophilic, yields anti-Markovnikov oxidation products from alkenes, and is effective for C–H bond activation.     

Initial Decomposition Mechanism of 3-Nitro-1,2,4-triazol-5-one (NTO) under Shock Loading: ReaxFF Parameterization and Molecular Dynamic Study

We report a reactive molecular dynamic (ReaxFF-MD) study using the newly parameterized ReaxFF-lg reactive force field to explore the initial decomposition mechanism of 3-Nitro-1,2,4-triazol-5-one (NTO) under shock loading (shock velocity >6 km/s). The new ReaxFF-lg parameters were trained from massive quantum mechanics data and experimental values, especially including the bond dissociation curves, valence angle bending curves, dihedral angle torsion curves, and unimolecular decomposition paths of 3-Nitro-1,2,4-triazol-5-one (NTO), 1,3,5-Trinitro-1,3,5-triazine (RDX), and 1,1-Diamino-2,2-dinitroethylene (FOX-7). The simulation results were obtained by analyzing the ReaxFF dynamic trajectories, which predicted the most frequent chain reactions that occurred before NTO decomposition was the unimolecular NTO merged into clusters ((C₂H₂O₃N₄)_n). Then, the NTO dissociated from (C₂H₂O₃N₄)_n and started to decompose. In addition, the paths of NO₂ elimination and skeleton heterocycle cleavage were considered as the dominant initial decomposition mechanisms of NTO. A small amount of NTO dissociation was triggered by the intermolecular hydrogen transfer, instead of the intramolecular one. For α-NTO, the calculated equation of state was in excellent agreement with the experimental data. Moreover, the discontinuity slope of the shock-particle velocity equation was presented at a shock velocity of 4 km/s. However, the slope of the shock-particle velocity equation for β-NTO showed no discontinuity in the shock wave velocity range of 3–11 km/s. These studies showed that MD by using a suitable ReaxFF-lg parameter set, could provided detailed atomistic information to explain the shock-induced complex reaction mechanisms of energetic materials. With the ReaxFF-MD coupling MSST method and a cheap computational cost, one could also obtain the deformation behaviors and equation of states for energetic materials under conditions of extreme pressure.     

The Tail Wags the Dog: The Far Periphery of the Coordination Environment Manipulates the Photophysical Properties of Heteroleptic Cu(I) Complexes

In this work we show, using the example of a series of [Cu(Xantphos)(N^N)]⁺ complexes (N^N being substituted 5-phenyl-bipyridine) with different peripheral N^N ligands, that substituents distant from the main action zone can have a significant effect on the physicochemical properties of the system. By using the C≡C bond on the periphery of the coordination environment, three hybrid molecular systems with −Si(CH₃)₃, −Au(PR₃), and −C₂HN₃(CH₂)C₁₀H₇ fragments were produced. The Cu(I) complexes thus obtained demonstrate complicated emission behaviour, which was investigated by spectroscopic, electrochemical, and computational methods in order to understand the mechanism of energy transfer. It was found that the −Si(CH₃)₃ fragment connected to the peripheral C≡C bond changes luminescence to long-lived intra-ligand phosphorescence, in contrast to MLCT phosphorescence or TADF. The obtained results can be used for the design of new materials based on Cu(I) complexes with controlled optoelectronic properties on the molecular level, as well as for the production of hybrid systems.     

Molecular dynamics simulation studies of the ε-CL-20/HMX co-crystal-based PBXs with HTPB

Molecular dynamics simulations were carried out to explore a ε-CL-20/HMX (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexazaisowurtzitane/1,3,5,7-tetranitro-1,3,5,7- tetrazacyclooctane) co-crystal-based polymer-bonded explosive (PBX) with HTPB (hydroxyl-terminated polybutadiene). The binding energies, pair correlation functions, and mechanical properties of the PBXs were reported. From the calculated binding energy, it was found that the order of the binding energies per unit surface between the crystalline surface and HTPB is (0 1 0) > (1 0 0) > (0 0 1). The pair correlation function revealed that the H···O and H···N H-bonds exist on the interfaces between the crystalline surfaces and HTPB, and the number of H???O hydrogen bonds (H-bonds) atom pairs is ten times more than that of H???N H-bonds. Additionally, the calculated mechanical data indicated that the stiffness of the co-crystal/HTPB PBX is weaker and its ductility is better than those of the co-crystal.     

Sensing the ortho Positions in C6Cl6 and C6H4Cl2 from Cl2− Formation upon Molecular Reduction

The geometrical effect of chlorine atom positions in polyatomic molecules after capturing a low-energy electron is shown to be a prevalent mechanism yielding Cl₂⁻. In this work, we investigated hexachlorobenzene reduction in electron transfer experiments to determine the role of chlorine atom positions around the aromatic ring, and compared our results with those using ortho-, meta- and para-dichlorobenzene molecules. This was achieved by combining gas-phase experiments to determine the reaction threshold by means of mass spectrometry together with quantum chemical calculations. We also observed that Cl₂⁻ formation can only occur in 1,2-C₆H₄Cl₂, where the two closest C–Cl bonds are cleaved while the chlorine atoms are brought together within the ring framework due to excess energy dissipation. These results show that a strong coupling between electronic and C–Cl bending motion is responsible for a positional isomeric effect, where molecular recognition is a determining factor in chlorine anion formation.     

A theoretical study of the C–HN hydrogen bond in the methane–ammonia complex

The C–HN hydrogen bond in the methane–ammonia complex is studied by determining its bond dissociation energy (BDE) and the n(N)→σ^*(C–H) interaction. At the MP2(Full)/6-311++G(3df,2p) level of theory with basis set superposition error (BSSE) correction, the BDE was determined to be 2.5 kJ mol⁻¹. The n(N)→σ^*(C–H) interaction at this level of theory was found to be 3.7 kJ mol⁻¹ by natural bond orbital (NBO) analysis. It was also found that the NBO values are in general higher than the BDE values with BSSE correction when they are compared at the same level of theory.     

端羟基聚丁二烯/增塑剂共混物相容性的分子动力学模拟和介观模拟

应用分子动力学(MD)和介观动力学(MesoDyn)模拟方法对固体推进剂中端羟基聚丁二烯(HTPB)与增塑剂癸二酸二辛酯(DOS)、硝化甘油(NG)的相容性进行了研究. 采用MD模拟方法在COMPASS力场下, 对纯物质、HTPB/增塑剂共混物的密度、内聚能密度、溶度参数和共混物分子间的Flory-Huggins作用参数及结合能等进行了模拟计算, 通过比较溶度参数差值(Δδ)的大小、模拟前后体系密度变化情况均可以预测HTPB与增塑剂的相容性, 结合能的分析揭示了HTPB/增塑剂共混物组分间的相互作用及本质. 将Flory-Huggins作用参数转化为MesoDyn模拟的输入参数, 采用MesoDyn模拟方法对HTPB/增塑剂共混体系的介观形貌与动力学演变过程进行了研究, 通过模拟得到的等密度图、自由能密度和有序度参数等可以判断共混体系的相容性. MD和MesoDyn模拟结果均表明: HTPB/DOS属于相容体系, 而HTPB/NG属于不相容体系, 其结论与实验结果一致.