Dissociative Transition Research Articles

Photoionization and Dissociative Photoionization of Acetaldehyde in the 10.0-13.7 eV Range by Synchrotron Photoelectron Photoion Coincidence Spectroscopy.

Photoionization and dissociative photoionization of acetaldehyde (CH3CHO) in the 10.0-13.7 eV energy range are studied by using synchrotron radiation double imaging photoelectron photoion coincidence spectroscopy (i2PEPICO). The X2A' and A2A" electronic states of CH3CHO+ as well as the Franck-Condon gap region between these two states have been populated with several vibrational sequences and assigned in the high-resolution slow photoelectron spectrum (SPES). The adiabatic ionization energies (AIEs) of the X2A' and A2A" states are measured at 10.228±0.006 and 12.52±0.05 eV, respectively. The present results show that the X2A' state is a stable state while the A2A" state is fully dissociative to produce CH3CO+, CHO+ and CH4 + fragment ions. The 0 K appearance energies (AE0K) of CH3CO+ and CHO+ fragment ions are determined through the modeling of the breakdown diagram, i. e., AE0K(CH3CO+)=10.89±0.01 eV (including a reverse barrier of ~0.19 eV) and AE0K(CHO+)=11.54±0.05 eV. In addition, the dissociation mechanisms of CH3CHO+ including statistical dissociation, direct bond breaking and isomerization are discussed with the support of the calculated dissociation limits and transition state energies.

Computational (DFT) and Experimental (NMR) Study of the Chelation of an Iridium Hyperpolarization Transfer Catalyst by Amino Acids

AbstractNon‐hydrogenative Para‐Hydrogen Induced Hyperpolarization (nhPHIP) is a Nuclear Magnetic Resonance (NMR) hyperpolarization technique which has experimentally been used to analyze complex biological samples containing amino acids using the Ir‐IMes hyperpolarization transfer catalyst. A computational study based on Density Function Theory (DFT) was performed on all relevant stereoisomers of [Ir(H)2(IMes)(AA)(Py)] (with AA=glycine, alanine, valine; Py=pyridine), for which R/S chirality and orientation of the amino acid chelation (C/A) were considered. A total of 30 structures were calculated comprising of 6 stereoisomers for achiral glycine, and 12 stereoisomers for each of the chiral amino acids. The abundances derived from the DFT energies confirmed the trends observed in thermal (non‐hyperpolarized) NMR experiments. Additionally, theoretical calculations of electronic (Wiberg bond indices, Natural Bond Orders, Frontier Orbital Analysis), bond dissociation energies, transition states, and activation energies related to interconversion between binding modes, and steric factors (Solid angle) were performed to provide detailed explanations for NMR experimental observations.

Solvent and solvation effects on reactivities and mechanisms of phospho group transfers from phosphate and phosphinate esters to nucleophiles.

Organophosphorus esters fulfil many industrial, agricultural, and household roles. Nature has deployed phosphates and their related anhydrides as energy carriers and reservoirs, as constituents of genetic materials in the form of DNA and RNA, and as intermediates in key biochemical conversions. The transfer of the phosphoryl (PO3) group is thus a ubiquitous biological process that is involved in a variety of transformations at the cellular level such as bioenergy and signals transductions. Significant attention has been paid in the last seven decades to understanding the mechanisms of uncatalyzed (solution) chemistry of the phospho group transfer because of the notion that enzymes convert the dissociative transition state structures in the uncatalyzed reactions into associative ones in the biological processes. In this regard, it has also been proposed that the rate enhancements enacted by enzymes result from the desolvation of the ground state in the hydrophobic active site environments, although theoretical calculations seem to disagree with this position. As a result, some attention has been paid to the study of the effects of solvent change, from water to less polar solvents, in uncatalyzed phospho transfer reactions. Such changes have consequences on the stabilities of the ground and the transition states of reactions which affect reactivities and, sometimes, the mechanisms of reactions. This review seeks to collate and evaluate what is known about solvent effects in this domain, especially their effects on rates of reactions of different classes of organophosphorus esters. The outcome of this exercise shows that a systematized study of solvent effects needs to be undertaken to fully understand the physical organic chemistry of the transfer of phosphates and related molecules from aqueous to substantially hydrophobic environments, since significant knowledge gaps exist.

Enhanced dehalogenation of brominated DBPs by catalyzed electrolysis using Vitamin B12 modified electrodes: Kinetics, mechanisms, and mass balances

Vitamin B12 (VB12) modified electrodes were prepared for the electrocatalytic reductive debromination of tribromoacetic acid (TBAA). Under galvanostatic conditions set as 5 mmol/L VB12 loading, 20 mmol/L Na2SO4 as electrolyte, 10.0 mA/cm2 current density, pH 3, and 298 K, the degradation efficiency of 200 μg/L TBAA at the VB12 modified electrode could reach 99.9 % after 6 h. The debromination of TBAA followed the first-order kinetic model. The masses of carbon and bromine elements were conserved before and after the reaction, together with the qualitative analysis of the degradation products showed the likely degradation pathways as TBAA→dibromoacetic acid (DBAA)→monobromoacetic acid (MBAA)→acetic acid (AA). ESR detection and quenching experiments confirmed the role of atomic H* in TBAA debromination. In-situ Raman spectroscopy showed that the Co-Br bond was strongly enriched to the electrode surface, accelerating the electron transfer. The H2O dissociation performance and transition states searching catalyzed by VB12 were calculated by Density Functional Theory (DFT) and proved that the composite electrode can effectively promote atomic H* generation. Material characterization and electrochemical performance tests showed that the VB12 modified electrode had excellent stability and atomic H* catalytic activity. The electrocatalytic debromination of TBAA at VB12 modified electrodes mainly involves two mechanisms, direct reduction by electron transfer and indirect reduction by the strongly reducing atom H*. The results provide an efficient way to achieve safe removal of brominated DBPs from drinking water after chlorination and before human consumption.

Mechanical stabilization of a bacterial adhesion complex.

The adhesions between Gram-positive bacteria and their host are exposed to varying magnitudes of tensile forces. Here, using an ultra-stable magnetic-tweezer-based single-molecule approach, we show catch-bond kinetics of the prototypical adhesion complex of SD-repeat protein G to a peptide from fibrinogen β, over a physiologically important force range from piconewton (pN) to tens of pNs which was not technologically accessible to previous studies. The dissociation transition pathway is determined as the unbinding of a critical β-strand peptide (“latch” strand of SdrG that secures the entire adhesion complex) away from its binding cleft, leading to the dissociation of the Fgβ ligand. At 37 oC, the lifetime of the complex exponentially increases from seconds at several pN to ∼1000 s as force reaches 30 pN, leading to mechanical stabilization of the adhesion. Similar mechanical stabilization behavior is also observed in several homologous adhesions, suggesting the generality of catch-bond kinetics in such bacterial adhesions. We reason that such mechanical stabilization allows dynamic bacterial adhesion under low mechanical stress and stable adhesion as force increases, which confers multiple advantages in the pathogenesis and adaptation of bacteria.

Penetrative Superplumes in the Mantle of Large Super‐Earth Planets: A Possible Mechanism for Active Tectonics in the Massive Super‐Earths

AbstractRecent theoretical studies suggest that the physical and rheological mantle properties in massive rocky planets fall outside conventional behaviors inferred for mantle properties at the Earth's mantle pressures. The vacancy diffusion occurring at low pressures is assumed to be followed by interstitial diffusion above ∼0.1 TPa resulting in viscosity reduction at higher pressures. In addition, the dissociation transition of MgSiO3 post‐perovskite (pPv) into new phases of minerals at 0.9 and 2.1 TPa, both with large negative Clapeyron slopes, has further impact(s) on the style of circulation in the mantle of super‐Earth planets. Further, the electronic contribution of conductivity increases exponentially with temperature at temperatures ∼5000 K and higher. We employ 3D‐controlled volume spherical convection models to explore the style of mantle circulation in large rocky super‐Earth planets with different core temperatures. Our numerical models resembling a GJ 876 d size super‐Earth reveal that due to the buffering influence of the pPv‐dissociation transition at ∼0.9 TPa, for deep mantle viscosities lower than ∼1022 Pa.s a small‐scale convective layer may develop at the top of the core‐mantle boundary (CMB). Penetrative superplumes originating from deep mantle‐layered regions can maintain the heat flux from the CMB required for the planet's geodynamo, and can survive for billions of years reaching shallow depths of the mantle without significant lateral migration. The strength of the focused penetrative superplumes that can potentially sustain surface volcanism and plate tectonics is enhanced with increasing CMB temperature, but diminished by higher rates of internal heating.

Dissociative Transition State in Hepatitis Delta Virus Ribozyme Catalysis.

Ribonucleases and small nucleolytic ribozymes are both able to catalyze RNA strand cleavage through 2'-O-transphosphorylation, provoking the question of whether protein and RNA enzymes facilitate mechanisms that pass through the same or distinct transition states. Here, we report the primary and secondary 18O kinetic isotope effects for hepatitis delta virus ribozyme catalysis that reveal a dissociative, metaphosphate-like transition state in stark contrast to the late, associative transition states observed for reactions catalyzed by specific base, Zn2+ ions, or ribonuclease A. This new information provides evidence for a discrete ribozyme active site design that modulates the RNA cleavage pathway to pass through an altered transition state.

Kinetics and Pathway Analysis Reveals the Mechanism of a Homogeneous PNP-Iron-Catalyzed Nitrile Hydrogenation.

Nitrile hydrogenation via the in situ-generated PNP-FeII(H)2CO (1) catalyst leads to a previously inexplicable loss of mass balance. Reaction kinetics, reaction progress analysis, in situ pressure nuclear magnetic resonance, and X-ray diffraction analyses reveal a mechanism comprising reversible imine self-condensation and amine-imine condensation cascades that yield >95% primary amine. Imine self-condensation has never been reported in a nitrile hydrogenation mechanism. The reaction is first order in catalyst and hydrogen and zero order in benzonitrile when using 2-propanol as the solvent. Variable-temperature analysis revealed values for ΔG298K⧧ (79.6 ± 26.8 kJ mol-1), ΔH⧧ (90.7 ± 9.7 kJ mol-1), and ΔS⧧ (37 ± 28 J mol-1 K-1), consistent with a solvent-mediated proton-shuttled dissociative transition state. This work provides a basis for future catalyst optimization and essential data for the design of continuous reactors with earth-abundant catalysts.

Prediction model on hydrolysis kinetics of phthalate monoester: A density functional theory study

As primary degradation products of phthalate esters, phthalate monoesters (MPEs) have been widely detected in various aquatic environments and drawn growing toxicological concerns. Hydrolysis kinetics that is of importance for assessing environmental persistence of chemicals remain elusive for MPEs. Herein, kinetics of base-catalyzed and neutral hydrolysis for 18 MPEs with different leaving groups was investigated by density functional theory calculation. Results indicate that MPEs with leaving groups having pKa of <10 prefer dissociative transition states. MPEs are more persistent than their parents, and their hydrolysis half-lives were calculated to vary from 3.4 min to 79.2 years (pH = 7–9). A quantitative structure-activity relationship model was developed for predicting the hydrolysis kinetics parameters. It was found that pKa of the leaving groups and electronegativity of the MPEs are key factors determining the hydrolysis kinetics. This work may lay a theoretical foundation for better understanding the chemical process that governs MPE persistence in aquatic environments.

Phosphoenolpyruvate Mutase-Catalyzed C-P Bond Formation: Mechanistic Ambiguities and Opportunities.

Phosphonates are produced across all domains of life and used widely in medicine and agriculture. Biosynthesis almost universally originates from the enzyme phosphoenolpyruvate mutase (Ppm), EC 5.4.2.9, which catalyzes O-P bond cleavage in phosphoenolpyruvate (PEP) and forms a high energy C-P bond in phosphonopyruvate (PnPy). Mechanistic scrutiny of this unusual intramolecular O-to-C phosphoryl transfer began with the discovery of Ppm in 1988 and concluded in 2008 with computational evidence supporting a concerted phosphoryl transfer via a dissociative metaphosphate-like transition state. This mechanism deviates from the standard 'in-line attack' paradigm for enzymatic phosphoryl transfer that typically involves a phosphoryl-enzyme intermediate, but definitive evidence is sparse. Here we review the experimental evidence leading to our current mechanistic understanding and highlight the roles of previously underappreciated conserved active site residues. We then identify remaining opportunities to evaluate overlooked residues and unexamined substrates/inhibitors.

Mechanical Stabilization of a Bacterial Adhesion Complex.

The adhesions between Gram-positive bacteria and their hosts are exposed to varying magnitudes of tensile forces. Here, using an ultrastable magnetic tweezer-based single-molecule approach, we show the catch-bond kinetics of the prototypical adhesion complex of SD-repeat protein G (SdrG) to a peptide from fibrinogen β (Fgβ) over a physiologically important force range from piconewton (pN) to tens of pN, which was not technologically accessible to previous studies. At 37 °C, the lifetime of the complex exponentially increases from seconds at several pN to ∼1000 s as the force reaches 30 pN, leading to mechanical stabilization of the adhesion. The dissociation transition pathway is determined as the unbinding of a critical β-strand peptide (“latch” strand of SdrG that secures the entire adhesion complex) away from its binding cleft, leading to the dissociation of the Fgβ ligand. Similar mechanical stabilization behavior is also observed in several homologous adhesions, suggesting the generality of catch-bond kinetics in such bacterial adhesions. We reason that such mechanical stabilization confers multiple advantages in the pathogenesis and adaptation of bacteria.

Atomically Dispersed CoN3 C1 -TeN1 C3 Diatomic Sites Anchored in N-Doped Carbon as Efficient Bifunctional Catalyst for Synergistic Electrocatalytic Hydrogen Evolution and Oxygen Reduction.

A encapsulation-adsorption-pyrolysis strategy for the construction of atomically dispersed Co-Te diatomic sites (DASs) that are anchored in N-doped carbon is reported as an efficient bifunctional catalyst for electrocatalytic hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). The as-constructed catalyst shows the stable CoN3 C1 -TeN1 C3 coordination structure before and after HER and ORR. The *OOH/*H intermediate species are captured by in situ Raman and in situ attenuated total reflectance-surface enhanced infrared absorption spectroscopy, indicating that the reactant O2 /H2 O molecule has a strong interaction with the Co site, revealing that Coδ+ is an effective active site. Theoretical calculations show that the Coδ+ has adsorption-activation function and the neighboring Teδ+ acts as an electron donor adjusting the electronic structure of Coδ+ , promoting the dissociation of H2 O molecules and the adsorption of H and oxygen-containing intermediates in HER and ORR. In the meanwhile, the nearest C atom around Co also profoundly affects the adsorption of H atoms. This results in the weakening of the OH adsorption and enhancement of H adsorption, as well as the more stable water molecule dissociation transition state, thus significantly boosting ORR and HER performance.

A spin promotion effect in catalytic ammonia synthesis

The need for efficient ammonia synthesis is as urgent as ever. Over the past two decades, many attempts to find new catalysts for ammonia synthesis at mild conditions have been reported and, in particular, many new promoters of the catalytic rate have been introduced beyond the traditional K and Cs oxides. Herein, we provide an overview of recent experimental results for non-traditional promoters and develop a comprehensive model to explain how they work. The model has two components. First, we establish what is the most likely structure of the active sites in the presence of the different promoters. We then show that there are two effects dictating the catalytic activity. One is an electrostatic interaction between the adsorbed promoter and the N-N dissociation transition state. In addition, we identify a new promoter effect for magnetic catalysts giving rise to an anomalously large lowering of the activation energy opening the possibility of finding new ammonia synthesis catalysts.

Collision-induced dissociation of homodimeric and heterodimeric radical cations of 9-methylguanine and 9-methyl-8-oxoguanine: correlation between intra-base pair proton transfer originating from the N1-H at a Watson-Crick edge and non-statistical dissociation.

It has been shown previously in protonated, deprotonated and ionized guanine-cytosine base pairs that intra-base pair proton transfer from the N1-H at the Watson-Crick edge of guanine to the complementary nucleobase prompts non-statistical dissociation of the base-pair system, and the dissociation of a proton-transferred base-pair structure is kinetically more favored than that of the starting, conventional base-pair structure. However, the fundamental chemistry underlying this anomalous and intriguing kinetics has not been completely revealed, which warrants the examination of more base-pair systems in different structural contexts in order to derive a generalized base-pair structure-kinetics correlation. The purpose of the present work is to expand the investigation to the non-canonical homodimeric and heterodimeric radical cations of 9-methylguanine (9MG) and 9-methyl-8-oxoguanine (9MOG), i.e., [9MG·9MG]˙+, [9MOG·9MG]˙+ and [9MOG·9MOG]˙+. Experimentally, collision-induced dissociation tandem mass spectrometry coupled with an electrospray ionization (ESI) source was used for the formation of base-pair radical cations, followed by detection of dissociation product ions and cross sections in the collisions with Xe gas under single ion-molecule collision conditions and as a function of the center-of-mass collision energy. Computationally, density functional theory and coupled cluster theory were used to calculate and identify probable base-pair structures and intra-base pair proton transfer and hydrogen transfer reactions, followed by kinetics modeling to explore the properties of dissociation transition states and kinetic factors. The significance of this work is twofold: it provides insight into base-pair opening kinetics in three biologically-important, non-canonical systems upon oxidative and ionization damage; and it links non-statistical dissociation to intra-base pair proton-transfer originating from the N1-H at the Watson-Crick edge of 8-oxoguanine, enhancing understanding towards the base-pair fragmentation assisted by proton transfer.

Kinetic and Computational Analysis of CO Substitution in a Dinuclear Osmium Carbonyl Complex: Intersection between Dissociative and Dissociative-Interchange Mechanisms.

The mechanism for the CO substitution reaction involving the diosmium carbonyl sawhorse complex Os2(μ-O2CH)2(CO)6, which contains an Os-Os single bond, two axial CO ligands, and four equatorial CO ligands, was investigated experimentally and theoretically. Kinetic measurements show 13CO axial substitution proceeding by a dissociative reaction that is first-order in the complex and zero-order in 13CO but with an unexpectedly negative entropy of activation. The corresponding electronic structure calculations yield an enthalpy of activation for axial CO dissociation that is much larger than that determined by the kinetic experiments, but in agreement with the complex's stability with respect to CO loss. Additional calculations yield a dissociative interchange transition state whose free energy, enthalpy, and entropy of activation are in good agreement with those obtained from the kinetic measurements for the apparently dissociative substitution. These results point to an exchange reaction mechanism that is surprisingly close to the poorly understood transition from a dissociative mechanism with a CO-loss intermediate to a dissociative interchange mechanism with a transition state involving both the entering and the leaving COs. The key to explain these findings is provided by the vibrational analysis, which shows very low energy wagging motions for the axial COs. Thus, the incoming CO only displaces the outgoing CO when the complex has an outgoing CO near the wag's turning point. This dissociative interchange mechanism predicted by the calculation explains the unexpected combination of kinetics and stability characteristics. Kinetics reveals that the reaction is first-order in the Os dimer with a negative Eyring entropy, while a stability study shows that the Os dimer's decomposition rate is several orders of magnitude slower than CO exchange.

Steady-State Dynamics of Exclusion Process with Local Reversible Association of Particles

Many biological processes are supported by special molecules, called motor proteins or molecular motors, that transport cellular cargoes along linear protein filaments and can reversibly associate to their tracks. Stimulated by these observations, we developed a theoretical model for collective dynamics of biological molecular motors that accounts for local association/dissociation events. In our approach, the particles interacting only via exclusion move along a lattice in the preferred direction, while the reversible associations are allowed at the specific site far away from the boundaries. Considering the association/dissociation site as a local defect, the inhomogeneous system is approximated as two coupled homogeneous sub-lattices. This allows us to obtain a full description of stationary dynamics in the system. It is found that the number and nature of steady-state phases strongly depend on the values of association and dissociation transition rates. Microscopic arguments to explain these observations, as well as biological implications, are also discussed. Theoretical predictions agree well with extensive Monte Carlo computer simulations.

Focused Penetrative Plumes: A Possible Consequence of the Dissociation Transition of Post‐Perovskite at ∼0.9 TPa in Massive Rocky Super‐Earths

AbstractBased on the standard view of depth‐increasing viscosity in the mantle of rocky planets, convection in the deep mantle of these planets is expected to become less likely as the size of planet increases. However, a recent theoretical study suggests that above 100 GPa pressure, the mantle viscosity can instead decrease with pressure at higher depths. We explore the potential impact of this type of viscosity structure on the nature of mantle convection in a super‐Earth planet of size GJ 876 d with a mass of ∼7.33 M⊕ (M⊕: Earth's mass). The pressures at the bottom of the mantle of GJ 876 d allow MgSiO3 post‐perovskite to dissociate into magnesium oxide (MgO) and relatively highly oxidized magnesium silicate MgSi2O5 at 0.9 TPa with highly negative Clapeyron slope. Our 3D‐spherical numerical model results suggest that for sufficiently low values of viscosity at the transition depth, a vigorous layered mantle convection may develop at the bottom of GJ 876 d‐size super‐Earth. Focused penetrative plumes originating from the deep mantle layered region can survive and stabilize over very long geological timescales and reach to the surface, which may induce unique circumstances of volcanism and tectonic activity.

Characterization and development of new anti‐toxins targeting Clostridium difficile

Clostridium difficile is a bacterial pathogen that causes serious and potentially fatal inflammatory disease of the colon. One strategy for targeting C. difficile infection is to development treatments that target bacterial virulence. Anti-virulence strategies to treat C. difficile infection are attractive options to antibiotic therapy because the human microbiota is spared and damage to host tissue is minimized. The Tcd toxins, TcdA and TcdB, are the major virulence factors produced by C. difficile and mediate host cytotoxicity by glucosylating and inactivating Rho family GTPases using UDP-glucose as a glucosyl donor. Glucosylation of Rho GTPases leads to cell rounding and eventually cell death. We used kinetic isotope effects (KIEs) and transition state theory to solve the transition state structure of the glycosyltransferase domain of TcdB (TcdB-GTD). This permits the design of transition state analogues which are potent enzyme inhibitors. KIEs compare the chemical steps of enzymatic reaction rates with isotopically labeled substrates and unlabeled substrates and provide critical information on the catalytic mechanism, bond lengths and geometry at the transition state. KIE analysis on the glycohydrolase reaction of TcdB-GTD supported the formation of a late, dissociative glucocation-like transition state where positive charge develops on the anomeric carbon. Glucocationic transition states are best mimicked by cationic nitrogen groups at or near the anomeric carbon. Iminosugars contain a nitrogen atom in the sugar ring in place of oxygen and mimic glucocationic transition states. We tested iminosugars for inhibition of TcdB-GTD and TcdA-GTD. Isofagomine (Ki TcdB-GTD = 1.4 µM and Ki TcdA-GTD = 0.24 µM) and noeuromycin (Ki TcdB-GTD = 10.6 µM and Ki TcdA-GTD = 4.7 µM) were the most potent iminosugars identified. Isofagomine and noeuromycin were shown to exhibit an uncompetitive mechanism of inhibition suggesting that both iminosugars act by binding to the enzyme-UDP complex. Isothermal titration calorimetry experiments with TcdB-GTD supported this finding where isofagomine and noeuromycin could only bind to TcdB-GTD in the presence of UDP with Kd = 0.133 µM and Kd = 0.4 µM respectively. In addition, the X-ray crystal structure of TcdB-GTD in complex with either Isofagomine or noeuromycin with UDP revealed the formation of an ion-pair interaction between the cationic nitrogen atom of the iminosugar and the β-phosphate of UDP. Finally, we confirmed that isofagomine and noeuromycin could prevent Tcd toxin-induced cytotoxicity of IMR90 cells. Firstly, both iminosugars were able to prevent Tcd toxin-induced cell rounding as visualized by light microscopy. Secondly, Western blot analysis of Tcd toxin treated IMR90 cell lysates revealed that both isofagomine and noeuromycin could prevent glucosylation of Rac1 by Tcd toxin. Isofagomine and noeuromycin are effective in preventing Tcd toxin action. Transition state analogues against TcdA and TcdB have the potential to be used as treatments for C. difficile infections.

First-principles equation of state of liquid hydrogen and dissociative transition

The properties of dense hot hydrogen, in particular the phase transition between the molecular insulating and atomic conductive states, are important in the fields of astrophysics and high-pressure physics. Previous ab initio calculations suggested the metallization in liquid hydrogen, accompanied by dissociation, is a first-order phase transition and ends at a critical point in temperature range between 1500 and 2000 K and pressure close to 100 GPa. Using density functional theoretical molecular dynamics simulations, we report a first-principles equation of state of hydrogen that covers dissociation transition conditions at densities ranging from 0.20 to 1.00 g/cc and temperatures of 600–9000 K. Our results clearly indicate that a drop in pressure and a sharp structural change still occur as the system transforms from a diatomic to monoatomic phase at temperatures above 2000 K, and support the first-order phase transition in liquid hydrogen would end in the temperature about 4500 K.

Evidence for a New Extended Solid of Nitrogen**Supported by the National Natural Science Foundation of China (Grant No. 11774247) and the Chinese Academy of Sciences (Grant No. 2019-SSRF-PT-009588).

A new extended solid nitrogen, referred to as post-layered-polymeric nitrogen (PLP-N, or Panda-N), was observed by further heating the layered-polymeric nitrogen (LP-N) to above 2300 K at 161 GPa. The new phase is found to be very optically transparent and exhibits ultra-large d-spacings ranging from 2.8 to 4.9 Å at 172 GPa, suggesting a lower-symmetry large-unit-cell 2D chain-like or 0D cluster-type structure with wide bandgap. However, the observed x-ray diffraction pattern and Raman scattering data cannot match any predicted structures in the published literature. This finding further complicates the phase diagram of nitrogen and also highlights the path dependence of the high-pressure dissociative transition in nitrogen. In addition, the phase transition from cubic gauche nitrogen (cg-N) to LP-N is observed at 157 GPa and 2000 K.