Bond Asymmetry Research Articles

Tailoring Polaron Dimensions in Lead-Tin Hybrid Perovskites.

Charge carriers in the soft and polar perovskite lattice form so-called polaron quasiparticles, charge carriers dressed with a lattice deformation. The spatial extent of a polaron is governed by the material's electron-phonon interaction strength, which determines charge carrier effective mass, mobility, and the so-called Mott polaron density, that is, the maximum stable density of charge carriers that a perovskite can support. Despite its significance, controlling polaron dimensions has been challenging. Here, experimental substantial tuning of polaron dimensions is reported by lattice engineering, through Pb/Sn substitution in CH3NH3SnxPb1-xI3. The polaron dimension is deduced from the Mott polaron density, which can be composition-tuned over an order of magnitude, while charge carrier mobility occurs through band transport, and remains substantial across all compositions, ranging from 10s to 100 scm2V s-1 at room temperature. The effective modulation of polaron size can be understood by considering the bond asymmetry after carrier injection as well as the random spatial distribution of Pb/Sn ions. This study underscores the potential for tailoring polaron dimensions, which is crucial for optimizing applications prioritizing either high charge carrier density or high mobility.

Effect of static electric fields on liquid water, its structure, dynamics, and hydrogen bond asymmetry: A molecular dynamics simulation study of TIP4P/2005 water model.

We study the effect of static electric fields of 0.1, 0.4, and 1.0 V/nm on the hydrogen bond structure and dynamics of TIP4P/2005 water at 1bar and at temperatures between 300 and 200K using molecular dynamics simulations. At all these temperatures, simulating liquid water with electric fields of 0.1 and 0.4 V/nm has no additional effect on its structural and dynamical changes, which otherwise already take place due to cooling. However, the introduction of 1.0 V/nm field enhances the slowing down of liquid water dynamics, crystallizes it to cubic ice at 240 and 220K, and amorphizes it at 200K. At 240 and 220K, crystallization occurs within 5 and 50ns, respectively. An electric field of 1 V/nm increases the relaxation times in addition to what cooling does. We note that when liquid water's metastability limit is reached, crystallization is averted and amorphization takes place. Both equilibrium (liquid-solid) and non-equilibrium (liquid-amorphous) transformations are observed at 1 V/nm. Moreover, with an increase in the electric field, H-bonds become stronger. However, the donor-acceptor asymmetry (the difference between the strengths of two donor/acceptor bonds) remains even when crystallization or amorphization takes place. At low temperatures, increasing electric fields on liquid water increases both its crystallization and amorphization tendencies.

Valence and Core Photoelectron Spectra of Aqueous I3- from Multi-Reference Quantum Chemistry.

The I3- molecule is known to undergo substantial structural reorganization upon solvation by a protic solvent, e.g., water. However, the details of this process are still controversially discussed in the literature. In the present study, we combined experimental and theoretical efforts to disentangle this controversy. The valence (5p), N4,5 (4d), and M4,5 (3d) edge photoelectron spectra were measured in an aqueous solution and computed using high-level multi-reference methods. Our previous publication mainly focused on obtaining reliable experimental evidence, whereas in the present article, we focused primarily on theoretical aspects. The complex electronic structure of I3- requires the inclusion of both static and dynamic correlation, e.g., via the multi-configurational perturbation theory treatment. However, the resulting photoelectron spectra appear to be very sensitive to problems with variational stability and intruder states. We attempted to obtain artifact-free spectra, allowing for a more reliable interpretation of experiments. Finally, we concluded that the 3d Photoelectron Spectrum (PES) is particularly informative, evidencing an almost linear structure with a smaller degree of bond asymmetry than previously reported.

A “nucleophilic” iodine in a halogen-bonded iodonium complex manifests an unprecedented I+···Ag+ interaction

Summary When an electron is removed from a halogen atom, it forms a halenium ion X+ (X = I, Br, Cl). In halogen bonding (XB), X+ is considered as a strong XB donor, and when interacting with two XB acceptors (e.g., pyridine), it forms a halonium XB complex with a [N–I–N] three-center-four-electron bond with the two XB acceptors. An unprecedented I+···Ag+ interaction occurs between a [L1–I–L1]+ halogen-bonded complex and a [L2–Ag–L2]+ complex in which the iodonium ion acts like a nucleophile and donates electrons to the silver(I) cation. The X-ray diffraction analysis reveals a short contact [3.4608(3) A] between the I+ and Ag+ cations, and ITC measurements give a ΔG of −6.321 kcal/mol and Ka∼37,000 M−1 for the 1:1 complex. The DFT computational study on the nature of the I+···Ag+ interaction indicates that the I+ is nucleophilic in character, manifesting an unusual and strongly attractive interaction between the I+ and Ag+ cations.

Absolute carrier-envelope-phase dependences of single and double ionization of methanol in a near-IR few-cycle laser field.

We investigate the carrier-envelope phase (CEP) dependences of the single and double ionization processes of methanol (CH3OH) in an intense near-IR few-cycle laser field (2.1 × 1014 W/cm2) by the asymmetry in the ejection direction of CH3 + for the non-hydrogen migration channels and CH2 + for the hydrogen migration channels created through the C-O bond breaking after the ionization. Based on the absolute CEP values at the laser-molecule interaction point, calibrated by the method using intense few-cycle circularly polarized laser pulses [Fukahori et al., Phys. Rev. A 95, 053410-1-053410-14 (2017)], we confirm that methanol cations are produced by tunnel ionization and methanol dications are produced by the recollisional double ionization. We obtain the phase offset for the double ionization accompanying no hydrogen migration to be 1.85π as the absolute CEP at which the extent of the asymmetry becomes maximum. We interpret the phase shift of 0.85π from the phase offset of 1.0π for the tunnel ionization, estimated by a tunnel ionization model incorporating the chemical bond asymmetry, as the corresponding time delay associated with the electron recollisional ionization. The positive phase shift of 0.13π for the single ionization in the non-hydrogen migration channel is interpreted as the additional time (165 as) with which a methanol cation can be excited electronically prior to the decomposition. The additional phase shift of 0.22π for the single ionization in the hydrogen migration channel is interpreted as the additional time (280 as) required for a methanol cation to be excited electronically leading to the hydrogen migration prior to the decomposition.

From Large to Small Polarons in Lead, Tin, and Mixed Lead-Tin Halide Perovskites.

The origin of the long carrier lifetime in lead halide perovskites is still under debate, and, among different hypotheses, the formation of large polarons preventing the recombination of charge couples is one of the most fascinating. Using state-of-the art ab initio calculations, we report a systematic study of the polaron formation process in metal halide perovskites, focusing on the influence of the chemical composition of the perovskite on the polaron properties. We examine variations in A-site cations (FA, MA, Cs, and Cs-MA), B-site cations (Pb, Sn, and Pb-Sn), and X-site anions (Br, I). Our study confirms that stronger structural distortions occur for Cs than for MA and FA, with the effect of different A-site cations being almost additive. For the same A cation, bromide features stronger distortions than iodide perovskites. The pure Sn phase has an almost double polaron stabilization energy compared with the pure Pb phase. Surprisingly, the trend of polaron stabilization energy is nonmonotonic in mixed Sn-Pb perovskites, with a maximum for small Sn percentages. Polaron formation is found to be promoted by bond asymmetry, ranging from small to large polarons in mixed Sn-Pb perovskites depending on the relative Sn percentage.

Halogen Bond Asymmetry in Solution.

Halogen bonding is the noncovalent interaction of halogen atoms in which they act as electron acceptors. Whereas three-center hydrogen bond complexes, [D···H···D]+ where D is an electron donor, exist in solution as rapidly equilibrating asymmetric species, the analogous halogen bonds, [D···X···D]+, have been observed so far only to adopt static and symmetric geometries. Herein, we investigate whether halogen bond asymmetry, i.e., a [D–X···D]+ bond geometry, in which one of the D–X bonds is shorter and stronger, could be induced by modulation of electronic or steric factors. We have also attempted to convert a static three-center halogen bond complex into a mixture of rapidly exchanging asymmetric isomers, [D···X–D]+ ⇄ [D–X···D]+, corresponding to the preferred form of the analogous hydrogen bonded complexes. Using 15N NMR, IPE NMR, and DFT, we prove that a static, asymmetric geometry, [D–X···D]+, is obtained upon desymmetrization of the electron density of a complex. We demonstrate computationally that conversion into a dynamic mixture of asymmetric geometries, [D···X–D]+ ⇄ [D–X···D]+, is achievable upon increasing the donor–donor distance. However, due to the high energetic gain upon formation of the three-center-four-electron halogen bond, the assessed complex strongly prefers to form a dimer with two static and symmetric three-center halogen bonds over a dynamic and asymmetric halogen bonded form. Our observations indicate a vastly different preference in the secondary bonding of H+ and X+. Understanding the consequences of electronic and steric influences on the strength and geometry of the three-center halogen bond provides useful knowledge on chemical bonding and for the development of improved halonium transfer agents.

Luminescence and decay characteristics of Tb3+-doped fluorophosphate glasses

ABSTRACTTb3+-doped fluorophosphate glasses with the composition of P2O5–K2O–SrF2–Al2O3–x Tb4O7 (where x = 0.1, 0.5, 1.0, 2.0 and 4.0 mol%) were prepared by a conventional high temperature melt quenching technique and characterized through absorption, emission, excitation and decay measurements. From the emission studies, a strong green emission at around 546 was observed, which corresponds to the 5D4 → 7F5 transition of Tb3+ ion. Green/blue intensity ratios (IG/IB) were evaluated as a function of Tb3+ concentration and vice versa. Higher IG/IB intensity ratio confirms the higher covalency between Tb–O bond and higher asymmetry around the Tb3+ ions in the present fluorophosphate glasses. The decay curves for the 5D4 level of Tb3+ ion were measured and found that they exhibited single exponential nature irrespective to the dopant concentration. The experimental lifetime was determined using single exponential fitting and found that it increased from 2.65 to 2.95 ms when Tb3+ concentration increased from 0.1 mol% to 4.0 mol%. The derived properties were compared to the other Tb3+-doped glasses in order to see the potentiality of the material for visible laser gain media at 546 nm.

Privileged Role of Thiolate as the Axial Ligand in Hydrogen Atom Transfer Reactions by Oxoiron(IV) Complexes in Shaping the Potential Energy Surface and Inducing Significant H-Atom Tunneling.

An H/D kinetic isotope effect (KIE) of 80 is found at -20 °C for the oxidation of 9,10-dihydroanthracene by [FeIV(O)(TMCS)]+, a complex supported by the tetramethylcyclam (TMC) macrocycle with a tethered thiolate. This KIE value approaches that previously predicted by DFT calculations. Other [FeIV(O)(TMC)(anion)] complexes exhibit values of 20, suggesting that the thiolate ligand of [FeIV(O)(TMCS)]+ plays a unique role in facilitating tunneling. Calculations show that tunneling is most enhanced (a) when the bond asymmetry between C-H bond breaking and O-H bond formation in the transition state is minimized, and (b) when the electrostatic interactions in the O---H---C moiety are maximal. These two factors-which peak for the best electron donor, the thiolate ligand-afford a slim and narrow barrier through which the H-atom can tunnel most effectively.

Spin-Liquid-to-Spin-Liquid Transition in Kitaev Magnets Driven by Fractionalization.

While phase transitions between magnetic analogs of the three states of matter-a long-range ordered state, paramagnet, and spin liquid-are extensively studied, the possibility of "liquid-liquid" transitions, namely, between different spin liquids, remains elusive. By introducing the additional Ising coupling into the honeycomb Kitaev model with bond asymmetry, we discover that the Kitaev spin liquid turns into a spin-nematic quantum paramagnet before a magnetic order is established by the Ising coupling. The quantum phase transition between the two liquid states accompanies a topological change driven by fractionalized excitations, the Z_{2} gauge fluxes, and is of first order. At finite temperatures, this yields a persisting first-order transition line that terminates at a critical point located deep inside the regime where quantum spins are fractionalized. It is suggested that similar transitions may occur in other perturbed Kitaev magnets with bond asymmetry.

Chemical Vapor Deposition of β-HgS Nanoparticles From a Precursor, bis(cinnamylpiperazinedithiocarbamato) Mercury(II)

Chemical vapor deposition method for the synthesis of β-HgS (meta cinnabar) nanoparticles is reported with bis(cinnamylpiperazinedithiocarbamato)mercury(II) as the single source precursor. Crystalline structure, size, morphology and composition of the products are characterized by X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analysis. PXRD shows (111), (220), (200), (311), (222), (400), (331), (420) reflections characteristic of β-HgS nanoparticles. SEM micrographs display the spherical nature of the b-HgS nanoparticles. EDX analysis showed the presence of Hg and S.HRTEM images confirm the spherical nature of the nanoparticles with their size in the range of 5–10 nm. The results are in agreement with those estimated from the XRD pattern. XPS signals observed at 162.6, 162.8, 222.3, 99.9, and 104.1 eV are due to S2p (3/2, 1/2), S2s (1/2), and Hg4f (7/2, 5/2) respectively. The precursor [Hg (cpzdtc)2] was synthesized and characterized by IR and NMR (1H and 13C). Single crystal X-ray crystallography of precursor shows the presence of HgS4 distorted tetrahedral coordination environment, with a distinct Hg–S bond asymmetry.

Oxygen deficient centers in silica: optical properties within many-body perturbation theory

The electronic and optical properties of neutral oxygen vacancies, also called oxygen deficient centers (ODC(I)s), have been investigated in pure and germanium doped silica (both amorphous and α-quartz) through first-principles calculations. By means of density functional theory and many-body perturbation theory (GW approximation and the solution of the Bethe–Salpeter equation), we obtain the atomic and electronic structures as well as the optical absorption spectra of pure and Ge-doped silica in the presence of ODCs (SiODC(I)s and GeODC(I)s); our study allows us to interpret and explain the very nature of the optical features in experimental absorption spectra. The theoretical optical absorption signatures of these defects show excellent agreement with experiments for the SiODC(I)s, i.e. two absorption bands arise around 7.6 eV due to transitions between the defect levels. Our theoretical results also explain the experimental difficulty in measuring the GeODC(I) absorption band in Ge-doped silica, which was in fact tentatively assigned to a broad and very weak absorption signature, located between 7.5 and 8.5 eV. The influence of Ge-doping induced disorder on the nature of the defect-related optical transitions is discussed. We find that even if the atomic and electronic structures of SiODC(I) and GeODC(I) defects are relatively similar, the slight network distortion induced by the presence of the Ge atom, together with the increase in the Ge–Si bond asymmetry, completely changes the nature of the optical absorption edge.

Solvothermal preparation of nano-β-HgS from a precursor, bis(dibenzyldithiocarbamato)mercury(II)

A simple solvothermal method for the selective synthesis of β-HgS (meta cinnabar) nanoparticles in aqueous solutions is reported with bis(dibenzyldithiocarbamato)mercury(II) as the precursor. Crystal structure, size, morphology and composition of the products are characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, high-resolution transmission electron microscopy (HRTEM), SAED and X-ray photoelectron spectroscopy (XPS). PXRD shows (111), (220), (200), (311), (222), (400), (331), (420) reflections characteristic of β-HgS. SEM micrographs display the spherical nature of the nano-β-HgS. EDX analysis showed the presence of Hg and S. HRTEM images indicate the spherical nature of the nanoparticles with their size in the range of 10–15 nm and the FFT pattern shows the crystalline nature of the spherical particles. The results are in agreement with those estimated from the XRD pattern. XPS signals observed at 162.6 and 162.8 eV are due to S2p 3/2 and S2p 1/2 electrons and the S2s was observed at 222.3 eV. The band gap of nano-β-HgS has been found to be 3.6 eV from the UV–visible spectral measurement. The blue-shifted band gap compared to the bulk HgS is a consequence of “size quantization” effect. A comprehensive characterization of the precursor by IR and single crystal X-ray crystallography shows the presence of HgS4 coordination environment, with a distinct Hg–S bond asymmetry.

Imaging the symmetry breaking of molecular orbitals in single-wall carbon nanotubes

Carbon nanotubes have attracted considerable interest for their unique electronic properties. They are fascinating candidates for fundamental studies of one dimensional materials as well as for future molecular electronics applications. The molecular orbitals of nanotubes are of particular importance as they govern the transport properties and the chemical reactivity of the system. Here, we show for the first time a complete experimental investigation of molecular orbitals of single wall carbon nanotubes using atomically resolved scanning tunneling spectroscopy. Local conductance measurements show spectacular carbon-carbon bond asymmetry at the Van Hove singularities for both semiconducting and metallic tubes, demonstrating the symmetry breaking of molecular orbitals in nanotubes. Whatever the tube, only two types of complementary orbitals are alternatively observed. An analytical tight-binding model describing the interference patterns of $\ensuremath{\pi}$ orbitals confirmed by ab initio calculations, perfectly reproduces the experimental results.

A DFT study of electronic structures of thiophene-based organosilicon compounds

Abstract Three density functionals, B3LYP, BB95 and MPWB1K, were adopted to investigate the geometries and spectral properties of bis(3-cyano-thienyl)-dimethylsilane, bis(2-thienyl)dimethylsilane and bis(3-thienyl)dimethylsilane, respectively. The best agreement between theoretical and experimental geometries is in the MPWB1K results; the next is in the B3LYP ones, followed by the BB95 ones. MPWB1K functionals can well predict the characteristic frequencies of most vibrational modes, except for the silylmethyl C–H and the C N bond stretching vibration. BB95 functionals can give the best electronic absorption spectra, followed by MPWB1K and B3LYP one. The CN substitution as well as the connection way between the silylene and thienyl unit exerts the significant effects on the silylmethyl C–H in-plane rocking, thienyl C–H out-of-plane and bridging C–Si–C bond asymmetry stretching vibration. The most obvious difference of the electronic absorption spectra among 1 , 2 and 3 is that 1 shows three strong peaks, while 2 as well as 3 shows only one.

High sensitivity of O17 NMR to p-d hybridization in transition metal perovskites: First principles calculations of large anisotropic chemical shielding

A first principles embedded cluster approach is used to calculate O chemical shielding tensors, σ̂, in prototypical transition metal oxide ABO3 perovskite crystals. Our principal findings are (1) a large anisotropy of σ̂ between deshielded σx≃σy and shielded σz components (z along the Ti–O bond); (2) a nearly linear variation, across all the systems studied, of the isotropic σiso and uniaxial σax components, as a function of the B-O-B bond asymmetry. We show that the anisotropy and linear variation arise from large paramagnetic contributions to σx and σy due to virtual transitions between O(2p) and unoccupied B(nd) states. The calculated isotropic δiso and uniaxial δax chemical shifts are in good agreement with recent BaTiO3 and SrTiO3 single crystal O17 NMR measurements. In PbTiO3 and PbZrO3, calculated δiso are also in good agreement with NMR powder spectrum measurements. In PbZrO3, δiso calculations of the five chemically distinct sites indicate a correction of the experimental assignments. The strong dependence of σ̂ on covalent O(2p)-B(nd) interactions seen in our calculations indicates that O17 NMR spectroscopy, coupled with first principles calculations, can be an especially useful tool to study the local structure in complex perovskite alloys.

Analysis of Infrared Reflection Spectra of (Mg1−xZnx)Al2O4 Microwave Dielectric Ceramics

Infrared reflection spectra of (Mg1−xZnx)Al2O4 ceramics were analyzed by Kramers–Kroning analysis and classical oscillator model simulation. The dielectric properties were extrapolated down to the microwave range using the classical oscillator model for fitting the dielectric function. According to structure analysis, the losses originating from bend vibration and stretch vibration of the bond between A‐site cation and oxygen anion dominated the whole dielectric losses of the spinel ceramics. The coexistence of Mg and Zn deteriorated the intrinsic dielectric properties due to the bond asymmetry thus introduced. The calculated Qf (∼105 GHz) was much higher than the measured ones (∼104 GHz), suggesting that the extrinsic loss was significant. Therefore, the microwave dielectric properties of MgAl2O4 and ZnAl2O4 could be improved much by microstructure modification, and the little superiority in their solution compared with the end‐members was due to microstructure improvement.

Measurement of Olefin Bond Rotation Barriers in a Series of Vinylaniline Complexes of the Cyclopentadienyliron(II) Dicarbonyl Cation and Their Correlation to Metal−Olefin Bond Asymmetry

1H NMR techniques (selective inversion, total line shape analysis, and T1ρ measurement) have been used to measure the olefin bond rotation rates in a series of compounds of the general formula CpFe(CO)2[η2-CH2C(H)NH(p−C6H4X)], where X = OMe (4), H (6), CN (9), NO2 (10), Cl (11), COOMe (12). Complexes 4, 6, 11, and 12 enabled the use of all three methods to yield data sets that spanned a wide temperature range and allowed for reliable determination of the thermodynamic activation parameters for olefin bond rotation. When these three methods were combined with a bootstrap statistical analysis of the Eyring plots, it was possible to resolve the small differences in the ΔH⧧ values between three of the four complexes. A clear trend is established between increasing electron donation from the aniline ligand and the facilitation of olefin bond rotation. These results are placed in the context of previous work, providing additional experimental evidence that electronic control from the para position of the anilin...

Solvent induced temperature dependencies of NMR parameters of hydrogen bonded anionic clusters

The solvent induced temperature dependence of NMR parameters (proton and fluorine chemical shifts, the two-bond scalar spin coupling constant across the hydrogen bridge, 2h J FF) for dihydrogen trifluoride anion, (FH) 2F −, in a polar aprotic solvent, CDF 3/CDF 2Cl, is reported and discussed. The results are interpreted in terms of a simple electrostatic model, accounting a decrease of electrostatic repulsion of two negatively charged fluorine atoms on placing into a dielectric medium. The conclusion is drawn that polar medium causes some contraction of hydrogen bonds in ionic clusters combined with a decrease of hydrogen bond asymmetry.

Synthesis and crystal structures of 2-substituted-2-phenylsulfonyloxiranes: evidence for a generalised anomeric effect in 2-phenylsulfonyloxiranes.

Crystal structures are reported for three new sulfonyloxiranes 5, 6b and 7. Taken together with the structures previously reported for the two sulfonyloxiranes 1 and 2, these structures provide evidence of the existence of a generalised anomeric effect in 2-phenylsulfonyloxiranes, accounting for the substantial bond asymmetry in the oxirane ring, and thus for the reactivity of these oxiranes towards nucleophiles.