Cationic States Research Articles

Photoexcited Intramolecular Charge Transfer in Dye Sensitizers: Predictive In Silico Screening for Dye-Sensitized Solar Cell Devices.

Efficient photoinduced intramolecular charge transfer (ICT) from donor to acceptor in dye molecules is the functional basis and key property in the working of a dye-sensitized solar cell (DSSC). To understand the ICT process in photoexcited dye molecules, we analyze the electronic properties and structural parameters of a chosen set of experimentally synthesized donor–acceptor (D–A) and donor−π-spacer−acceptor (D−π–A) type dye molecules in their ground, excited, and cationic states. The correlation between structural modification and charge redistribution in different parts of the molecule helps to identify the extent of π-conjugation and spatial rearrangement of electron density localization along the molecular skeleton. We find that prominent twisting of several groups and the resulting molecular bond rearrangements in larger parts of the molecule promote efficient donor to acceptor ICT, such as in D–A type ADEKA1 and C275 dyes. Thus, based on the modest computation of structural and electronic properties of dye molecules in their respective ground, excited, and cationic states, we identify the desired structural changes that facilitate tunable intramolecular charge transfer to highlight a simple and direct prescription to screen out probable efficient dye molecules among many samples. Our approach complements recent experimental evidence of capturing the structural view of the excited-state charge transfer in molecules.

Carbon/chlorinate deposition on MnOx-CeO2 catalyst in chlorobenzene combustion: The effect of SCR flue gas

• Chlorine accumulation has great impact on the MnO x -CeO 2 catalyst deactivation. • SCR flue gas alleviate the poisoning process in CB catalytic oxidation. • Carbon deposition in MPC process is less than that of CB oxidation. • Chlorine adsorption on MnO x -CeO 2 catalyst surface is suppressed by NH 3 . NO x and chlorobenzene (CB) generally coexist together in the industrial flue gas, and MnO x -CeO 2 catalyst is reported as one of possible candidates to remove them simultaneously. However, the influences of complex gas components on the catalysts are not studied well. Herein, coke deposition and chlorine accumulation are systematically investigated. Compared with coke deposition, the surface chlorine accumulation has a greater impact on the CB oxidation performance. NO and NH 3 could reduce the surface coke loading and mitigate the coke deposition extent. The deposited coke mainly consists of various intermediates derived from the nucleophilic and electrophilic substitution of CB, and it could be released in 200–600 °C as CO 2 . The surface chlorine influences the oxidation state of Mn cations rather than Ce, which restrains the redox property. These species bond to the surface up to 600 °C. DFT results demonstrate that the chlorine anchors on the oxygen vacancies of the MnO x -CeO 2 models and hinders the adsorption and activation of O 2 , which could inhibit the CB oxidation. The adsorbed NH 3 weakens the bonding energies of chlorine to the surface, accounting for the mitigation of SCR flue gas on CB oxidation.

Redox-Active Hydrocarbons: Isolation and Structural Determination of Cationic States toward Advanced Response Systems.

Organic chemistry has developed rapidly as a carbon-based science. Particularly, hydrocarbons with aromatic rings have attracted much attention as molecular materials for use in organic electronics. In principle, aromatic species can be constructed if carbon and hydrogen atoms are available. Therefore, revealing the nature of pure hydrocarbons with an (anti)aromatic nature should pave the way for the development of as yet unexplored organic materials. In this Review, we focus on pure hydrocarbons composed of only carbon and hydrogen atoms, and present recent studies on their intrinsic electrochemical and spectroscopic properties and structural characterization of their cationic states. We also address more sophisticated hydrocarbon-based response systems that are capable of modulating their properties in response to external stimuli such as light, heat, and electric potential.

Threshold Photoelectron Spectroscopy of Quinoxaline, Quinazoline, and Cinnoline

The threshold photoelectron spectra of cinnoline, quinazoline, and quinoxaline, three small naphthalene-analogue polycyclic nitrogen-containing hydrocarbons of C8H6N2 composition, were recorded. The spectra are assigned to understand their electronic structure and the role of isomerism. Furthermore, this work provides reference data for the selective identification of such species as gas-phase reaction products at low number densities. Imaging photoelectron photoion coincidence spectroscopy was used at the VUV beamline of the Swiss Light Source to record the spectra from the ionization onset to 12 eV. To assign and interpret the spectral features, we relied on (time-dependent) density functional theory and EOM-IP-CCSD calculations and computed vertical and adiabatic ionization energies as well as Franck-Condon factors to simulate ground- and excited-state spectra. Vibrational progressions belonging to four electronic states could be simulated in each of the samples, and we report a total of 12 adiabatic ionization energies, including the ones to the ground and excited cation states. Such a wealth of spectral information, as well as the reliable ab initio modeling, is promising with regards to analytical applications. While cinnoline can be easily distinguished by its lowest adiabatic ionization energy, quinazoline and quinoxaline show different vibrational fingerprints, which can be used to distinguish the three isomers even in complex reaction mixtures. Finally, we also relate the cation electronic states to the neutral molecular orbitals and note that Koopmans' approximation fails in these N2-containing species very much like it does in N2.

Tuning the thiophilicity of Pr6O11 (PrO1.833) through Pb-substitution to append efficient adsorption characteristics

With the intent of enhancing the thiophilic character of Pr6O11 (PrO1.833), pure and Pb-substituted samples were synthesized by combustion synthesis and characterized extensively. We could introduce up to 30 mol % of Pb in PrO1.833, retaining fluorite structure as revealed by powder X-ray diffraction and Raman spectroscopy measurements. The non-linear variation of the cubic lattice parameter of Pb-substituted samples suggested mixed oxidation states of cations, further established with XPS analysis. Pr0.70Pb0.30O2-δ showed porous flaky morphology and had a surface area of 344.4 m2/g. Charge transfer bands dominated in the UV–visible spectra of these samples. The Pb-substituted samples were examined as adsorbents for the harmful dye molecules (Congo red and Xylenol orange) having SO3 groups in their structural skeleton. The Langmuir model described the adsorption process well. Pr0.70Pb0.30O2-δ showed a maximum adsorbent capacity of 909 (CR) and 500 (XO) mg/g, attaining equilibrium rapidly (4 min). The thermodynamics and kinetics of the adsorption process have been examined, and a suitable adsorption mechanism is proposed. After the adsorption experiments, sulfur and PrS1.7 formation on the surface of Pr0.70Pb0.30O2-δ illustrated the exciting red-ox chemistry at the interface.

Dynamics of Methyl Radical Formation Following 266 nm Dissociative Photoionization of Xylenes and Mesitylene.

The 266 nm dissociative photoionization of three xylene isomers and mesitylene leading to the formation of methyl radical was examined. The total translational energy distribution profiles [P(ET)] for the methyl radical were almost identical for all of the three isomers of xylene and mesitylene, while a substantial difference was observed for the corresponding P(ET) profile of the co-fragment produced by loss of one methyl group in m-xylene. This observation is attributed to the formation of the methyl radical from alternate channels induced by the probe. The P(ET) profiles were rationalized based on the dissociation of {sp2}C-C{sp3} bond in the cationic state, wherein the {sp2}C-C{sp3} bond dissociation energy is substantially lower relative to the neutral ground state. The dissociation in the cationic state follows a resonant three-photon absorption process, resulting in a maximum translational energy of about 1.6-1.8 eV for the photofragments in the center-of-mass frame. Fitting of the P(ET) profiles to empirical function reveals that the dynamics of {sp2}C-C{sp3} bond dissociation is insensitive to the position of substitution but marginally dependent on the number of methyl groups.

The Role of Scandium Substitution in Babingtonite Group Minerals

Sc-rich babingtonite from Heftetjern, Norway and Baveno, Italy were examined using electron microprobe analysis and X-ray single-crystal refinement in order to re-examine the behavior of Sc3+ and analyze its effect on the crystal structure of babingtonite. The Sc2O3 content is 13.78 wt.% in the Heftetjern specimen, and 8.44 wt.% in the Baveno one. In contrast, the latter has higher Fe content (11.13 wt.% as FeO) rather than the former one (8.63 wt.% as FeO). Characteristically, both specimens contain sodium. Although the oxidation state of octahedral cations in babingtonites is in general Me2+:Me3+ = 1:1, trivalent cations in the Heftetjern specimen attain 1.14 apfu. This excess of trivalent cations must be counterbalanced by monovalent Na substituted for Ca. The unit-cell parameters are a = 7.5272(1), b = 11.7175(1), c = 6.7613(1) Å, α = 91.710(1), β = 93.637(1), γ = 104.522(1)°, and V = 575.49(2) Å3 for the Heftetjern specimen, and a = 7.5199(2), b = 11.7145(3), c = 6.7408(2) Å, α = 91.756(2), β = 93.786(2), γ = 104.549(2)°, and V = 573.83(3) Å3 for the Baveno one. The structural formulae are A1Ca1.00A2(Ca0.879Na0.121)M1(Sc3+0.42Fe2+0.37Mn2+0.21)M2(Sc3+0.68Fe2+0.27Mg0.03Fe3+0.02)Si5O14(OH) for Heftetjern, and A1Ca1.00A2(Ca0.819Na0.181)M1(Sc3+0.43Mn2+0.36Fe2+0.21)M2(Fe3+0.36Fe2+0.30Sc3+0.26Sn4+0.05Al0.03)Si5O14(OH) for Baveno. Due to Sc3+ substitution, the <M2–O> distance, 2.09–2.11 Å, is longer than that of Sc-free babingtonite, 2.03–2.05 Å. The M2O6 expansion leads to the lengthened O4–O10 edge shared between the M1O6 and M2O6 octahedra, and causes the stronger angular distortion of M2O6. This can be explained by the increase of the O4–M1–O10 angle and decrease of the O4–M1–O8 angle with lengthening of the O4–O10 edge.

The effect of the incorporation of catalase mimetic activity cations on the structural, thermal and chemical durability properties of the 45S5 Bioglass®

Understanding the effect of the incorporation of doping ions into the structure, thermal properties and chemical durability of bioactive glasses is fundamental for the design of new compositions with tailored biological functions and applications. In this work, we have applied a combined experimental and computational approach to unravel the effect of adding metal oxides of Ce, Ti, V, Mn, Fe, Co, Cu, and Zr that impart catalase mimetic activity to the 45S5 Bioglass® on its density, thermal properties and chemical durability. UV-Vis-NIR spectroscopy and temperature-programmed reduction (TPR) experiments allowed us to determine the oxidation states of the doping cations in the bulk of the glasses, Differential Thermal Analysis has been used to determine the glass transition and crystallization temperature, whereas the chemical durability in water was determined by following the hydrolytic resistance of glass grains at 98 °C standard method. The experimental results have been interpreted at the atomic level by exploiting reliable bulk and surface structural models of the investigated glasses generated by using Molecular Dynamics Simulations. Structure-property relationships helpful for the rational design of new glass compositions have also been inferred.

Direct Crystallization of Diamine Radical Cations: Carbon-Nitrogen Bond Formation from the Reaction of Triphenylamine with TiCl4 , TiBr4 , or SnCl4 versus Carbon-Carbon Bond Formation with SbCl5.

Direct synthesis of diamine radical cations in crystalline form proceeding through oxidation of triphenylamine followed by the formation of a new C-N bond is reported. Although the oxidative coupling of triphenylamine is well studied, diamine products are rarely captured in their radical cation state. The neutral diamine most frequently obtained from this reaction pathway is N,N,N',N'-tetraphenylbenzidine. Herein, the capture of radical cations of diamines in crystalline form in one step starting with neutral triphenylamine was demonstrated, and the formation of two products (the radical cations of N,N,N',N'-tetraphenyl-1,4-benzenediamine or N,N,N',N'-tetraphenylbenzidine) depending on the oxidizing agent used was observed. The radical species are characterized by single-crystal X-Ray diffraction, electron paramagnetic resonance spectroscopy, and optical spectroscopy.

Investigation of the low-lying electronic states of sulfur monofluoride cation

High-level ab initio computations are performed for the low-lying Λ-S and Ω electronic states of SF+, with the icMRCI+Q methodology coupled with basis sets of quintuple-ζ quality. The manifolds of electronic states correlating with the first and second dissociation limits are well determined. Electronic structures of all excited states investigated are characterized for the first time. Dipole moments of all states are obtained, and trends of polarity variations are discussed. Transition probabilities, such as transition dipole moments, Franck-Condon factors, vibrational branching ratios and radiative lifetimes, are also calculated for selected transition bands. We have also performed a parallel calculation for the SF radical to verify our computational accuracy. From comparative studies on associated parameters derived in this work and those reported in literature, a mutually satisfactory agreement is noticed, indicating our data and results are trustworthy.

Chemical Structure of Conductive Filaments in Tantalum Oxide Memristive Devices and Its Implications for the Formation Mechanism

AbstractResistive switching in metal oxides is believed to be caused by a temperature and electric field driven redistribution of oxygen vacancies within a nanometer sized conductive filament. Accordingly, gaining detailed information about the chemical composition of conductive filaments is of key importance for a comprehensive understanding of the switching process. In this work, spectromicroscopy is used to probe the electronic structure of conductive filaments in Ta2O5‐based memristive devices. It is found that resistive switching leads to the formation of a conductive filament with an oxygen vacancy concentration of ≈20%. Spectroscopic insights provide detailed information about the chemical state of the tantalum cations and show that the filament is not composed of a metallic Ta0 phase. As an extreme case, devices after an irreversible dielectric breakdown are investigated. These devices feature larger conductive channels with higher oxygen vacancy concentrations. Using the experimental data as input for finite element simulations, the role of thermodiffusion for the formation process of conductive filaments is revealed. It is demonstrated that thermodiffusion is not the dominating effect for the filament formation here but might play a role in accelerating the forming process, as well as in the stabilization of the filament.

Nuclear dissociation after the O 1s → ()3sσ excitation in O2 molecules

We investigate the dissociation dynamics of core-excited O2 molecules using a high-resolution energy-resolved electron-ion coincidence experimental setup. The excited cationic states with two valence holes and one Rydberg electron are created after spectator Auger decay induced by O 1s → ()3sσ core excitation in O2. From the energy correlation between the kinetic energy of the Auger electron and the ion kinetic energy release, we distinguish several dissociation channels. Rather complex dissociation channels of the spectator Auger final states are disclosed, which can be explained by the increased number of the crossing point due to the existence of Rydberg electron. The quantum system will evolve into different dissociation limits at each crossing point between the potential energy curves.

Aromatic Interactions in Glycochemistry: From Molecular Recognition to Catalysis.

Aromatic platforms are ubiquitous recognition motifs occurring in protein carbohydrate- binding domains (CBDs), RNA receptors and enzymes. They stabilize the glycoside/ receptor complexes by participating in stacking CH/π interactions with either the α- or β- face of the corresponding pyranose units. In addition, the role played by aromatic units in the stabilization of glycoside cationic transition states has started being recognized in recent years. Extensive studies carried out during the last decade have allowed the dissection of the main contributing forces that stabilize the carbohydrate/aromatic complexes, while helping delineate not only the standing relationship between the glycoside/ aromatic chemical structures and the strength of this interaction but also their potential influence on glycoside reactivity.

Data-Driven Quest for Two-Dimensional Non-van der Waals Materials.

Two-dimensional (2D) materials are frequently associated with the sheets forming bulk layered compounds bonded by van der Waals (vdW) forces. The anisotropy and weak interaction between the sheets have also been the main criteria in the computational search for new 2D systems, predicting ∼2000 exfoliable compounds. However, some representatives of a new type of non-vdW 2D systems, without layered 3D analogues, were recently manufactured. For this novel materials class, data-driven design principles are still missing. Here, we outline a set of 8 binary and 20 ternary candidates by filtering the AFLOW-ICSD database according to structural prototypes. The oxidation state of the surface cations regulates the exfoliation energy with low oxidation numbers leading to weak bonding─a useful descriptor to obtain novel 2D materials also providing clear guidelines for experiments. A vast range of appealing electronic, optical, and magnetic properties make the candidates attractive for various applications and particularly spintronics.

Mechanism of liquid-phase metal infiltration into pyridine-containing polymeric thin films

This research studied the liquid-phase selective metal infiltration of a pyridine-containing block copolymer (BCP). The results demonstrated that the metal cations interact differently with the poly(2-vinylpyridine) in polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP); the oxidation state and type of cation play an important role in the infiltration process, which is also affected by the solvent. Silver (Ag+) did not present an effective infiltration, while nickel (Ni2+) and chromium (Cr3+) were able to reproduce the pattern of the BCP. These differences can be associated with the coordination of the pyridinic nitrogen and the metal, in parallel with π-interactions. Furthermore, in the case of chromium, the presence of oxyanions led to an improved infiltration and high fidelity on the formation of the ordered metal oxide layer.

Effective degradation of hydrolyzed polyacryamide (HPAM) in a simultaneous combination of acoustic cavitation and microbubbles ozonation: Process optimization and degradation mechanism

Managing environmental contamination with hydrolyzed polyacryamide (HPAM) is essential due to its persist long with slow biodegradability influence on the environment. In this study, the simultaneous combination of acoustic cavitation and microbubbles ozonation (US/O3) was applied to generate additional highly reactive hydroxyl radicals (·OH) and thus to enhance the degradation of HPAM. Compared with the two separated degradation process methods, the coupled method exerts a synergistic effect on the decomposition of HPAM, with an enhancement factor of 1.50. Effects of aeration pattern, ultrasound irradiation, operating temperature, initial HPAM concentration, and valance state of cations on the removal of HPAM were investigated intensively. An increase in valance state of cations contributes to HPAM removal. The maximum HPAM degradation, chemical oxygen demand (CODcr) removal and viscosity reduction of the HPAM wastewater were 97.35%, 89.01% and 93.25%, respectively. The degradation of HPAM conformed to the first-order reaction kinetic model. Removal of HPAM followed hydroxyl radical mechanism. The degradation mechanism of HPAM was also discussed with the change of FTIR and UV–Visible spectra of HPAM in investigated processes. The main reaction intermediates, such as heptanoic anhydride, oleamide, myristamide, acetic acid, acetamide, and propanamide, are identified and a possible degradation pathway is proposed during the US/O3 process. The process was proved to be a suitable technique for dealing with HPAM-containing wastewater.

Time-resolved luminescence spectroscopy of ultrafast emissions in BaGeF6

Phase-pure crystalline micropowder samples of BaGeF6 were prepared and studied under excitation by tuneable synchrotron radiation and 10 keV electron beam. Time-resolved photoluminescence emission and excitation spectra and a set of single emission decay curves were recorded at 7 K for the exciting photon energy region of 4.3–45 eV. Several intrinsic emissions were revealed in BaGeF6 and their origin investigated. A single broad emission band peaking at 455 nm is assigned to be of excitonic origin due to its long decay time in the μs range and due to the presence of an intense excitation peak at 10.1 eV right in the region of the host absorption onset. The energy gap width of BaGeF6 was determined experimentally from the photoluminescence excitation spectra of the 455 nm emission to be 10.9 eV. Several emission bands, including distinct peaks at 270 and 455 nm, with the main decay component of ∼180 ps were revealed across the wavelength range of 200–500 nm. The revealed ultrafast emissions were studied by means of time-resolved photoluminescence spectroscopy and their origin was assigned to cross-luminescence resulting from radiative transitions between the Ba 5p core level and sub-bands of the valence band (Ge 4s, Ge 4p and F 2p hybridized states) and to intraband luminescence between the valence band sub-bands. Photoluminescence excitation spectra of the ultrafast emissions revealed a gently sloping onset at 17 eV, related to transitions from the Ge 4s states. It is followed by a distinct peak at 19.4 eV, which corresponds to the ionization of the Ba 5p cation states and is related to the excitation threshold of cross-luminescence.

Structures and electronic states of trimer radical cations of coronene: DFT-ESR simulation study.

Coronene (C24H12), a charge transfer complex with low-cost and high-performance energy storage, has recently attracted attention as a model molecule of graphene nano-flakes (GNFs). The stacking structures of the trimer radical cation correlate strongly with the conduction states of the GNFs. In the present paper, the structures and electronic states of the monomer, dimer and trimer radical cations of coronene were investigated by means of density functional theory calculations. In particular, the proton hyperfine coupling constants of these species were determined. The radical cation of coronene+ (monomer) showed two structures corresponding to the 2Au and 2B3u states due to the Jahn-Teller effect. The 2Au state was more stable than the 2B3u state, although the energy difference between the two states was only 0.03 kcal mol-1. The dimer and trimer radical cations took stacking structures distorted from a full overlap structure. The intermolecular distances of the molecular planes were 3.602 Å (dimer) and 3.564 and 3.600 Å (trimer). The binding energies of the dimer and trimer were calculated to be 8.7 and 13.3 kcal mol-1, respectively. The spin density was equivalently distributed on both coronene planes in the dimer cation. In contrast, the central plane in the trimer cation had a larger spin density, ρ = 0.72, than the upper and lower planes, both with ρ = 0.14. The proton hyperfine coupling constants calculated from these structures and the electronic states of the monomer, dimer, and trimer radical cations of coronene were in excellent agreement with previous ESR spectra of coronene radical cations. The structures and electronic states of (coronene)n+ (n = 1-3) were discussed on the basis of the theoretical results.

The Jahn-Teller and pseudo-Jahn-Teller effects in the propyne radical cation.

The Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) effects in the X̃2E, Ã2E and B̃2A1 electronic states of the propyne radical cation are investigated with the aid of ab initio quantum chemistry calculations and first principles quantum dynamics simulations. For the latter, both time-independent and time-dependent quantum mechanical methods are employed. Standard vibronic coupling theory is used to construct a symmetry consistent vibronic Hamiltonian in a diabatic electronic basis. Taylor series expansion of the elements of the diabatic electronic Hamiltonian is carried out and the parameters that appear in the expansion are derived from the ab initio calculated adiabatic electronic energies. It is found that the JT effect is weak in the X̃2E state as compared to that in the Ã2E state. Because of large energy separation, the PJT coupling among the JT-split components of the X̃2E state with the neighboring states is also very weak. However, the PJT coupling of the B̃2A1 state with the JT split components of the Ã2E state has some impact on the dynamics in the coupled Ã2E-B̃2A1 electronic states. The vibronic spectrum of each of these states is calculated and compared with the experimental results. The nonradiative internal conversion dynamics of excited cationic states is examined. Interesting comparison is made with the JT and PJT coupling effects in the nuclear dynamics of the X̃2E-Ã2E-B̃2B2 electronic states of the isomeric allene radical cation.

HCl-Induced Hg0 Transformation over CuMn2O4 Sorbent

CuMn2O4 spinel has been regarded as a highly efficient sorbent for Hg0 capture from flue gas. The regenerability and recyclability of CuMn2O4 sorbent are mainly associated with the mercury speciation adsorbed on its surface. However, the effect mechanism of HCl on Hg0 transformation over CuMn2O4 sorbent is still elusive. Experiments were conducted to understand the effect of HCl on Hg0 transformation over CuMn2O4 sorbent. The results indicate that CuMn2O4 sorbent is a mesoporous material and possesses a good thermal stability. CuMn2O4 shows >95% Hg0 removal efficiency in a wide temperature window of 50–350°C. The favorable electron-transfer environment caused by the mixed valence states of Cu and Mn cations is responsible for the excellent Hg0 removal performance of CuMn2O4 sorbent. CuMn2O4 shows a higher Hg0 adsorption capacity of 4774.57 μg/g. Hg0 adsorption process over CuMn2O4 sorbent can be well described by the developed kinetic model. Hg0 removal efficiency of CuMn2O4 sorbent does not depend on the presence of HCl. Mercury species adsorbed on the CuMn2O4 sorbent in the presence of HCl mainly exist in the forms of HgO and HgCl2O8 · H2O. HCl shows a significant effect on mercury speciation over CuMn2O4 sorbent. Most of HgO species will be transformed into HgCl2O8 · H2O in the presence of HCl.