Periodic density functional theory study of the high‐pressure behavior of crystalline 7,2′‐anhydro‐β‐d‐arabinosylorotidine

In this work, a detailed study of the structural, electronic, and absorption properties of crystalline 7,2′‐anhydro‐β‐d ‐arabinosylorotidine (Cyclo ara‐O) in the pressure range of 0–350 GPa is performed by density functional theory calculations. The detail analysis of the crystal with increasing pressure shows that complex transformations occur in Cyclo ara‐O under compression. In addition, the b‐direction is much stiffer than the a‐ and c‐axis at 0–330 GPa, suggesting that the Cyclo ara‐O crystal is anisotropic in the certain pressure region. In the pressure range of 110–290 GPa, repeated formations and disconnections of covalent bonds in O7–O6* and C3–C6* occur several times, resulting in a new six‐atom ring that forms at 220, 270, and 290 GPa, while a five‐atom ring and seven‐atom ring form between two adjacent molecules at 300 and 340 GPa, respectively. Then, the analysis of the band gap and DOS (PDOS) of Cyclo ara‐O indicates that its electronic character has changed at 300 GPa into an excellent insulator, but the electron transition is much easier at 350 GPa. Moreover, the relatively high optical activity with the pressure increases of Cyclo ara‐O is seen from the absorption spectra, and two obvious structural transformations are also observed at 180 and 230 GPa, respectively. Copyright © 2016 John Wiley & Sons, Ltd.     

Theoretical studies on structure and performance of [1,2,5]‐oxadiazolo‐[3,4‐d]‐pyridazine‐based derivatives

Based on energetic compound [1,2,5]‐oxadiazolo‐[3,4‐d]‐pyridazine, a series of functionalized derivatives were designed and first reported. Afterwards, the relationship between their structure and performance was systematically explored by density functional theory at B3LYP/6‐311 g (d, p) level. Results show that the bond dissociation energies of the weakest bond (N–O bond) vary from 157.530 to 189.411 kJ · mol^?1. The bond dissociation energies of these compounds are superior to that of HMX (N–NO_2, 154.905 kJ · mol^?1). In addition, H1, H2, H4, I2, I3, C1, C2, and D1 possess high density (1.818–1.997 g · cm^?3) and good detonation performance (detonation velocities, 8.29–9.46 km · s^?1; detonation pressures, 30.87–42.12 GPa), which may be potential explosives compared with RDX (8.81 km · s^?1, 34.47 GPa ) and HMX (9.19 km · s^?1, 38.45 GPa). Finally, allowing for the explosive performance and molecular stability, three compounds may be suggested as good potential candidates for high‐energy density materials. Copyright © 2016 John Wiley & Sons, Ltd.     

共振价键波函数在高压液氢量子蒙卡模拟中的适用性研究

在共振价键理论基础上, 选取高压液氢电子主要占据轨道的线性组合作为基组, 构建由Jastrow项和反对称孪生函数乘积项 (AGP) 组成的波函数. 考虑电子关联作用的共振价键 (RVB) 波函数得出的能量值低于LDA能量值; 当满足r_s<1.75或T >15000 K时引入backflow项以改善波函数结点面, 改善后的能量值下降约1 mHa/atom, 能量方差值变小. 将构建的RVB波函数与电子-离子耦合的蒙特卡罗法 (CEIMC) 相结合, 计算结果与实验及其他ab-initio结果相符合, 获得的液氘单次冲击Hugoniot曲线基本通过所有加载类型实验误差棒, 液氘在50.3 GPa处具有最大压缩率4.48, 在100–120 GPa内未发现压缩率有急剧增大的现象. 构建的RVB 波函数能够适用于较宽密度与温度范围内(1.0< r_s<2.2, 2800 K< T<60000 K)液氢的模拟, 与CEIMC法相结合可提高液氢冲击特性的模拟精度. 关键词： 共振价键理论 波函数 量子蒙卡法 液氢     

Stress measurement under high pressure using Kawai‐type multi‐anvil apparatus combined with synchrotron radiation

A system for stress measurement under high pressure has been developed at beamline BL04B1, SPring‐8, Japan. A Kawai‐type multi‐anvil apparatus, SPEED‐1500, was used to pressurize polycrystalline KCl to 9.9 GPa in a mechanically anisotropic cell assembly with the KCl sample sandwiched between dense Al₂O₃ pistons. The variation of deviatoric stress was determined from the lattice distortion measured using two‐dimensional X‐ray diffraction with monochromatic synchrotron X‐rays. The low‐pressure B1 phase transformed to the high‐pressure polymorph B2 during compression. The deviatoric stress increased with increasing pressure in both the B1 and B2 phases except for the two‐phase‐coexisting region at a pressure of 2–3 GPa. This new system provides one of the technical foundations for conducting precise rheological measurements at conditions of the Earth's lower mantle.     

Theoretical studies on 2‐(5‐amino‐3‐nitro‐1,2,4‐triazolyl)‐3,5‐dinitropyridine (PRAN) and its derivatives

In this work, a set of derivatives of 2‐(5‐amino‐3‐nitro‐1,2,4‐triazolyl)‐3,5‐dinitropyridine (PRAN) with different energetic substituents (?N₃, –NO₂, –NH₂, –NF₂) have been studied at the Becke, three‐parameter, Lee–Yang–Parr/aug‐cc‐pvdz, Becke, three‐parameter, Lee–Yang–Parr/6‐31G(d), Becke, three‐parameter, Perdew 86/6‐31G(d), and Becke three‐parameter, Perdew–Wang 91/6‐31G(d,p) levels of density functional theory. The gas‐phase heats of formation were predicted with isodesmic reactions and the condensed‐phase HOFs were estimated with the Politzer approach. The effects of different functionals and basis sets were analyzed. –N₃ and –NO₂ greatly increase while –NH₂ and –NF₂ slightly decrease heats of formation. An analysis of the bond dissociation energies and impact sensitivity shows that all compounds have good stability. The crystal densities (1.82–2.00 g/cm³) computed from molecular packing calculations are big for all compounds and that of the –NF₂ derivative is the largest. All derivatives have higher detonation velocity and detonation pressure than PRAN. Compounds 3 and 4 (R = NO₂ and NF₂) have better performance than hexahydro‐1,3,5‐trinitro‐1,3,5‐trizine and the performance of 4 is quite close to that of 1,3,5,7‐tetranitro‐1,3,5,7‐tetraazacyclooctane, they are promising candidates of high energy compounds and worth further investigations. Copyright © 2012 John Wiley & Sons, Ltd.     

Metastable Conducting Crystalline Hydrogen at High Pressure

The quantum molecular dynamics method within the density functional theory has been used to calculate the equation of state, pair correlation function, and static electrical conductivity of solid hydrogen in the region of formation of a conducting phase. Hysteresis has been revealed on the density dependence of the pressure at a temperature of 100 K under compression and subsequent tension. The overlapping of branches of the isotherms of the molecular and nonmolecular phases of solid hydrogen corresponds to the region of existence of metastable states. The width of this region is 275 GPa. It has been shown that conducting crystalline nonmolecular hydrogen with P2₁/c symmetry can exist at extension to a pressure of 350 GPa.

     

Microwave‐assisted facile synthesis of liquid crystalline non‐symmetrical hexaalkoxytriphenylenes containing a branched chain and their characterization

Two novel series of liquid crystalline non‐symmetrical hexaalkoxytriphenylenes containing a branched alkyl chain have been prepared using microwave dielectric heating. Series 1 contains 2‐ethyl hexyloxy group as the branched chain whereas series 2 contains 3,7‐dimethyl octyloxy as the branched chain along with five normal alkoxy chains. The number of carbon atoms varies from four to eight in the normal alkoxy chains. Mesophase behaviour of the compounds has been characterized by polarizing optical microscopy, differential scanning calorimetry and mesophase structure has been characterized by X‐ray diffractometry. All the compounds show enantiotropic mesophase transitions with columnar hexagonal structure. In series 1 ( 4a – e ) the mesophase range and transition temperatures of all the compounds are lowered as compared to the parent compounds whereas in series 2 ( 5a – e ) the transition temperatures of all the compounds are lowered, mesophase range for lower members are decreased, however, higher members show more mesophase stability. Both melting and clearing temperatures of series 2 ( 5a – e ) show strong odd–even effect. The intercolumnar distance increases as expected for compounds of both the series with increase in alkyl chain length with some degree of interdigitation of the alkyl chains. The intercolumnar distances for the compounds of the series 2 are slightly higher than the compounds of the series 1 . Compound 4c displays homeotropic alignment without using any special technique for alignment of the columnar phase or application of any external force. Copyright © 2007 John Wiley & Sons, Ltd.     

Pressure‐induced incompressibility and point singularity of mechanical properties of TaC in NiAs‐type structure

The structural and elastic properties of TaC in NiAs‐type structure under high pressure have been investigated using first principles calculations based on density functional theory. Results indicate that the incompressibility along the c‐axis of TaC exceeds that of diamond under higher pressure. Particularly, an interesting point singularity exists in its mechanical properties as the pressure increases from 20 GPa to 40 GPa. The minimal shear modulus, Young's modulus, Debye temperature, and maximum Poisson ratio of TaC are simultaneously obtained at 28 GPa. The calculations of hardness indicate that the NiAs‐type TaC crystal possesses excellent mechanical properties. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Evolution of metallization and superconductivity in solid hydrogen

Inspired by the recent experimental reports of metallic hydrogen [Science 355 (2017) 715, Nature, 577 (2020) 631], we have reexamined the metallization and superconductivity of solid hydrogen in the pressure range of interest. Based on high quality calculations with zero-point vibrations and van der Waals (vdW) corrections, hydrogen is disclosed to metallize at about 485 GPa via the phase transition from insulate molecular C2/c-24 to metallic molecular Cmca-4, then dissociate into atomic phase with increasing pressure to 600 GPa. Further analyses demonstrate that vdW interaction can reduce the H-H distances in the metallic molecular Cmca-4, thus pulling down the contributions from phonon frequencies and electronic structures to electron-phonon coupling λ, and resulting in the declining superconducting $T_{\mathrm{c}}$. Meanwhile, slightly influence on these cases can be found in metallic atomic I4₁/amd by vdW correction. Our results indicate that aspirational room-temperature superconductivity in solid hydrogen requires a high pressure beyond 600 GPa.     

Theoretical investigations on 4,4′,5,5′‐tetranitro‐2,2′‐1H,1′H‐2,2′‐biimidazole derivatives as potential nitrogen‐rich high energy materials

The ―NH₂, ―NO₂, ―NHNO₂, ―C(NO₂)₃ and ―CF(NO₂)₂ substitution derivatives of 4,4′,5,5′‐tetranitro‐2,2′‐1H,1′H‐2,2′‐biimidazole were studied at B3LYP/aug‐cc‐pVDZ level of density functional theory. The crystal structures were obtained by molecular mechanics (MM) methods. Detonation properties were evaluated using Kamlet–Jacobs equations based on the calculated density and heat of formation. The thermal stability of the title compounds was investigated via the energy gaps (?E_{LUMO ? HOMO}) predicted. Results show that molecules T5 (D = 10.85 km·s^?1, P = 57.94 GPa) and T6 (D = 9.22 km·s^?1, P = 39.21 GPa) with zero or positive oxygen balance are excellent candidates for high energy density oxidizers (HEDOs). All of them appear to be potential explosives compared with the famous ones, octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetraazocane (HMX, D = 8.96 km·s^?1, P = 35.96 GPa) and hexanitrohexaazaisowurtzitane (CL‐20, D = 9.38 km·s^?1, P = 42.00 GPa). In addition, bond dissociation energy calculation indicates that T5 and T6 are also the most thermally stable ones among the title compounds. Copyright © 2014 John Wiley & Sons, Ltd.     

Pressure effects on structural,electronic, absorption,and thermodynamic properties of crystalline 2,4,6‐triamino‐3,5‐dinitropyridine‐1‐oxide: A DFT study

Density functional theory calculations have been performed to study the structural, electronic, absorption, and thermodynamic properties of crystalline 2,4,6‐triamino‐3,5‐dinitropyridine‐1‐oxide (TANPyo) in the pressure range of 0–50 GPa. The variation trends of the lattice constants, bond lengths, bond angles, intramolecular H‐bonds, and dihedral angles under compression show that there are two structural transformations at 17 and 38 GPa, respectively. The remarkable changes in the bond lengths indicate that there are two possible initiation decomposition mechanisms of TANPyo under compression. As the pressure increases, the intramolecular H‐bond strengthens. The obvious changes of the dihedral angles show that the planar structure of the TANPyo molecule is damaged under compression. Its absorption spectra show that as the pressure increases, the absorption coefficient of the N–H stretching decreases, while that of the O–H stretching increases. TANPyo has relatively high optical activity at high pressure. An analysis of thermodynamic properties indicates that both two structural transformations are endothermic and not spontaneous at room temperature. Copyright © 2013 John Wiley & Sons, Ltd.     

Resonance Raman intensity analysisof the excited‐state photochemical dynamicsof dimethyl 1,3‐dithiole‐2‐thione‐4,5‐dicarboxylate in the A‐band absorption

Dimethyl 1,3‐dithiole‐2‐thione‐4,5‐dicarboxylate (DDTD) was synthesized and characterized using NMR, Fourier transform (FT)‐Raman, Fourier transform‐infrared (FT‐IR) and UV spectroscopies. Resonance Raman spectra (RRs) were obtained with 341.5, 354.7 and 368.9 nm excitation wavelengths and density functional calculations were carried out to elucidate the π (S C S) →π* (S C S) electronic transitions and the RRs of DDTD in cyclohexane solution. The RRs indicate that the Franck–Condon region photo dynamics have a multidimensional character with motion predominantly along the CS stretch and the C S symmetric stretch modes in the five‐member heterocycle. A preliminary resonance Raman intensity analysis was carried out and the results for DDTD were compared with previously reported results for 1,3‐dithiole‐2‐thione (DTT). Differences and similarities of the spectra in terms of molecular symmetry and electron density are also discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

液氮冲击压缩特性的理论计算

 用修正的WCA理论计算了液氮冲击压缩至70 GPa的一次冲击Hugoniot数据。计算中引入与体系比容有关的分子离解因子，计算结果表明，在33GPa以上液氮体系发生的相变为分子离解相变，该过程对体系热力学状态有较大影响，分子离解是对冲击能量的吸收过程，导致体系冲击压力和温度增长率下降。     

Surface phase transitions of multiple-site associating fluids

Surface phase transitions of Lennard–Jones (LJ) based two- and four-site associating fluids have been studied for various associating strengths using grand-canonical transition matrix Monte Carlo simulations. Our results suggest that, in the case of a smooth surface, represented by a LJ 9-3-type potential, multiple-site associating fluids display a prewetting transition within a certain temperature range. However, the range of the prewetting transition decreases with increasing associating strength and increasing number of sites on the fluid molecules. With the addition of associating sites on the surface, a quasi-2D vapor–liquid transition may appear, which is observed at a higher surface site density for weaker associating fluids. The prewetting transition at lower associating strength is found to shift towards the quasi-2D vapor–liquid transition with increasing surface site density. However, for highly associating fluids, the prewetting transition is still intact, but shifts slightly towards the lower temperature range. Adsorption isotherms, chemical potentials and density profiles are used to characterize surface phase transitions.     

Exploring the possible interlinked structures in single‐wall carbon nanotubes under pressure by Raman spectroscopy

High‐pressure Raman measurements on single‐wall carbon nanotubes (SWNTs) have been carried out in a diamond anvil cell by using two wavelength lasers: 830 and 514.5 nm. Irrespective of using a pressure transmitting medium (PTM) or not, we found that nanotubes undergo similar transformations under pressure. The pressure‐induced changes in Raman signals at around 2 and 5 GPa are attributed to the nanotube cross‐section transitions from circle to ellipse and then to a flattened shape, respectively. Especially with pressure increasing up to 15–17 GPa, we observed that the third transition takes place in both the Raman wavenumber and the linewidth of G‐band. We propose explanations that the interlinked configuration with sp³ bonds forms in the bundles of SWNTs under pressure, which was the cause for the occurrence of those Raman anomalies, similar to the structural‐phase transition of graphite above 14 GPa. Our TEM observations and Raman measurements on the decompressed samples support this transition picture. Copyright © 2012 John Wiley & Sons, Ltd.     

Fluid dynamics analysis of a gas attenuator for X‐ray FELs under high‐repetition‐rate operation

Newtonian fluid dynamics simulations were performed using the Navier–Stokes–Fourier formulations to elucidate the short time‐scale (µs and longer) evolution of the density and temperature distributions in an argon‐gas‐filled attenuator for an X‐ray free‐electron laser under high‐repetition‐rate operation. Both hydrodynamic motions of the gas molecules and thermal conductions were included in a finite‐volume calculation. It was found that the hydrodynamic wave motions play the primary role in creating a density depression (also known as a filament) by advectively transporting gas particles away from the X‐ray laser–gas interaction region, where large pressure and temperature gradients have been built upon the initial energy deposition via X‐ray photoelectric absorption and subsequent thermalization. Concurrent outward heat conduction tends to reduce the pressure in the filament core region, generating a counter gas flow to backfill the filament, but on an initially slower time scale. If the inter‐pulse separation is sufficiently short so the filament cannot recover, the depth of the filament progressively increases as the trailing pulses remove additional gas particles. Since the rate of hydrodynamic removal decreases while the rate of heat conduction back flow increases as time elapses, the two competing mechanisms ultimately reach a dynamic balance, establishing a repeating pattern for each pulse cycle. By performing simulations at higher repetition rates but lower per pulse energies while maintaining a constant time‐averaged power, the amplitude of the hydrodynamic motion per pulse becomes smaller, and the evolution of the temperature and density distributions approach asymptotically towards, as expected, those calculated for a continuous‐wave input of the equivalent power.     

A comparative study on the gas‐phase and liquid‐phase thermal isomerization reaction of α‐pinene

In this paper, a method of preparation of ocimene is investigated, which is obtained from isomerization reaction of α‐pinene. Two kinds of experimental apparatus are established for the investigation of the thermal isomerization reaction of α‐pinene. The behavior of thermal isomerization reaction of α‐pinene is respectively discussed in the gas phase and in the liquid phase. Under gas phase conditions, the conversion of α‐pinene is 80% and the selectivity of ocimene is 30%–33%. Under liquid phase conditions, the conversion of α‐pinene is 60% and the selectivity of ocimene is 50%–54%. According to the kinetic‐molecular theory of ideal gases, two kinds of reaction models are proposed to visualize the reaction process. In addition, the mechanism and kinetics of thermal isomerization reaction of α‐pinene are respectively discussed. The conclusion is that the gas phase reaction temperature is calculated to be 390–450 °C and the liquid phase reaction temperature is calculated to be 450–550 °C. From a bond dissociation energy point of view, results support the hypothesis that the reaction involves biradical intermediates. Copyright © 2012 John Wiley & Sons, Ltd.     

Ultrasonic study of the phase diagram of methanol

The phase diagram of methanol is studied by an ultrasonic technique over the temperature range 90–290 K at pressures up to 1.2 GPa. The pressure and temperature dependence of the velocity of longitudinal ultrasonic waves and the density of crystalline and liquid phases has been determined. Weak anomalies in the velocity of ultrasound in the liquid phase of methanol and the corresponding anomalous additional compression of the liquid at 230–250 K and 0.2–0.6 GPa have been found, and they are likely attributable to structural changes in the liquid phase.     

Reexploration of Structural Changes in Element Bromine through Pressure-Induced Decomposition of Solid HBr

Simple molecular solids have been an important subject in condensed matter physics,particularly for research of pressure-induced molecular dissociation.We re-explore the structural changes of element bromine through pressure-induced decomposition of solid HBr.The phase changes in HBr are investigated by Raman spectroscopy and synchrotron x-ray diffraction up to 125 GPa at room temperature.By applying pressure,HBr decomposes into solid bromine in the pressure range of 18.7-38 GPa.The solid bromine changes from molecular phase to incommensurate phase at 81 GPa,and finally to monatomic phase at 91 GPa.During the process of pressureinduced molecular dissociation, the intermediate incommensurate phase of element bromine is confirmed for the first time from the x-ray diffraction studies.The decomposition of HBr is irreversible since HBr cannot form again upon pressure decompression.