The role of the sextet potential energy surface in O2 + N inelastic collision processes.

We have performed molecular dynamics simulations of inelastic collisions between molecular oxygen and atomic nitrogen, employing the quasi-classical trajectory method on the new doublet, quartet, and sextet analytical potential energy surfaces of NO2. A complete database of vibrationally detailed rate coefficients is constructed in a wide temperature range for high vibrational states up to ν = 25. In particular, the present work shows that the sextet potential energy surface plays a crucial role in the rovibrational relaxation process of O2 + N collisions. The state-to-state rate coefficients increase by a factor of 2 to 6 when we consider the contribution of this sextet potential energy surface according to the corresponding weight factor, especially for vibrational energy transfer processes in single quantum jumps and/or high-temperature regimes. Furthermore, we also provide Arrhenius-type accurate fits for the vibrational state-specific rate coefficients of this collision system to achieve the flexible application of rate coefficients in numerical codes concerning air kinetics. Our results have implications for understanding the relaxation mechanism of the collision system with degenerate electronic states.

Real-time dynamic simulation of laser-induced N2 dissociation on two-dimensional graphene sheets.

Due to its relatively high inertness, nitrogen dissociation at ambient temperature and pressure has always been a challenging task. Plasmon driven photocatalysis has proved to be an effective method. Owing to their unique physical, chemical, and electronic properties, two-dimensional planar materials have become the most promising candidates to replace noble metal catalytic nitrogen reduction. In this study, real-time dynamics of N2 dissociation on graphene sheets under femtosecond laser irradiation was studied by using time-dependent density functional theory. We confirm that electrons generated by plasmon excitation of graphene transfer to the N2 molecular antibonding orbital and activate the N-N bond. The threshold of laser intensity of N2 dissociation can be effectively reduced by mixing CO molecules. This work provides basic insights for understanding the plasmon induced N2 activation process at the atomic scale and proves that graphene can be used as one of the candidate materials for N2 reduction photocatalysts with excellent performance.

Dissociation cross sections and rates in O2 + N collisions: molecular dynamics simulations combined with machine learning.

The collision-induced dissociation reaction of O2 (v, j) + N, a fundamental process in nonequilibrium air flows around reentry vehicles, has been studied systematically by applying molecular dynamics simulations on the 2A', 4A' and 6A' potential energy surfaces of NO2 in a wide temperature range. In particular, we have directly investigated the role of the 6A' surface in this process and discussed the applicability of the simplified approximate rate models proposed by Esposito et al. and Andrienko et al. based on the lowest two surfaces. The present work indicates that the state-selected dissociation of O2 + N is dominated by the 6A' surface for all except for the low-lying O2 states. Furthermore, a complete database of rovibrationally detailed cross sections and rate coefficients is a prerequisite for modeling the relevant nonequilibrium air flows in spacecraft reentry. Here, the combination of the quasi-classical trajectory (QCT) and the neural network (NN) has been proposed to predict all state-selected dissociation cross sections and further construct dissociation parameter sets. All NN-based models established in this work accurately reproduce the results calculated from QCT simulations over a wide range of rovibrational quantum numbers with R2 > 0.99. Compared with the explicit QCT simulations, the computational requirement for predicting cross sections and rates based on the NN models significantly reduces. Finally, thermal equilibrium rate coefficients computed from NN models match remarkably well the available theoretical and experimental results in the whole temperature range explored.

Exhaustive state-specific dissociation study of the N2(Σg+1)+N(S4) system using QCT combined with a neural network method.

This work studies the exhaustive rovibrational state-specific collision-induced dissociation properties of the N2+N system by QCT (quasi-classical trajectory) combined with a neural network method based on the ab initio PES recently published by Varga et al. [Phys. Chem. Chem. Phys. 23, 26273 (2021)]. The QCT combined with a neural network for state-specific dissociation (QCT-NN-SSD) model is developed and used to predict the dissociation cross sections and their energy dependence on the thermal range from a sparsely sampled noisy dataset. It is conservatively estimated that using this method can reduce the cost of the calculation by 96.5%. The rate coefficient of thermal non-equilibrium between different energy modes is obtained by combining the QCT-NN-SSD model with the multi-temperature model. The results show that, for the equilibrium state, dissociation mainly occurs at high vibrational and moderately low rotational levels. When the system is in non-equilibrium, there is no obvious vibrational level preference and highly rotationally excited molecules play a major role in promoting the dissociation by compensating for the lack of vibrational energy. The use of neural network training to generate complete datasets based on limited and discrete data provides an economical and reliable way to obtain a complete kinetic database needed to accurately simulate non-equilibrium flows.

Study on the Ti K, L2,3, and M Edges of SrTiO3 and PbTiO3.

The multiplet theory of free ions combined with crystal field theory is used to study core electron excitation of perovskites. This combined method is helpful to further identify transition peaks and comprehend the transition mechanism. In this paper, the core electron excitation of Ti K edge, L2,3 edges, and M edge in SrTiO3 and PbTiO3 is studied, and the identification of peak and the analysis of spectral shape are emphasized. Especially at the M edge, we can identify the dipole and higher-order multiple transition peaks of Ti4+. At the L2,3 edges, the transition strength corresponding to the 2p3d configuration coupling affects the shape of the spectra. The correction of the crystal field can make the theoretical spectra more consistent with the experimental spectra. For the K edge in PbTiO3, by comparing the spectral shapes and energy of Ti2+, Ti3+, and Ti4+ with experiments, the spectrum is mainly composed of Ti3+ and Ti4+, and the proportion of Ti4+ is determined to be over 91%.

GCMC investigation into adamantane-based aromatic frameworks with diamond-like structure as high-capacity hydrogen storage materials.

A new class of 3D adamantane-based aromatic framework (AAF) with diamond-like structure was computationally designed with the aid of density functional theory (DFT) calculation and molecular mechanics (MM) methods. The hydrogen storage capacities of these AAFs were studied by the method of grand canonical Monte Carlo (GCMC) simulations. The calculated pore sizes of three AAFs reveal that AAF-1 and AAF-2 belong to microporous materials, while AAF-3 is a member of mesoporous materials. The GCMC results reveal that at 77 K and 100 bar, AAF-3 exhibits the highest gravimetric hydrogen uptake of 29.50 wt%, while AAF-1 shows the highest volumetric hydrogen uptake of 63.04 g L(-1). In particular, the gravimetric hydrogen uptake of AAF-3 reaches the Department of Energy's target of 6 wt% at room temperature. The extraordinary performances of these new AAFs in hydrogen storage have made them enter the list of top hydrogen storage materials up to now.

Reactive molecular dynamics simulation of solid nitromethane impact on (010) surfaces induced and nonimpact thermal decomposition.

Which is the first step in the decomposition process of nitromethane is a controversial issue, proton dissociation or C-N bond scission. We applied reactive force field (ReaxFF) molecular dynamics to probe the initial decomposition mechanisms of nitromethane. By comparing the impact on (010) surfaces and without impact (only heating) for nitromethane simulations, we found that proton dissociation is the first step of the pyrolysis of nitromethane, and the C-N bond decomposes in the same time scale as in impact simulations, but in the nonimpact simulation, C-N bond dissociation takes place at a later time. At the end of these simulations, a large number of clusters are formed. By analyzing the trajectories, we discussed the role of the hydrogen bond in the initial process of nitromethane decompositions, the intermediates observed in the early time of the simulations, and the formation of clusters that consisted of C-N-C-N chain/ring structures.

Theoretical prediction of the reaction of s-triazine with nitrate radical: a new possible route to generate oxy-s-triazine.

Oxy-s-triazine (OST) is one of the important Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) decomposition products, while it is yet not fully clear how it is formed up to now. The study systematically investigates the reaction of s-triazine (TAZ) with nitrate radical (NO(3)) using computational chemistry methods, for which three entrance channels are devised, resulting in the formation of four isomers of OST. Based on the analysis of the barrier heights and the reaction exothermicities, the pathway to form OST3 through hydrogen atom abstraction and rebound mechanism is likely to be the main channel in the reactions of TAZ with NO(3) radical. Our study puts forward a new possible route to generate OST.

Separation of C1/C3 binary organic gas mixtures via flexible nanoporous carbon molecular sieves: DFT calculations and MD simulations.

The C1/C3 hydrocarbon gas separation characteristics of nanoporous carbon molecular sieves (CMS) are studied using DFT calculations and MD simulations. To efficiently separate the equimolar CH4/C3H8 and CH4/C3H6 gas mixtures, CNT gas transport channels are embed between the polyphenylene membranes which created structural deformation for both CNT and membrane. The adsorption and permeation of gas molecules via CMS and the effect of nanochannel density and electric field on gas selectivity are analyzed. In addition to the direct permeation, gas molecules that adsorbed on the NPG surface also making a significant contribution to the gas permeability comes from a surface mechanism. Results also uncovered that the gas selectivity is enhanced by the electric field along the + x and +y axes, whereas reduced by the electric field along the + z and -z axes. Plainly, this CMS provides a feasible way for the efficient separation of the C1/C3 organic gas mixtures.

Kohn anomaly and van Hove singularity in IVB and VB group transition metals nitrides and carbides.

The superconducting behavior in IVB-VB group transition metal nitrides and carbides has generally been associated with the phonon anomaly and Fermi surface nesting. However, the origin of phonon anomaly has remained ambiguous (i.e. longitudinal acoustic or transverse acoustic modes). We performed first-principles calculations to investigate the phononic properties of these materials and theoretically confirmed that the Kohn anomaly originates from the lower transverse acoustic mode along the ГX direction, thereby revealing the frequency derivative discontinuity of the mode. In particular, the Kohn anomaly region is found to move from the interior to the boundary X point of the Brillouin zone with increasing number of valence electrons. We deduced that the Kohn anomaly originated from the electrons of the filled energy level near the van Hove singularity. These results suggest that the screening of the ionic electric field decreases, while the coupling of conduction electrons with the highly degenerate modes between the TA∥ and LA via Umklapp scattering process increases. The Fermi surface nesting also plays a role in enhancing the superconductivity. The electronic excitation effect induces a stabilization of the V2 group transition metal nitrides.

MD simulation of methane adsorption properties on pillared graphene bubble models.

Pillared graphene bubble framework is selected as the methane storage vessel in this article. All investigations of methane adsorption are executed by using the MD simulations. The average adsorption energy of methane on different bubble models is between - 4.3 and - 5.2 kcal/mol, which is desirable for absorbing and desorbing gas molecules. The methane adsorption properties of bubble models are obviously different from those of pillared graphene. The effect of graphene interlayer spacing on methane adsorption in selected bubble models can be negligible. Nevertheless, bubble density and temperature have a significant influence on methane adsorption. The amount of adsorbed methane on pillared bubble models at room temperature can reach up to 18.2 mmol/g. This performance of methane adsorption on pillared graphene bubble structures may bring new enlightenment to the investigations of gas storage materials.

[Line intensities of upsilon2 perpendicular band and the change of intensities with temperature for H12C14N].

The total internal partition sums (TIPS) were calculated for H12C14N with the product approximation. For rotational partition sums Q(rot), the centrifugal distortion corrections were taken into account. The calculation method for the vibrational partition sums Q(vib) is the harmonic oscillator approximation. The line intensities of upsilon2 perpendicular band (0110-0000 transition) of H12C14N were calculated at normal temperatures and several high temperatures by using the calculated partition functions and experimental transition moment squared and Herman-Waills factor coefficients. Results showed that our line intensities data at 296 and 3 000 K are in excellent agreement with the data in HITRAN, which provide a strong support for the calculations of partition function and line intensity at high temperature. Thereby, the line intensities and spectral simulations of upsilon2 perpendicular band at the higher temperatures 4 000 and 5 000 K were presented and the chang in line intensities with the temperature was discussed. For those transitions corresponding to rotational quantum number J > or = 32 (including P, Q and R branch), the line intensities increase when temperature gradually increase from 296 K. The line intensities are up to the largest at around 1000 K and then weaken rapidly. For J < 32 (also including P, Q and R branch), the line intensities are the largest at 296 K and then weaken rapidly as temperature gradually increase.

Simulations on methane uptake in tunable pillared porous graphene hybrid architectures.

In this article, 3D pillared carbon nanotube (CNT)-porous graphene (PG) nanomesh architectures are computationally investigated as methane storage nanocontainer. The purpose of this article is to screen the configurations of 3D pillared CNT-PG materials and to select the optimal one for maximizing the methane storage capacity. Molecular mechanics (MM) calculations and MD simulations are executed to depict the structural characteristics and methane adsorption properties. The calculated structural parameters coincide well with the empirical conclusions. The methane adsorption simulations are systematic investigated as a function of geometry variables such as PG interlayer spacing, distance of CNTs, and the number of PG sheets in a wide range of pressure. The average adsorption energy of methane in different configurations is concentrated between 2 and 4 kcal mol-1. The results revealed that the applications of 3D CNT-PG models can significantly enhance methane adsorption performance in comparison to pillared graphene: the maximum amount of adsorbed methane of 3D CNT-PG displays 21.3 mmol/gr (interlayer spacing of 1.2 nm and bilayer PG), which is about 25% higher than that of pillared graphene. Meanwhile, the deformation of (6, 6) carbon nanotubes can significantly improve the methane storage capacity. This provides a viable structure modification method, which is suitable for enhancement of methane storage.

Calculations of bond dissociation energies and dipole moments in energetic materials using density-functional methods.

We compare the effectiveness of six exchange/correlation functional combinations (Becke/Lee, Yang and Parr; Becke-3/Lee, Yang and Parr; Becke/Perdew-Wang 91; Becke-3/Perdew-Wang 91; Becke/Perdew 86; Becke-3/Perdew 86) for computing C-N, O-O and N-NO2 dissociation energies and dipole moments of five compounds. The studied compounds are hexabydro-1,3,5-trinitro-1,3,5-triazine (RDX), dimethylnitramine, cyanogen, nitromethane and ozone. The Becke-3/Perdew 86 in conjunction with 6-31G** is found to give the best results, although for the dipole moments of RDX, there is a slightly difference that B3P86/6-31G** is less reliable than B3P86/6-31+G**.

Theoretical studies of some bimolecular reactions during the decomposition of CH3NO2: reactions between NO2 and nine intermediates.

Nitromethane (NM, CH3NO2) is a widely studied energetic material, and its decomposition mechanism attracts great interest. In this work, bimolecular reactions between NO2 and nine intermediates generated during the decomposition of NM were investigated by computational chemistry methods. The mechanisms of the reactions were analyzed. The results revealed that these reactions possess small barriers and can easily occur, so they may be responsible for NO2 loss during the decomposition of NM.

[Study on micro-behaviours shock ignition for epoxypropane using spectrum techniques].

The micro-behaviours of shock ignition of epoxypropane were studied by OMA (optical multii channal system) and monochromator techniques. The radicals O, CH2O, C2, CH, CH3O, CO2 and H2O were observed by OMA spectrometer. The delay time and critical condition of shock ignition were determined using three monochromators and gauge. The emergence of intermediate product of O for epoxypropane after shock ignition is always the earliest.

Theoretical study of the reaction mechanism of CH3NO2 with NO2, NO and CO: the bimolecular reactions that cannot be ignored.

The intriguing decompositions of nitro-containing explosives have been attracting interest. While theoretical investigations have long been concentrated mainly on unimolecular decompositions, bimolecular reactions have received little theoretical attention. In this paper, we investigate theoretically the bimolecular reactions between nitromethane (CH3NO2)-the simplest nitro-containing explosive-and its decomposition products, such as NO2, NO and CO, that are abundant during the decomposition process of CH3NO2. The structures and potential energy surface (PES) were explored at B3LYP/6-31G(d), B3P86/6-31G(d) and MP2/6-311 + G(d,p) levels, and energies were refined using CCSD(T)/cc-pVTZ methods. Quantum chemistry calculations revealed that the title reactions possess small barriers that can be comparable to, or smaller than, that of the initial decomposition reactions of CH3NO2. Considering that their reactants are abundant in the decomposition process of CH3NO2, we consider bimolecular reactions also to be of great importance, and worthy of further investigation. Moreover, our calculations show that NO2 can be oxidized by CH3NO2 to NO3 radical, which confirms the conclusion reached formerly by Irikura and Johnson [(2006) J Phys Chem A 110:13974-13978] that NO3 radical can be formed during the decomposition of nitramine explosives.