Electronic Structure Of Complexes Research Articles

Diabatic Valence-Hole States in the C2 Molecule: “Putting Humpty Dumpty Together Again”

Despite the long history of spectroscopic studies of the C$_2$ molecule, fundamental questions about its chemical bonding are still being hotly debated. The complex electronic structure of C$_2$ is a consequence of its dense manifold of near-degenerate, low-lying electronic states. A global multi-state diabatic model is proposed here to disentangle the numerous configuration interactions within four symmetry manifolds of C$_2$ ($^{1}\Pi_g$, $^{3}\Pi_g$, $^{1}\Sigma_u^+$, and $^{3}\Sigma_u^+$). The key concept of our model is the existence of two "valence-hole" configurations, $2\sigma_g^22\sigma_u^11\pi_{u}^33\sigma_g^2$ for $^{1,3}\Pi_g$ states and $2\sigma_g^22\sigma_u^11\pi_{u}^43\sigma_g^1$ for $^{1,3}\Sigma_u^+$ states that derive from $3\sigma_g\leftarrow2\sigma_u$ electron promotion. The lowest-energy state from each of the four C$_2$ symmetry species is dominated by this type of valence-hole configuration at its equilibrium internuclear separation. As a result of their large binding energy (nominal bond order of 3) and correlation with the 2s$^2$2p$^2$+2s2p$^3$ separated-atom configurations, the presence of these valence-hole configurations has a profound impact on the $global$ electronic structure and unimolecular dynamics of C$_2$.

Giant nonlinear anomalous Hall effect induced by spin-dependent band structure evolution

The anomalous Hall effect (AHE) is a key transport signature revealing the topological properties of magnetic compounds. In quantum materials, the classical linear dependence of the AHE on magnetization often breaks down, which is typically ascribed to the presence of topological magnetic or electronic textures. However, the complex electronic structure of these compounds may offer alternative, unexplored mechanisms. Here, we show that a giant nonlinear AHE can originate from a series of magnetic-field-induced Lifshitz transitions in the spin-dependent band structure. In our experiments on ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$ the AHE contributes to 97% of the total Hall response, corresponding to a record anomalous Hall angle of 21%. Our scaling analysis and first-principles calculations demonstrate that the electronic structure is extremely sensitive to spin canting, with the magnetic field causing band crossing and band inversion and introducing a band gap when oriented along specific directions. Our results not only provide an ideal platform for Berry curvature engineering but reveal a general effect that may be applied to other material systems.

A DFT+U approach: Superior charge transfer characteristics and optoelectronic properties of GQD@TiO2 rutile (110) surface for improved hydrogen evolution

The understanding on complex electronic structure and charge transfer characteristics is crucial in the development of heterostructure-based photocatalyst such as graphene quantum dots (GQD) and titanium dioxide (TiO2). Simulation studies will be useful in gaining valuable insight on the complex system at atomistic level. Herein, for the first time, a theoretically designed heterogeneous GQD modified TiO2 (110) interface model using Hubbard's modified first-principles density functional theory (DFT+U) is presented. The structural properties were simulated with Perdew–Burke–Ernzerhof assisted generalized gradient approximation (GGA+PBE) and optoelectronic properties with Hubbard's modified (GGA+U) exchange correlation functional. The addition of GQD reduces the bandgap energy of the TiO2 rutile (110) surface from 2.95 eV to 1.86 eV, thereby improving the visible light response as it reduces the electron transition energy. Charge density difference map and Mulliken population analysis demonstrate frequent transfer of charges from GQD to the TiO2 surface, resulting in reduction of charge recombination rate. Moreover, the energy band edge estimation confirms a suitable band edge position in accordance with the redox potential of water. The collective effect of the heightened absorption of visible light and effective charge separation led to a significant photocatalytic performance of the hybrid photocatalyst.

Synthesis, crystal structure, Hirshfeld surface analysis, and DFT studies on (2,2’-bipyridine)chlorobis(N,N-bis(thiophen-2-ylmethyl)dithiocarbamato-S,S’)zinc(II) complex

Bis(N,N-bis(thiophen-2-ylmethyl)dithiocarbamato-S,S’)zinc(II) complex (1) and (2,2’-bipyridine)chlorobis(N,N-bis(thiophen-2-ylmethyl)dithiocarbamato-S,S’)zinc(II) complex (2) were synthesized. Complex 2 (final product) was structurally characterized by single crystal X-ray diffraction studies. Complex 2 (C21H18ClN3S4Zn) crystallized in triclinic crystal system with space group P-1 (no. 2), a = 8.7603(4) Å, b = 10.7488(6) Å, c = 13.0262(7) Å, α = 103.965(2)°, β = 91.913(2)°, γ = 104.944(2)°, V = 1144.07(10) Å3, Z = 2, T = 302(2) K, μ(MoKα) = 1.569 mm-1, Dcalc = 1.572 g/cm3, 14892 reflections measured (4.838° ≤ 2Θ ≤ 56.52°), 5570 unique (Rint = 0.0188, Rsigma = 0.0230) which were used in all calculations. The final R1 was 0.0810 (I > 2σ(I)) and wR2 was 0.2788 (all data). Complex 2 displays distorted square pyramidal coordination geometry. Crystal structure analysis of complex 2 shows that the crystal packing is mainly stabilized by C-H···π (chelate) and C-H···Cl interactions. Hirshfeld surface analysis was carried out to explore deeply into the nature and type of non-covalent interactions. The molecular and electronic structures of complexes 1 and 2 were also studied by DFT quantum chemical calculations.

Incorporating research literature and chemistry textbooks in 5E instructional model to reveal ambiguous oxidation state formalism of CuS for pre-service science teachers

AbstractThis paper implements the 5E instructional model to reveal authentic concepts in chemistry, in particular the ambiguous oxidation state formalism of copper sulfide (CuS) for pre-service science teachers (PSTs). We discuss the process and outcomes of learning phases of the 5E (engage, explore, explain, elaborate, and evaluate) for authentic chemistry learning. The puzzle activity of engage phase demonstrate PSTs prior-knowledge, understanding, problem-solving skills in the fundamental concepts of chemistry. However, we observed misconceptions in chemistry concepts, in particular the oxidation state formalism of CuS. Next, the explore phase describe how the scientific evidence from research literature give insight into whether the PSTs conceptions are in accordance with research evidence. The research evidence from collaborative literature review revealed the uncertainty in the oxidation state formalism of CuS. In the explain phase, we explained the complex electronic structure of CuS. In the fourth phase, the elaborate phase, we involve the PSTs in the book review to elaborate and analyze the uncertain concept. The results of the book review provide insight into the coverage of oxidation state formalism of CuS in nine chemistry textbooks. Finally, in evaluate phase, the results of questionnaire describe the PSTs perspectives and experiences in the student-centered chemistry learning.

Relativistic Effects in Modeling the Ligand K-Edge X-ray Absorption Near-Edge Structure of Uranium Complexes.

Accurate modeling of the complex electronic structure of actinide complexes requires full inclusion of relativistic effects. In this study, we examine the effect of explicit inclusion of spin-orbit coupling (SOC) versus scalar relativistic effects on the predicted spectra for heavy-element complexes. In this study, we employ a relativistic two-component Hamiltonian in the X2C form with all of the electrons in the system being considered explicitly to compare and contrast with previous studies that included the relativistic effects by means of relativistic effective core potentials (RECPs). A few uranium complexes are chosen as model systems. Comparison of the computed Cl K-edge X-ray absorption spectra with experimental data shows significantly improved agreement when a variational relativistic treatment of SOC is performed. In particular, we note the importance of SOC terms to obtain not only correct transition energies but also correct intensities for these heavy-element complexes because of the redistribution of ligand bonding character among the valence MOs. While RECPs generally agree well with all-electron scalar relativistic calculations, there are some differences in the predicted spectra of open-shell systems. These methods are still suitable for broad application to analyze the qualitative nature of transitions in X-ray absorption spectra, but caution is recommended for quantitative analysis, as SOC can be non-negligible for both open- and closed-shell heavy-element systems.

Kohn anomaly and elastic softening in body-centered cubic molybdenum at high pressure

Transition metals in body-centered cubic (bcc) structures under compression can display several novel physical properties because of their complex electronic structures and electron-phonon interactions. Here, we used inelastic x-ray scattering experiments in a diamond-anvil cell up to \ensuremath{\sim}45 GPa and density-functional theory calculations up to 210 GPa to investigate the phonon dispersions, and electronic and elastic properties of single-crystal molybdenum (Mo). Our results show a pressure-induced Kohn anomaly at $q\ensuremath{\sim}0.5$ along the [\ensuremath{\xi}00] direction in the longitudinal acoustic mode at \ensuremath{\sim}45 GPa; this anomaly is triggered by the pressure-enhanced Fermi-surface nesting effect. Theoretical calculations show that electron redistributions in the $s$-to-$d$ orbitals of bcc-Mo contribute to the shear modulus anomaly at \ensuremath{\sim}50 GPa. In contrast, the Young's modulus anomaly in bcc-Mo at \ensuremath{\sim}210 GPa results from a Lifshitz-type electronic topological transition. Our results shed light on the complex electronic behaviors that are associated with macroscopic elastic properties in typical bcc $d$-block transition metals under compression.

Post-Density Matrix Renormalization Group Methods for Describing Dynamic Electron Correlation with Large Active Spaces.

The ab initio density matrix renormalization group (DMRG) method has been well-established and has become one of the most accurate numerical methods for the precise electronic structure solution of large active spaces. In the past few years, to capture the missing dynamic correlation, various post-DMRG approaches have been proposed through the combination of DMRG and multireference quantum chemical methods or density functional theory. With this in mind, this work provides a brief overview of ab initio DMRG principles and the new developments within post-DMRG methods. For clarity, post-DMRG methods are classified into two main categories depending on whether high-order n-electron reduced density matrices are used, and their merits and disadvantages are properly discussed. Finally, we conclude by discussing unsolved bottlenecks and giving development perspectives of post-DMRG approaches, which are expected to yield quantitative descriptions of complex electronic structures in large strongly correlated molecules and materials.

The Effect of Geometry, Spin, and Orbital Optimization in Achieving Accurate, Correlated Results for Iron-Sulfur Cubanes.

Iron-sulfur clusters comprise an important functional motif in the catalytic centers of biological systems, capable of enabling important chemical transformations at ambient conditions. This remarkable capability derives from a notoriously complex electronic structure that is characterized by a high density of states that is sensitive to geometric changes. The spectral sensitivity to subtle geometric changes has received little attention from correlated, large active space calculations, owing partly to the exceptional computational complexity for treating these large and correlated systems accurately. To provide insight into this aspect, we report the first Complete Active Space Self Consistent Field (CASSCF) calculations for different geometries of the [Fe(II/III)4S4(SMe)4]-2 clusters using two complementary, correlated solvers: spin-pure Adaptive Sampling Configuration Interaction (ASCI) and Density Matrix Renormalization Group (DMRG). We find that the previously established picture of a double-exchange driven magnetic structure, with minute energy gaps (<1 mHa) between consecutive spin states, has a weak dependence on the underlying geometry. However, the spin gap between the singlet and the spin state 2S + 1 = 19, corresponding to a maximal number of Fe-d electrons being unpaired and of parallel spin, is strongly geometry dependent, changing by a factor of 3 upon slight deformations that are still within biologically relevant parameters. The CASSCF orbital optimization procedure, using active spaces as large as 86 electrons in 52 orbitals, was found to reduce this gap compared to typical mean-field orbital approaches. Our results show the need for performing large active space calculations to unveil the challenging electronic structure of these complex catalytic centers and should serve as accurate starting points for fully correlated treatments upon inclusion of dynamical correlation outside the active space.

Electronic structure of the solvated benzene radical anion

The benzene radical anion is a molecular ion pertinent to several organic reactions, including the Birch reduction of benzene in liquid ammonia. The species exhibits a dynamic Jahn-Teller effect due to its open-shell nature and undergoes pseudorotation of its geometry. Here, we characterize the complex electronic structure of this condensed-phase system based on ab initio molecular dynamics simulations and GW calculations of the benzene radical anion solvated in liquid ammonia. Using detailed analysis of the molecular and electronic structure, we find that the spatial character of the excess electron of the solvated radical anion follows the underlying Jahn-Teller distortions of the molecular geometry. We decompose the electronic density of states to isolate the contribution of the solute and to examine the response of the solvent to its presence. Our findings show the correspondence between instantaneous molecular structure and spin density; provide important insights into the electronic stability of the species, revealing that it is, indeed, a bound state in the condensed phase; and offer electronic densities of states that aid in the interpretation of experimental photoelectron spectra.

First principles study of hydrogen adsorption and dissociation behavior on γ-U (100)/Mo surface

&lt;sec&gt;Uranium (U) is one of the most natural radioactive elements widely used in the nuclear industry. In the civilian field, uranium is the most important nuclear fuel in nuclear reactors; militarily, uranium is an important raw material for nuclear weapons. In addition, uranium is also used for radiation shielding and ship ballast due to its high-density properties. Depending on the temperature, U has three kinds of allotrope phases: the orthogonal α phase at temperature below 940 K, the body-centered tetragonal (BCT) β phase at temperature ranging from 940 K to 1050 K, and the body-centered cubic (BCC) γ phase at temperature above 1050 K. Compared with the other two structures, the crystal structure of γ phase has good symmetry and excellent mechanical properties. However, γ-U is extremely unstable at low temperature. No matter what heat treatment method is adopted, γ-U will undergo phase transformation and become α phase. It is shown that adding certain alloying elements, such as Mo, Nb, Zr, Ti and Hf, into uranium can stabilize γ-U to room temperature and improve the mechanical properties of uranium greatly. As an important uranium alloy, γ-U by doping Mo atom has excellent mechanical properties, structural stability and thermal conductivity, and is an important nuclear reactor fuel.&lt;/sec&gt;&lt;sec&gt;However, uranium has special physical and chemical properties due to its complex electronic structure and strong correlation of 5f electrons. Because of its special valence electron structure, it is highly susceptible to chemical and electrochemical reactions of environmental media. After the reaction between uranium and hydrogen, hydrogen embrittlement will occur, and even easily break into powder, which reduces the performance of uranium in service and brings hidden trouble to its storage. With the increase of service life, surface corrosion becomes more serious, and the safety and reliability of U alloys are seriously affected. The adsorption and dissociation of hydrogen on U alloy surface is the primary process of hydrogenation corrosion.&lt;/sec&gt;&lt;sec&gt;Based on density functional theory, first-principles study of hydrogen adsorption and dissociation on γ-U(100) surface by Mo atoms coatings is carried out in this work. The model of γ-U(100) and Mo atoms coatings on γ-U(100) surface are established, and the structural parameters, adsorption energy, Bader charge, surface work function, and electron state density of H&lt;sub&gt;2&lt;/sub&gt; at highly symmetrical adsorption sites are calculated. The results show that H&lt;sub&gt;2&lt;/sub&gt; molecule occurs when physical dissociation adsorption takes place on γ-U(100) and U(100)/Mo surface. The electron state density shows that H&lt;sub&gt;2&lt;/sub&gt; does not bond to the surface atoms and no new hybridization peak appears. However, in the hollow parallel adsorption configuration, H&lt;sub&gt;2&lt;/sub&gt; is completely dissociated into two H atoms and occurs chemical adsorption and dissociation on γ-U(100) and U(100)/Mo surface. The H/1s orbital electrons are hybridized with the U/6p, U/6d, Mo/5s, Mo/4p, Mo/4d orbital electrons, and the H atom forms stable chemical bonds with the Mo atoms. Bader charge distribution results show that the change of chemical adsorption net charge of H&lt;sub&gt;2&lt;/sub&gt; on U(100)/Mo is more than that of physical adsorption. Because the adsorption energy of H&lt;sub&gt;2&lt;/sub&gt; in the most stable configuration (H&lt;sub&gt;Mo&lt;/sub&gt;-Hor) on U(100)/Mo is less than that of the most stable configuration (H&lt;sub&gt;U&lt;/sub&gt;-Hor) on γ-U(100), the adsorption of H&lt;sub&gt;2&lt;/sub&gt; on U(100)/Mo is more stable than that of γ-U(100) surface.&lt;/sec&gt;

Enhanced thermoelectric performance in Sb-Br codoped Bi2Se3 with complex electronic structure and chemical bond softening.

Prior experimental work showed that Bi2Se3, as a sister compound of the best room-temperature thermoelectric material Bi2Te3, has remarkably improved thermoelectric performance by Sb–Br codoping. But the relationship between its crystalline structure and thermoelectric properties is still unclear to date. Here, we use first-principles calculations to explore the possible reasons for such improvement. The electronic structures of Bi2−xSbx(Se1−yBry)3 (x = 0, 1, 2; y = 0, 0.08) are systematically investigated. Significant effects of 8% Br codoping in BiSbSe3 are found. First, the Br atom acts as an electron donor, thus greatly increasing the carrier concentration. Second, similar to the effect of Sb doping, Br codoping further improves greatly the degeneracy of the conduction band edge, which leads to a remarkably increased density-of-states effective mass without deterioration of the carrier mobility, and simultaneously preserves a large Seebeck coefficient of ∼−254 μV K−1 at 800 K. In addition, the Br codoping softens the chemical bonds, which enhances anharmonic scattering and further reduces the lattice thermal conductivity. We predict that the maximum zT of BiSb(Se0.92Br0.08)3 at 800 K can reach 0.96 with the carrier concentration of 9.22 × 1019 cm−3. This study rationalizes a potential strategy to improve the thermoelectric performance of Bi2Se3-based thermoelectric materials.

Computational mechanistic studies of ruthenium catalysed methanol dehydrogenation.

Homogeneous ruthenium catalysed methanol dehydrogenation could become a key reaction for hydrogen production in liquid fuel cells. In order to improve existing catalytic systems, mechanistic insight is paramount in directing future studies. Herein, we describe what computational mechanistic research has taught us so far about ruthenium catalysed dehydrogenation reactions. In general, two mechanistic pathways can be operative in these reactions: a metal-centered or a metal–ligand cooperative (Noyori–Morris type) minimum energy reaction pathway (MERP). Discerning between these mechanisms on the basis of computational studies has proven to be highly input dependent, and to circumvent pitfalls it is important to consider several factors, such as solvent effects, metal–ligand cooperativity, alternative geometries, and complex electronic structures of metal centres. This Frontiers article summarizes the reported computational research performed on ruthenium catalyzed dehydrogenation reactions performed in the past decade, and serves as a guide for future research.

Study of transition metal dichalcogenides/chromium trihalides van der Waals heterostructure by band unfolding method

The transition metal dichalcogenides &lt;i&gt;MX&lt;/i&gt;&lt;sub&gt;2&lt;/sub&gt;/Chromium Trihalides Cr&lt;i&gt;X&lt;/i&gt;&lt;sub&gt;3&lt;/sub&gt; van der Waals heterostructures can control the valley polarization of of &lt;i&gt;MX&lt;/i&gt;&lt;sub&gt;2&lt;/sub&gt; effectively, which makes them possess promising potential applications in valleytronics. In the present work, the stacking order and electronic structure of MoSe&lt;sub&gt;2&lt;/sub&gt;/CrI&lt;sub&gt;3&lt;/sub&gt;, MoSe&lt;sub&gt;2&lt;/sub&gt;/CrBr&lt;sub&gt;3&lt;/sub&gt; and WS&lt;sub&gt;2&lt;/sub&gt;/CrBr&lt;sub&gt;3&lt;/sub&gt; are investigated based on the first-principle calculation and k-projection band unfolding method. The underlying mechanism of valley splitting is also explored. The stacking energy surfaces are calculated and the stable stacking configurations are determined. The effects of the breaking of time-symmetry and spatial-symmetry on electronic structure are also revealed. Because of the orbital hybridization, the conduction band of heterostructure becomes complicated and the valence band maximum changes drastically. It is thus difficult to compare the electronic structure of vdW heterostructure with that of free-standing &lt;i&gt;MX&lt;/i&gt;&lt;sub&gt;2&lt;/sub&gt; directly. Through the unfolding energy band, the electronic structure change of &lt;i&gt;MX&lt;/i&gt;&lt;sub&gt;2&lt;/sub&gt; induced by Cr&lt;i&gt;X&lt;/i&gt;&lt;sub&gt;3&lt;/sub&gt; is revealed clearly, and the valley splitting of &lt;i&gt;MX&lt;/i&gt;&lt;sub&gt;2&lt;/sub&gt; is obtained quantitatively. Moreover, the interlayer distance and strain are found to be able to tune the valley splitting effectively. When the interlayer distance reduces to 2.6 Å, the valley splitting of MoSe&lt;sub&gt;2&lt;/sub&gt;/CrI&lt;sub&gt;3&lt;/sub&gt; is enhanced to 10.713 meV with the increase of &lt;i&gt;AB&lt;/i&gt; stacking, which is 8.8 times as large as the value of equilibrium structure. This work breaks through the limit of the complex electronic structure in supercell, providing an important reference for studying other magnetic vdW heterostructure.

Decoding the conductance of disordered nanostructures: a quantum inverse problem

Obtaining conductance spectra for a concentration of disordered impurities distributed over a nanoscale device with sensing capabilities is a well-defined problem. However, to do this inversely, i.e., extracting information about the scatters from the conductance spectrum alone, is not an easy task. In the presence of impurities, even advanced techniques of inversion can become particularly challenging. This article extends the applicability of a methodology we proposed capable of extracting composition information about a nanoscale sensing device using the conductance spectrum. The inversion tool decodes the conductance spectrum to yield the concentration and nature of the disorders responsible for conductance fluctuations in the spectra. We present the method for simple one-dimensional systems like an electron gas with randomly distributed delta functions and a linear chain of atoms. We prove the generality and robustness of the method using materials with complex electronic structures like hexagonal boron nitride, graphene nanoribbons, and carbon nanotubes. We also go on to probe distribution of disorders on the sublattice structure of the materials using the proposed inversion tool.

Electronic Structure and Magnetic Properties of a Low-Spin CrII Complex: trans-[CrCl2(dmpe)2] (dmpe = 1,2-Bis(dimethylphosphino)ethane).

Octahedral coordination complexes of the general formula trans-[MX2(R2ECH2CH2ER2)2] (MII = Ti, V, Cr, Mn; E = N, P; R = alkyl, aryl) are a cornerstone of both coordination and organometallic chemistry, and many of these complexes are known to have unique electronic structures that have been incompletely examined. The trans-[CrCl2(dmpe)2] complex (dmpe = Me2PCH2CH2PMe2), originally reported by Girolami and co-workers in 1985, is a rare example of a six-coordinate d4 system with an S = 1 (spin triplet) ground state, as opposed to the high-spin (S = 2, spin quintet) state. The ground-state properties of S = 1 systems are challenging to study using conventional spectroscopic methods, and consequently, the electronic structure of trans-[CrCl2(dmpe)2] has remained largely unexplored. In this present work, we have employed high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy to characterize the ground-state electronic structure of trans-[CrCl2(dmpe)2]. This analysis yielded a complete set of spin Hamiltonian parameters for this S = 1 complex: D = +7.39(1) cm-1, E = +0.093(1) (E/D = 0.012), and g = [1.999(5), 2.00(1), 2.00(1)]. To develop a detailed electronic structure description for trans-[CrCl2(dmpe)2], we employed both classical ligand-field theory and quantum chemical theory (QCT) calculations, which considered all quintet, triplet, and singlet ligand-field states. While the high density of states suggests an unexpectedly complex electronic structure for this "simple" coordination complex, both the ligand-field and QCT methods were able to reproduce the experimental spin Hamiltonian parameters quite nicely. The QCT computations were also used as a basis for assigning the electronic absorption spectrum of trans-[CrCl2(dmpe)2] in toluene.

Nickel-Borolide Complexes and Their Complex Electronic Structure.

Borolides (BC42-) can be considered as dianionic heterocyclic analogues of monoanionic cyclopentadienides. Although both are formally six-π-electron donors, we herein demonstrate that the electronic structure of their corresponding transition metal complexes differs significantly, leading to altered properties. Specifically, the 18-electron sandwich complex Ni(iPr2NBC4Ph2)2 (1) features an ∼90° angle between the Ni-B-N planes and is best described as a combination of three limiting resonance structures with the major contribution stemming from a formally Ni2+ species bound to two monoanionic radical (BC4•-) ligands. Compound 1 displays two sequential one-electron oxidation events over a small potential range of <0.2 V, which strikingly contrasts the large potential separations between redox partners in the family of metallocenes, and the potential reasons for this unusual observation are discussed.

Electron Flow Characterization of Charge Transfer for Carbonic Acid to Strong Base Proton Transfer in Aqueous Solution.

Protonation of the strong base methylamine CH3NH2 by carbonic acid H2CO3 in aqueous solution, HOCOOH···NH2CH3 → HOCOO-···+HNH2CH3, has been previously studied ( J. Phys. Chem. B 2016, 109, 2271-2280; J. Phys. Chem. B 2016, 109, 2281-2290) via Car-Parinnello molecular dynamics. This proton transfer (PT) reaction within a hydrogen (H)-bonded complex was found to be barrierless and very rapid, with key reaction coordinates comprising the proton coordinate, the H-bond separation RON, and a solvent coordinate, reflecting the water solvent rearrangement involved in the neutral to ion pair conversion. In the present work, the reaction's charge flow aspects are analyzed in detail, especially a description via Mulliken charge transfer for PT (MCTPT). A natural bond orbital analysis and some extensions of them are employed for the complex's electronic structure during the reaction trajectories. Results demonstrate that consistent with the MCTPT picture, the charge transfer (CT) occurs from a methylamine base nonbonding orbital to a carbonic acid antibonding orbital. A complementary MCTPT reaction product perspective of CT from the antibonding orbital of the HN+ moiety to the nonbonding orbital of the oxygen in the H-bond complex is also presented. σOH and σHN+ bond order expressions show this CT to occur within the H-bond OHN triad, an aspect key for simultaneous bond-breaking and -forming in the PT reaction.

Ion Dynamics and Polymorphism in Cu20Te11Cl3.

Coinage metal polychalcogenide halides are an intriguing class of materials, and many representatives are solid ion conductors and thermoelectric materials. The materials show high ion mobility, polymorphism, and various attractive interactions in the cation and anion substructures. Especially the latter feature leads to complex electronic structures and the occurrence of charge-density waves (CDWs) and, as a result, the first p-n-p switching materials. During our systematic investigations for new p-n-n switching materials in the Cu-Te-Cl phase diagram, we were able to isolate polymorphic Cu20Te11Cl3, which we characterized structurally and with regard to its electronic and thermoelectric properties. Cu20Te11Cl3 is trimorphic, with phase transitions occurring at 288 and 450 K. The crystal structures of two polymorphs, the α phase, stable above 450 K, and the β polymorph (288-450 K), are reported, and the complex structure chemistry featuring twinning upon a phase change is illustrated. We identified a dynamic cation substructure and a static anion substructure for all polymorphs, characterizing Cu20Te11Cl3 as a solid Cu-ion conductor. Temperature-dependent measurements of the Seebeck coefficient and total conductivity were performed and substantiated a linear response of the Seebeck coefficient, a lack of CDWs, and no p-n-p switching. Reasons for a lack of CDWs in Cu20Te11Cl3 are discussed and illustrated in the context of existing p-n-p switching materials.

A high-pressure Raman study of FeTiO3 ilmenite: Fermi resonance as a manifestation of Fe-Ti charge transfer

We investigated the 300 K high-pressure behavior of ilmenite using Raman spectroscopy to 54 GPa. Upon compression, we observe a Fermi resonance between the lowest frequency Ag symmetry peaks (ν4 and ν5) between ~ 10 and ~ 30 GPa: bands that involve major components of Ti–O and Fe–O-related displacements, respectively. The peaks’ relative intensities switch at ~ 18 GPa and they also reach their minimum separation at ~ 20 GPa, indicating that their maximum resonance occurs between 18 and 20 GPa. The negative shift of the Ti–O-associated ν4 vibration under compression is fully consistent with a shift in valence of Ti from 4 + to 3 + under compression. Anomalously small mode shifts of other, more localized vibrations are also consistent with a charge transfer from Fe to Ti under compression. At higher pressures, we have not found definitive evidence for a transition to the perovskite-structure at 300 K, which has been well characterized at high pressures and temperatures. At 40 GPa, we observe an apparent reversible disordering that persists up to our highest pressure. The 300 K mode shifts of the Raman active modes in FeTiO3 under pressure are notably different from those of other ABO3 compounds (where A = Mg, Mn and B = Ti, Si); in other ilmenite-structured compounds, the peaks shift at a faster rate and there has not been any observation of Fermi resonance. Thus, iron’s complex electronic structure, and its charge transfer with titanium, appears to play a primary role in the behavior of phonons in FeTiO3 ilmenite.