Cationic Rearrangement Research Articles

Exploring the orthorhombic-tetragonal phase transition in CH3NH3PbI3: the role of atom kinetics.

Methylammonium lead tri-iodide is a polymorphic material with two temperature-induced phase transitions at 165 K and 327 K, accompanied by an orthorhombic-to-tetragonal and a tetragonal-to-cubic lattice modification. Understanding the origins of these transitions as well as their implications on the crystal structure of the material is fundamental for its technological optimization. Here, we use the density functional theory along with ab initio molecular dynamics to study the low-temperature phase transition of CH3NH3PbI3. Considering two kinetically robust models for the orthorhombic and the tetragonal phase, we show that the vibrational features of the material can be strongly affected by the orientations of the methylammonium ions. We argue that the orthorhombic-tetragonal transition is characterized by a partial rearrangement of the organic cations that locally relaxes the stress induced by the thermal movement of atoms. We finally propose a macroscopic model for the tetragonal phase that consists of rotated noncentrosymmetric domains, where the methylammonium ions are quasi-two-dimensionally confined around the a-b crystallographic plane.

Isolation and chemistry of clerodane diterpenes from Dodonaea ceratocarpa

A new clerodane was isolated from Dodonaea ceratocarpa and obtained in appreciable quantities (0.5% w/w plant). A variety of reactions on this clerodane provided access to a range of new derivatives, including cationic skeletal rearrangements and reduction reactions with lanthanide salt additives.

Structural transformations in Pb1 − xBi4 + xTi4 − xMnxO15(x = 0.2 and 0.4): a Raman scattering study

The temperature evolution of the Raman scattering of Pb1 − xBi4 + xTi4 − xMnxO15 (x = 0.2 and 0.4) on cooling from 850 to 140–130 K was studied in order to elucidate the effect of B-site Mn3+ doping accompanied by the increase in the A-site Bi3+ content on the structural transformations in four-layer Aurivillius-type compound (An − 1Bi2BnO3n + 3, n = 4). The ferroelectric–paraelectric transition (Tc ~800 K for x = 0.2 and ~765 K for x = 0.4) is well mirrored by the Raman scattering near 60 cm−1 arising from the so-called rigid layer mode. The temperature dependence of the phonon mode near 42 cm−1 arising from A-site Pb/Bi displacements indicates a second structural transformation near 570 K and 400 K for x = 0.2 and 0.4, respectively, similar to that observed for Mn-free PbBi4Ti4O15. This structural alteration resembles a thermodynamically second-order phase transition for all three compounds and the critical temperature (Ta) decreases with the increase in the A-site Bi3+ amount, related to the heterovalent substitution (Mn3+ for Ti4+) on the B site; Ta = 600, 570, 400 K for x = 0, 0.2, 0.4, respectively. The BO6 tilting and bending mode near 220 cm−1 also shows an anomaly at Ta, and, thus, this second structural transformation was attributed to subtle rearrangements of A-site cations accompanied by octahedral BO6 tilting in the perovskite-like blocks. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.

The Cytochrome P450-Catalyzed Oxidative Rearrangement in the Final Step of Pentalenolactone Biosynthesis: Substrate Structure Determines Mechanism.

The final step in the biosynthesis of the sesquiterpenoid antibiotic pentalenolactone (1) is the highly unusual cytochrome P450-catalyzed, oxidative rearrangement of pentalenolactone F (2), involving the transient generation and rearrangement of a neopentyl cation. In Streptomyces arenae this reaction is catalyzed by CYP161C2 (PntM), with highly conserved orthologs being present in at least 10 other Actinomycetes. Crystal structures of substrate-free PntM, as well as PntM with bound substrate 2, product 1, and substrate analogue 6,7-dihydropentalenolactone F (7) revealed interactions of bound ligand with three residues, F232, M77, and M81 that are unique to PntM and its orthologs and absent from essentially all other P450s. Site-directed mutagenesis, ligand-binding measurements, steady-state kinetics, and reaction product profiles established there is no special stabilization of reactive cationic intermediates by these side chains. Reduced substrate analogue 7 did not undergo either oxidative rearrangement or simple hydroxylation, suggesting that the C1 carbocation is not anchimerically stabilized by the 6,7-double bond of 2. The crystal structures also revealed plausible proton relay networks likely involved in the generation of the key characteristic P450 oxidizing species, Compound I, and in mediating stereospecific deprotonation of H-3re of the substrate. We conclude that the unusual carbocation intermediate results from outer shell electron transfer from the transiently generated C1 radical to the tightly paired heme-•Fe(3+)-OH radical species. The oxidative electron transfer is kinetically dominant as a result of the unusually strong steric barrier to oxygen rebound to the neopentyl center C-1si, which is flanked on each neighboring carbon by syn-axial substituents.

Routes of phlogopite weathering by three fungal strains

Fungi dissolve soil minerals by acidification and mechanical disruption. Dissolution may occur at the microscale (contact between fungus and mineral) and medium scale (entire mineral grains). Mineral weathering by fungi and other microorganisms is thought to be of significant global contribution, perhaps producing specific weathering signatures. We report fungal dissolution of phlogopite mica in experiments with three fungal strains (Alternaria tenuissima, Cladosporium cladosporioides, Stilbella sp.) on solid medium for 30 days at 21 °C and 96–100% relative humidity. The study used variable-pressure SEM-EDS equipped with charge contrast imaging. Statistical analysis of the results discriminated between the weathering activities of the three fungal species, which increased from Stilbella sp. to C. cladosporioides to A. tenuissima, in agreement with the respective decreasing pH in the media (6.4, 5.8, 5.2 ± 0.03). Phlogopite weathering features were irregular and variable, apparently not caused by direct contact with fungal hyphae. EDS values indicated two or more dissolution mechanisms, one of them suggesting cation rearrangement in the mica towards formation of Al-rich smectite. Intimate fungus-mineral interaction was observed, and the lack of observable dissolution traces from such contact interaction is interpreted as the result of effacing by the more intense acid leaching operating at larger scale.

Synthesis of a Small-Molecule Library with Skeletal Diversity from Hemslecin A via the Reaction-Discovery Strategy.

An efficient reaction tool box was developed for the synthesis of skeletally diverse and stereochemically complex templates for a small-molecule library based on the common synthon Q, which was prepared from hemslecin A in four steps. The reaction tool box comprises three acid-promoted rearrangements: semipinacol, Wagner-Meerwein, and cyclopropylmethyl cation rearrangements. More importantly, a Mn-mediated C-H oxidation was developed to achieve a high level of complexity, which provides a new entry for C-H functionalization of inert angular methyl groups in the chemistry of triterpenes. Our reaction-discovery strategy based on hemslecin A provides a basis for the inherent chemistry of triterpenes and could be applied for the further transformation of triterpenes.

Ion irradiation-induced crystal structure changes in inverse spinel MgIn2O4

400keV Ne and 200keV He ion irradiations were performed on fully inverse MgIn2O4 samples at cryogenic temperature (~77K), in order to examine the influence of radiation-induced cation disordering on crystal structure. In the case of MgIn2O4 samples irradiated with Ne ions to a peak displacement damage dose of 4 displacements per atom (dpa), a spinel-to-rocksalt phase transformation was observed. By contrast, for MgIn2O4 samples irradiated with He ions to a peak displacement damage dose of 5dpa, the only observed structural effect involved cation rearrangements from an inverse to a “random” spinel structure.

Response of Methylammonium Lead Iodide to External Stimuli and Caloric Effects from Molecular Dynamics Simulations

The power conversion efficiency for solar cells fabricated using organometal halide perovskites (OMHPs) has risen to more than 20% in a short span of time, making OMHPs promising solar materials for harnessing energy from sunlight. The hybrid perovskite architecture that consists of organic molecular cations and an inorganic lattice could also potentially serve as a robust platform for materials design to realize functionalities beyond photovoltaic applications. Taking methylammonium lead iodide (MAPbI3) as an example, we explore the response of organometal halide perovskites to various stimuli, using all-atom molecular dynamics simulations with a first-principles-based interatomic potential. We find that a large electric field is necessary to introduce a sizable molecular ordering at room temperature in unstrained MAPbI3. Molecular dipoles in epitaxially strained MAPbI3 are more susceptible to an electric field. We also report various caloric effects in MAPbI3. The adiabatic thermal change is estimated directly by introducing different driving fields in the simulations. We find that MAPbI3 exhibits both electrocaloric and mechanocaloric effects at room temperature. Local structural analysis reveals that the rearrangement of molecular cations in response to electric and stress fields is responsible for the caloric effects. The enhancement of caloric response could be realized through strain engineering and chemical doping.

Reaction Mechanism of Alluaudite Sodium Iron Sulphate As High Energy Density Cathode Material for Na-Ion Battery

The sodium-ion battery system is now growing as a potential alternative to the lithium-ion battery for large energy storage systems. The ubiquitous element Na allows us to realize stable production of large scale storages for broader applications. Electrode materials for the Na system is under intensive development by many battery researchers to make properties of the Na-ion battery comparable to those of Li-ion battery. In such pursuit we developed an ideal cathode material with abundant constituents sodium, iron and sulfate ions.1 Alluaidite-type sodium iron sulfate Na2+2x Fe2–x (SO4)3 exhibits high energy density due to high working potential of 3.8 V vs. Na/Na+. Large tunnel structure in the alluaudite framework derives high Na-ion conductivity. Thus Na2+2x Fe2–x (SO4)3 electrode allows high-rate battery operation.In this poster, we present detailed reaction mechanisms of Na2+2x Fe2–x (SO4)3 for Na batteries by means of crystal structural characterizations. As mentioned in our first report of this system, Na2+2x Fe2–x (SO4)3 (NFS) electrode undergoes irreversible change during first charing of NFS/Na half cells. Subsequent discharging and charging cycles are reversible. We carried out ex situ structural analysis to understand about the irreversible and reversible changes during electrochemical reaction. Crystal structure of NFS powder (x ~ 0.28) was successfully analyzed by the alluaudite structure model with C2/c symmetry.2 Ex situ structural analysis of NFS electrodes were carried out by powder X-ray diffraction using synchrotron radiation at Photon Factory in KEK, Japan. Electrochemically reacted samples were also successfully analyzed by using a monoclinic lattice with C2/c symmetry, which is isomorphic with the pristine lattice. Volume change is less than 5% in whole reaction range. The refined structure shows that the desodiation/resodiation reactions are essentially topochemical, while partially irreversible cation rearrangement is observed. One of the Na sites, Na3, is preferentially extracted due to higher site-potential than the other two sites. This model is consistent with computational evaluation of relative defect formation energy at the Na sites.1 Subsequent extraction sequence shows complex behaviors involving Fe migrations. [1] P. Barpanda, G. Oyama, S. Nishimura, S.-C. Chung, and A. Yamada, Nat. Commun., 5:4358, 2014. [2] G. Oyama, S. Nishimura, Y. Suzuki, M. Okubo, and A. Yamada. ChemElectroChem, 2(7):1019–1023, 2015. [3] G. Oyama, O. Pecher, K. J. Griffith, S. Nishimura, R. Pigliapochi, C. P. Grey and A. Yamada, in preparation. Figure 1

Low-Temperature Cationic Rearrangement in a Bulk Metal Oxide.

Cationic rearrangement is a compelling strategy for producing desirable physical properties by atomic-scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn2 FeMoO6 at unparalleled low temperatures of 150-300 (o) C. The irreversible ionic motion at ambient pressure, as evidenced by real-time powder synchrotron X-ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni3 TeO6 -type to an ordered-ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures.

Low‐Temperature Cationic Rearrangement in a Bulk Metal Oxide

AbstractCationic rearrangement is a compelling strategy for producing desirable physical properties by atomic‐scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn2FeMoO6 at unparalleled low temperatures of 150–300 oC. The irreversible ionic motion at ambient pressure, as evidenced by real‐time powder synchrotron X‐ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni3TeO6–type to an ordered‐ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures.

Unusual Rearrangements of Radical Cations: The Role of Vibronic Coupling.

Radicalcations often undergo very unexpected rearrangements. Three examples of such rearrangements are given, and it is shown how vibronic coupling between the ground and low-lying excited states may cause certain bonds that are quite solid in the neutral molecules to become so weak that they break spontaneously, even though the bond order does not change (or changes very little) on ionization. In radical cations where spin and charge are delocalized over two similar halves, vibronic coupling can lead to localization of spin and charge, which may greatly affect the reactivity. Finally, it is shown how vibronic coupling can lead to avoidance of state crossings, a feature that appears very frequently in rearrangements of radical cations.

Phase structure evolution and chemical durability studies of Ce-doped zirconolite–pyrochlore synroc for radioactive waste storage

The Ca1−x Zr1−x Ce2x Ti2O7+δ (0 ≤ x ≤ 0.4) series is synthesized via chemical routes, with microstructure/phase structure characterized by XPS, XRD, HRTEM, and SEM as well. The partial reduction of Ce4+ in Ce3+ in Ca1−x Zr1−x Ce2x Ti2O7+δ ceramics is observed from the Ce 3d XPS spectra. The XRD investigations and Rietveld refinements show that there are three crystal structures, namely monoclinic zirconolite-2M or zirconolite-4M, tetragonal perovskite, and cubic pyrochlore. The Ca and Zr sites of zirconolite structure are occupied by Ce3+ and Ce4+ ions, respectively. The zirconolite-2M → zirconolite-4M → pyrochlore transformation is based on the extent of Ce3+ and Ce4+ substitution. The zirconolite-4M phase tends to form at slightly higher substitution levels of Ce3+ and Ce4+ ions. At high substitution levels of Ce3+ and Ce4+ ions, cation rearrangement occurs resulting in the formation of pyrochlore structure. The chemical durability test is measured by the ASTM Product Consistency Test leaching method, and the normalized mass loss (NL Ce) of Ce in the Ca1−x Zr1−x Ce2x Ti2O7+δ ceramics is shown in the order of 10−6–10−7 g/m2.

ChemInform Abstract: Lewis Acid Catalyzed Synthesis of Allocolchicinoids.

AbstractA three‐step access to (biphenyl‐2‐yl)prop‐2‐en‐1‐ones is described which serve as substrates for the cyclization to allocolchicinoids.

Acid-promoted bicyclization of arylacetylenes to benzobicyclo[3.2.1]octanes through cationic rearrangements.

Acid-promoted efficient, site- and stereo-selective bicyclization of alkynes to polycyclic compounds (benzobicyclo[3.2.1]octanes) was realized with atom- and step-economy. The reaction proceeded through two C-C bonds formed on remote alkyl C-H bonds via twice long-distance cationic rearrangement.

Enhanced Magnetic Properties of Mn–Ni Codoped Cobalt Ferrite Nanoparticles Corroborated with Microstructural Analysis

Magnetic nanoparticles including ferrites are very potential candidates for its versatile application in the fields of engineering, bio medicine and electronics. Here we report the enhancement of magnetic properties of Mn-Ni co-doped cobalt ferrite nanoparticles which have been synthesized using coprecipitation technique. TEM study depicts the single phase nanocrystalline ferrite formation. Cation distribution between the tetrahedral and octahedral sites changes due to doping which is responsible for enhancement of magnetic properties. An exhaustive Rietveld analysis confirmed the rearrangement of the cations between the tetrahedral and octahedral sites. Also the results suggests strengthening of A-B and A-A interactions while weakening of B-B interaction for doped sample. ``Law of Approach'' technique is found to be useful and utilized to extract the detail of magnetic parameters like anisotropy constant, anisotropy field, coercivity.

Structural, thermal, dielectric and phonon properties of perovskite-like imidazolium magnesium formate.

We report the synthesis and characterisation of a magnesium formate framework templated by protonated imidazole. Single-crystal X-ray diffraction data showed that this compound crystallizes in the monoclinic structure in the P21/n space group with lattice parameters a = 12.1246(4) Å, b = 12.2087(5) Å, c = 12.4991(4) Å and β = 91.39(1)°. The antiparallel arrangement of the dipole moments associated with imidazolium cations suggests the antiferroelectric character of the room-temperature phase. The studied compound undergoes a structural phase transition at 451 K associated with a halving of the c lattice parameter and the disappearance of the antiferroelectric order. The monoclinic symmetry is preserved and the new metrics are a = 12.261(7) Å, b = 12.290(4) Å, c = 6.280(4) Å, and β = 90.62(5)°. Raman and IR data are consistent with the X-ray diffraction data. They also indicate that the disorder of imidazolium cations plays a significant role in the mechanism of the phase transition. Dielectric data show that the phase transition is associated with a relaxor nature of electric ordering. We also report high-pressure Raman scattering studies of this compound that revealed the presence of two pressure-induced phase transitions near 3 and 7 GPa. The first transition is most likely associated with a rearrangement of the imidazolium cations without any significant distortion of these cations and the magnesium formate framework, whereas the second transition leads to strong distortion of both the framework and imidazolium cations. High-pressure data also show that imidazolium magnesium formate does not show any signs of amorphization up to 11.4 GPa.

The cubyl cation rearrangements.

Born-Oppenheimer molecular dynamics simulations and high-level ab initio computations predict that the cage-opening rearrangement of the cubyl cation to the 7H(+)-pentalenyl cation is feasible in the gas phase. The rate-determining step is the formation of the cuneyl cation with an activation barrier of 25.3 kcal mol(-1) at the CCSD(T)/def2-TZVP//MP2/def2-TZVP level. Thus, the cubyl cation is kinetically stable enough to be formed and trapped at moderate temperatures, but it may be rearranged at higher temperatures.

Towards pi-extended cycloparaphenylenes as seeds for CNT growth: investigating strain relieving ring-openings and rearrangements.

Despite significant multidisciplinary effort over many years, the preparation of uniform carbon nanotubes (CNTs) is still an unsolved problem in the scientific community. This inaccessibility hampers the commercial use of CNTs in electronic devices due to the sensitive connection between their electronic properties and molecular structure. The [n]cycloparaphenylenes ([n]CPPs), the smallest horizontal segment of an armchair CNT, hold great promise as "seeds", or templates, for the preparation of homogenous batches of CNTs. Initial reports towards this goal, however, suggest that it would be advantageous to pi-extend these structures through traditional organic synthesis before their use in CNT growth. Towards this, several strategies have been reported attempting to utilize the Scholl reaction on aryl-substituted cycloparaphenylenes to yield a small CNT for use as a template for larger tubes. Prominently used in polyaromatic hydrocarbon chemistry, the Scholl reaction has afforded numerous extraordinary targets, such as graphene nanoribbons and graphene propellors. In this work, both experimental and computational studies are provided to unravel the complex cationic rearrangements and ring-openings associated with the Scholl reaction in the context of the cycloparaphenylenes-systems that are thermodynamically and kinetically different from flat graphene fragments. Additionally, this work demonstrates the unique reactivity of cycloparaphenylenes in the context of cationic or radical cationic intermediates, which are common reaction pathways for numerous transformations.

Use of Trifluoromethyl Groups for Catalytic Benzylation and Alkylation with Subsequent Hydrodefluorination

AbstractThe electrophilic organofluorophosphonium catalyst [(C6F5)3PF][B(C6F5)4] is shown to effect benzylation or alkylation by aryl and alkyl CF3 groups with subsequent hydrodefluorination, thus resulting in a net transformation of CF3 into CH2–aryl fragments. In the case of alkyl CF3 groups, Friedel–Crafts alkylation by the difluorocarbocation proceeded without cation rearrangement, in contrast to the corresponding reactions of alkyl monofluorides.