Geometries and stabilities of chromium doped nitrogen clusters: mass spectrometry and density functional theory studies.

Metal-doped nitrogen clusters serve as effective models for elucidating the geometries and electronic properties of nitrogen-rich compounds at the molecular scale. Herein, we have conducted a systematic study of VIB-group metal chromium (Cr) doped nitrogen clusters through a combination of mass spectrometry techniques and density functional theory (DFT) calculations. The laser ablation is employed to generate CrNn+ clusters. The results reveal that CrN8+ cluster exhibits the highest signal intensity in mass spectrometry. The photodissociation experiments with 266 nm photons confirm that the chromium heteroazide clusters are composed of chromium ions and N2 molecules. Further structural searches and electronic structure calculations indicate that the cationic CrN8+ cluster possesses an X shaped geometry with D2 symmetry and exhibits robust stability. Molecular orbital and chemical bonding analyses demonstrate the existence of strong interactions between Cr+ cation and N2 ligands. The present findings enrich the geometries of metal doped nitrogen clusters and provide valuable guidance for the rational design and synthesis of novel transition metal nitrides.

Finite temperature string by K-means clustering sampling with order parameters as collective variables for molecular crystals: application to polymorphic transformation between ß-CL-20 and ε-CL-20.

Polymorphic transformation of molecular crystals is a fundamental phase transition process, and it is important practically in the chemical, material, biopharmaceutical, and energy storage industries. However, understanding of the transformation mechanism at the molecular level is poor due to the extreme simulating challenges in enhanced sampling and formulating order parameters (OPs) as the collective variables that can distinguish polymorphs with quite similar and complicated structures so as to describe the reaction coordinate. In this work, two kinds of OPs for CL-20 were constructed by the bond distances, bond orientations and relative orientations. A K-means clustering algorithm based on the Euclidean distance and sample weight was used to smooth the initial finite temperature string (FTS), and the minimum free energy path connecting ß-CL-20 and ε-CL-20 was sketched by the string method in collective variables, and the free energy profile along the path and the nucleation kinetics were obtained by Markovian milestoning with Voronoi tessellations. In comparison with the average-based sampling, the K-means clustering algorithm provided an improved convergence rate of FTS. The simulation of transformation was independent of OP types but was affected greatly by finite-size effects. A surface-mediated local nucleation mechanism was confirmed and the configuration located at the shoulder of potential of mean force, rather than overall maximum, was confirmed to be the critical nucleus formed by the cooperative effect of the intermolecular interactions. This work provides an effective way to explore the polymorphic transformation of caged molecular crystals at the molecular level.

All-Nitrogen Energetic Material Cubic Gauche Polynitrogen: Plasma Synthesis and Thermal Performance.

Numerous theoretical calculations have demonstrated that polynitrogen with an extending polymeric network is an ultrahigh-energy all-nitrogen material. Typical samples, such as cubic gauche polynitrogen (cg-N), have been synthesized, but the thermal performance of polynitrogen has not been unambiguously determined. Herein, macroscopic samples of polynitrogen were synthesized utilizing a coated substrate, and their thermal decomposition behavior was investigated. Polynitrogen with carbon nanotubes was produced using a plasma-enhanced chemical vapor deposition method and characterized using infrared, Raman, X-ray diffraction X-ray photoelectron spectroscopy and transmission electron microscope. The results showed that the structure of the deposited polynitrogen was consistent with that of cg-N and the amount of deposition product obtained with coated substrates increased significantly. Differential scanning calorimetry (DSC) at various heating rates and TG-DSC-FTIR-MS analyses were performed. The thermal decomposition temperature of cg-N was determined to be 429 °C. The apparent activation energy (Ea) of cg-N calculated by the Kissinger and Ozawa equations was 84.7 kJ/mol and 91.9 kJ/mol, respectively, with a pre-exponential constant (lnAk) of 12.8 min-1. In this study, cg-N was demonstrated to be an all-nitrogen material with good thermal stability and application potential to high-energy-density materials.

Geometric and electronic structures of medium-sized boron clusters doped with plutonium.

Doping metal heteroatoms is an effective strategy to regulate the geometric and electronic structure of boron based nanoclusters. However, the exploration of the ground state structures of metal-boron-based nanoclusters is still a challenge duo to the complexity of the bonding interactions between heterogeneous atoms and boron cluster and the number of isomers on the potential energy surface increases exponentially with cluster size. Here, we use the CALYPSO cluster structural search method in combination with density functional theory calculations to study the geometries and electronic properties of anionic boron clusters doped with plutonium (PuBn-,n= 10-20). Our results show that the medium-sized PuB14-cluster exhibits excellent stability with highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap of 2.30 eV. The remarkable stability of the anionic PuB14-cluster is due to the robust interactions between the Pu metal and the B14skeleton, along with the strong covalent interactions between the B atoms. These findings enrich the geometric structure database of metal doped clusters and provide valuable insights for the future synthesis of boron based nanomaterials.

Predicted the structural diversity and electronic properties of Pt-N compounds under high pressure.

The nitrogen-rich transition metal nitrides have attracted considerable attention due to their potential application as high energy density materials. Here, a systematic theoretical study of PtNxcompounds has been performed by combining first-principles calculations and particle swarm-optimized structure search method at high pressure. The results indicate that several unconventional stoichiometries of PtN2, PtN4, PtN5, and Pt3N4compounds are stabilized at moderate pressure of 50 GPa. Moreover, some of these structures are dynamically stable even when the pressure release to ambient pressure. TheP1-phase of PtN4and theP1-phase of PtN5can release about 1.23 kJ g-1and 1.71 kJ g-1, respectively, upon the decomposition into elemental Pt and N2. The electronic structure analysis shows that all crystal structures are indirect band gap semiconductors, except for the metallic Pt3N4withPcphase, and the metallic Pt3N4is a superconductor with estimated critical temperatureTcvalues of 3.6 K at 50 GPa. These findings not only enrich the understanding of transition metal platinum nitrides, but also provide valuable insights for the experimental exploration of multifunctional polynitrogen compounds.

Cyclic Behavior of Autoclaved Aerated Concrete External Panel with New Connector.

In this paper, a new slip-type crossing connector is proposed for autoclaved aerated concrete (ALC) panels with steel frames, and the proposed connector is also studied deeply in terms of seismic performance. The research included pseudo-static tests and finite element simulations. First, the seismic performance of slip-type crossing connectors and standard L-hooked bolts was studied comparatively, including the stability, bearing capacity, stiffness, energy dissipation, and hysteresis performance. ABAQUS 2020 software was used to establish finite element models, and the results of the experiments were verified with simulations on the basis. According to the simulations, a parameter analysis of connector optimization was carried out. The effects of connector thickness and connector plate length on the seismic performance were further investigated. From the experimental and simulation results, the slip-type crossing connector has excellent performance and good assembly efficiency, it can improve the deficiencies of the existing connectors. The comparison demonstrated that the slip-type crossing connector has a complete hysteresis curve, a high energy dissipation capacity, and a 9.7% increase in bearing capacity. The appropriate reduction in connector thickness and plate length can ensure superior seismic performance while saving resources. The finite analysis method can guide the design and implementation of new external ALC panel connectors.

Polymeric Hydronitrogen N4H: A Promising High-Energy-Density Material and High-Temperature Superconductor.

Solid nitrogen-rich compounds are potential high-energy-density materials (HEDMs). The enormous challenge in this area is to synthesize and stabilize these energetic materials at moderate pressure and better under near-ambient conditions. Here, we perform an extensive theoretical study on hydronitrogens by the reverse design method considering both energies and energy densities. Four hydronitrogens with different stoichiometries, that is, N4H, N3H, N2H, and NH, are found to be stable at pressures of about 80-300 GPa and metastable with pressure releasing to ambient pressure. The energy densities of these hydronitrogens are about 5.6-6.5 kJ/g and 1.3-1.5 times larger than that of trinitrotoluene (TNT). Most importantly, the Pbam phase of the N4H compound is an excellent high-temperature superconductor with a Tc of 37.7 K at 72 GPa. The present findings enrich new phases of hydronitrogens under high pressure and characterize their structural and energetic properties and superconductivity, which offer crucial insights for further design and synthesis of exceptional materials with high energy density and high-temperature superconductivity.

Structure and Chemical Bonding in Medium-Size Boron Clusters Doped with Praseodymium.

After reports of unusually low oxidation states of lanthanide elements in Ln-B clusters and their inverse sandwich geometrical topologies, the interest shifted from boride clusters doped with transition metal (TM) elements to the boride clusters doped with lanthanide atoms. In this work, the results obtained by a combined approach consisting of CALYPSO structure predictions and density functional theory (DFT) calculations for the neutral and anionic PrBn series, n = 7-16, are reported. A close agreement between our calculated vertical detachment energies and experimental data supports the accuracy of the results obtained. Contrary to the medium-size TM-doped medium boron clusters, which prefer three types of structural configurations, all lowest-energy states of the medium-size Pr-doped boron clusters have half-sandwich geometries. An interesting structural evolution pattern was found for both neutral and anionic PrBn clusters at n = 7, 10, 13, and 16, which includes quasi-planar B7 units half-sandwiching the Pr atom. Unusual oxidation numbers of +2 and +1 were found for the Pr atom in the PrB7- and PrB8- anions, respectively. Chemical bonding analysis for the neutral PrB7 and PrB13 clusters revealed that their high stability stems from interactions between Pr 5d and B 2p orbitals. A stable tubular-shaped PrB30 cluster is proposed as a promising building block for boron-based nanotubes.

Optimization Strategy for Parks and Green Spaces in Shenyang City: Improving the Supply Quality and Accessibility.

In this study, we evaluated the supply quality of parks and green spaces within the Third Ring Road area in Shenyang city by combining a landscape pattern index analysis with a principal component analysis. Moreover, a network analysis based on the ArcGIS platform was used to measure the accessibility of parks and green spaces. The research results showed that the overall supply quality of parks and green spaces (−9.55) must be improved. The supply quality levels of the four analyzed park types could be ranked as follows: garden parks (118.00) > community parks (73.67) > comprehensive parks (−16.64) > specific parks (−32.17). Among the analyzed recreation parks, the accessibility of daily recreation parks was poor, while the overall service efficiency of weekly recreation parks was better, except in a few regions. These research results can provide suggestions for future green space planning in Shenyang city. In addition, from the perspective of landscape patterns, studying the service quality of parks and green spaces can provide new ideas for further research on accessibility.

Identification of octahedral coordinated ZrN12+ cationic clusters by mass spectrometry and structure searches.

Cationic zirconium-doped nitrogen clusters were produced by laser ablation of a Zr : BN mixture target and were detected by TOF mass spectrometry. It is found that the mass peak of the ZrN12+ cluster is dominant in the spectrum. The ZrN12+ cluster was further dissociated with 266 nm photons. Extensive structure searches of a cationic ZrN12+ cluster indicate that the ground state structure of ZrN12+ consists of a central Zr atom and six N2 pairs with Oh symmetry. The calculated binding energy of the ZrN12+ cluster is about 0.96 eV, which is in accordance with the result of the photodissociation experiment. The neutral ZrN12 cluster has almost the same geometry, but with D3h symmetry. NBO analysis showed that the molecular orbitals of ZrN12+/0 clusters are mainly composed of Zr 4d and N 2p orbitals. These findings provide rich information for understanding the geometries and the electronic properties of zirconium-doped N clusters, which will offer valuable guidance for the exploration of other metal doped nitrogen clusters.

Tuning of Structure Evolution and Electronic Properties through Palladium-Doped Boron Clusters: PdB16 as a Motif for Boron-Based Nanotubes.

Transition metal-doped electronic deficiency boron clusters have led to a vast variety of electronic bonding properties in chemistry and materials science. We have determined the ground state structures of PdBn0/- (n = 10-20) clusters by performing CALYPSO search and density functional theory (DFT) optimization. The identified lowest energy structures for both neutral and anionic Pd-doped boron clusters follow the structure evolution from two dimensional (2D) planar configurations to 3D distorted Pd-centered drum-like or tubular structures. Photoelectron spectra are simulated by time-dependent DFT theoretical calculations, which is a powerful method to validate our obtained ground-state structures. More interestingly, two "magic" number clusters, PdB12 and PdB16, are found with enhanced stability in the middle size regime studied. Subsequently, molecular orbital and adaptive natural density partitioning analyses reveal that the high stability of the PdB16 cluster originates from doubly σ π aromatic and bonding interactions of d-type atomic orbitals of the Pd atom with tubular B16 units. The tubular C8v PdB16 cluster, with robust relative stability, is an ideal embryo for forming finite and infinite nanotube nanomaterials.

Structural evolution of LiN n + (n = 2, 4, 6, 8, and 10) clusters: mass spectrometry and theoretical calculations.

Mixed nitrogen-lithium cluster cations LiN n + were generated by laser vaporization and analyzed by time-of-flight mass spectrometry. It is found that LiN8 + has the highest ion abundance among the LiN n + ions in the mass spectrum. Density functional calculations were conducted to search for the stable structures of the Li-N clusters. The theoretical results show that the most stable isomers of LiN n + clusters are in the form of Li+(N2) n/2, and the order of their calculated binding energies is consistent with that of Li-N2 bond lengths. The most stable structures of LiN n + evolve from one-dimensional linear type (C ∞v, n = 2; D ∞h, n = 4), to two-dimensional branch type (D 3h, n = 6), then to three-dimensional tetrahedral (T d, n = 8) and square pyramid (C 4v, n = 10) types. Further natural bond orbital analyses show that electrons are transferred from the lone pair on Nα of every N2 unit to the empty orbitals of lithium atom in LiN2-8 +, while in LiN10 +, electrons are transferred from the bonding orbital of the Li-Nα bonds to the antibonding orbital of the other Li-Nα bonds. In both cases, the N2 units become dipoles and strongly interact with Li+. The average second-order perturbation stabilization energy for LiN8 + is the highest among the observed LiN n + clusters. For neutral LiN2-8 clusters, the most stable isomers were also formed by a Li atom and n/2 number of N2 units, while that of LiN10 is in the form of Li+(N2)3(η1-N4).

Mass Spectrometry and Theoretical Investigation of VN n+ ( n = 8, 9, and 10) Clusters.

VN n+ clusters were generated by laser ablation and analyzed by mass spectrometry. The results showed that VN8+, VN9+, and VN10+ clusters were formed, and the mass peak of VN8+ is dominant in the spectrum. The VN8+ cluster was further investigated by a photodissociation experiment with 266 nm photons. Density functional theory calculations were conducted at the M06-2X/6-311+G(d,p) level to search for stable structures of VN n+ ( n = 8, 9, and 10) and their neutral counterparts. The theoretical calculations revealed that the most stable structure of VN8+ is in the form of V(N2)4+ with D 4h symmetry. The binding energy from the calculation is in good agreement with that obtained from the photodissociation experiments. The global minimum structures of VN8, VN9+/0, and VN10+/0 contain a similar substructure of the N4 ring and exhibit energy properties. The most stable structure of VN9+ is in the form of (η2-N4)V+N(N2)2 with C1 symmetry, while that of VN10+ is in the form of (η4-N4)V+(N2)3 with C s symmetry. For neutral VN8, VN9, and VN10, (η4-N4)V(N2)2, (η4-N4)V(N3)(N2), and (η4-N4)V(N2)3 are their ground-state structures, with decomposition into one V atom, and corresponding quantities of N2 can release energies of about 50.20, 96.28, and 57.76 kcal/mol, respectively.

The sequential structure of tripyridiniumylporphyrin pendants in water-soluble copolymers and their association behaviour with tetrasulfonatophenylporphyrin guests: UV-vis absorption and fluorescence emission spectra study.

A novel cationic tripyridiniumylporphyrin monomer, 5-[4-[2-(acryloyloxy)ethoxy]phenyl]-l0,l5,20-tris(N-methyl-4-pyridiniumyl)porphyrinate zinc(ii) (ZnTrMPyP), was synthesized, and its self-aggregation in water was studied by UV-vis absorption. The monomer was copolymerized with acrylamide in water and DMSO, respectively, to prepare the water-soluble polymers P-W and P-D. The aggregation behaviour of the copolymers in aqueous solution was investigated by UV-vis absorption and fluorescence emission spectra. The polymer P-D displayed very similar absorption and emission spectra to those of ZnTrMPyP in water, indicating that the polymer chains in P-D have no significant effect on the aggregate structure of ZnTrMPyP in aqueous media. In comparison, two new absorption bands appeared in the Q band range of polymer P-W and its fluorescence spectra red shifted and the fluorescence quantum yield decreased obviously. These characteristics remained unchanged even in a good solvent for the monomer, suggesting that a new aggregation structure for the porphyrin pendants fixed by the covalent bond was formed. According to the different dispersed states of the porphyrin monomer in water and DMSO, the porphyrin pendants should distribute randomly in the P-D polymer chains while having micro-blocky sequences in polymer P-W. The association behaviour between the copolymers and tetra(p-sulfonatophenyl)porphyrin, TSPP, bearing opposite charged substituents were studied by absorption and emission Spectra and further analyzed by the Benesi-Hildebrand and the Stern-Volmer methods. The results showed that relatively discrete porphyrin pendants in P-D formed a 1 : 1 stoichiometric complex with TSPP and both static and dynamic mechanisms were active in this quenching process, while the tightly associated porphyrin pendants in P-W interacted with TSPP as an entirety and static quenching was dominant in this process. This observation was in accordance with their sequential structure. The polymer P-W has a wider absorption range and higher absorption intensity in the long wavelength region than the porphyrin monomer, which can more efficiently absorb light to accomplish light harvesting in water.

Experimental observation of TiN12+ cluster and theoretical investigation of its stable and metastable isomers.

TiN n+ clusters were generated by laser ablation and analyzed experimentally by mass spectrometry. The results showed that the mass peak of the TiN12+ cluster is dominant in the spectrum. The TiN12+ cluster was further investigated by photodissociation experiments with 266, 532 and 1064 nm photons. Density functional calculations were conducted to investigate stable structures of TiN12+ and the corresponding neutral cluster, TiN12. The theoretical calculations found that the most stable structure of TiN12+ is Ti(N2)6+ with Oh symmetry. The calculated binding energy is in good agreement with that obtained from the photodissociation experiments. The most stable structure of neutral TiN12 is Ti(N2)6 with D3d symmetry. The Ti-N bond strengths are greater than 0.94 eV in both Ti(N2)6+ and its neutral counterpart. The interaction between Ti and N2 weakens the N-N bond significantly. For neutral TiN12, the Ti(N3)4 azide, the N5TiN7 sandwich structure and the N6TiN6 structure are much higher in energy than the Ti(N2)6 complex. The DFT calculations predicted that the decomposition of Ti(N3)4, N5TiN7, and N6TiN6 into a Ti atom and six N2 molecules can release energies of about 139, 857, and 978 kJ mol-1 respectively.