Confined beryllium atom electronic structure and physicochemical properties

This topical review focuses on the distinct role of carbon support coordination environment of single-atom catalysts (SACs) for electrocatalysis. The article begins with an overview of atomic coordination configurations in SACs, including a discussion of the advanced characterization techniques and simulation used for understanding the active sites. A summary of key electrocatalysis applications is then provided. These processes are oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), nitrogen reduction reaction (NRR), and carbon dioxide reduction reaction (CO2 RR). The review then shifts to modulation of the metal atom-carbon coordination environments, focusing on nitrogen and other non-metal coordination through modulation at the first coordination shell and modulation in the second and higher coordination shells. Representative case studies are provided, starting with the classic four-nitrogen-coordinated single metal atom (MN4 ) based SACs. Bimetallic coordination models including homo-paired and hetero-paired active sites are also discussed, being categorized as emerging approaches. The theme of the discussions is the correlation between synthesis methods for selective doping, the carbon structure-electron configuration changes associated with the doping, the analytical techniques used to ascertain these changes, and the resultant electrocatalysis performance. Critical unanswered questions as well as promising underexplored research directions are identified.

The redox capacities of 5-hydroxy-1,4-naphthaquinone (VI), 5-8-dihydroxy-1,4-naphthaquinone (VII), and 5,8-dihydroxy-1,4-naphthaquinone imine (VIII) as model systems for the pharmacophore of aclacinomycin A, adriamycin/daunomycin, and 5-iminodaunomycin (5IDN), respectively, along with 1,4-naphthaquinone (V), 1,4-benzoquinone (IV), and 1,4-benzoquinone imine (IX), have been investigated by the AM1 semiempirical method. The reduction activation of the parent (Q) model systems to their various redox states [quinone radical anion (Q−2), semiquinone (QH˙), semiquinone anion (QH−), and hydroquinone (QH2)], the redox capacities of the redox states, and the intermolecular electron self-exchange processes between the redox states and electron transfer reactions from the redox states to molecular oxygen have been examined using reaction enthalpies, adiabatic ionization potentials and electron affinities, and absolute and adiabatic electronegativities. Keto—enol transformations and the effects of solvation and H bonding on keto–enol tautomers of VI and of the hydroquinones of VI and VIII have also been assessed. The results indicate that the reactivity of VIII, relative to that of VII, may not be diminished. VI, however, appears to be less reactive than VII, and this suggests clues for the reduced toxicities of aclacinomycin A. Overall, the results suggest that the experimentally observed reduced cardiotoxic effects of 5IDN may be explained by changes in electron configuration and/or electron density and in geometry, such as changes in planarity that accompany enol to keto transformations in the reduction by product of 5IDN (Bird et al.7)—that is, between the hydroquinone (II) of naphthacenedione (I) and naphthacenone (III). Moreover, the results suggest that the two-electron reduction product, Q−2, of the drugs can be the reductant that produces reactive dioxygen species, such as O2−˙ and HO2˙, via electron transfer to molecular oxygen as opposed to QH˙ and QH2, which have been postulated to be responsible for electron transfer. This possibly new role for Q−2 may be important in cardioxicity, particularly in aprotic and/or hydrophobic media. © 1996 by John Wiley & Sons, Inc.

IT has long been realized in the field of molecular spectra that there is need of some form of molecule-building principle equivalent to the Atombau principle of Bohr. This would enable the electron configuration and term type of at least the ground-state of any molecule to be predicted from its position in the Periodic Table. Mulliken1 attempted to trace the change in electron configuration from molecule to molecule for the lighter members of the Table, but his pioneering work was hampered by the absence of many necessary spectrum data, and no progress has since been made. In particular, very little knowledge has been obtained on the configuration of the heavier diatomic molecules.

The coherent wavepacket dynamics observed in 35 fs crosscorrelation pumpprobe spectra of 2-(2’-hydroxyphenyl)benzothiazole render a novel and detailed picture of the excited state intramolecular proton transfer, the contributing skeletal deformations and the electronic configuration changes.

Single-atom catalysts (SACs) represent a promising class of low-cost materials with high catalytic activity and 100% atom-utilization efficiency, widely applied in energy conversion fields.[1-2] Benefit from tunable coordination environment, the uniformly dispersed SACs on diverse supports exhibit satisfied catalytic ability. More importantly, different from the multiple crystal facets of nanomaterials can serve as active sites. There is only one active site consisting of the single metal atom and coordinated atoms in SACs, thereby improving the selectivity of products some catalytic reactions. Moreover, atomically dispersed metal catalysts exist in cations, which exhibit better stability than traditional nanoparticulated counterparts.[3] However, SACs are susceptible to undergo restructuring during the reactions because of high surface free energy. Therefore, exploring the active sites of SACs through in-situ characterization techniques plays a critical role in studying the reaction mechanism and guiding the design of optimum SACs.As a synchrotron-based element-specific technique, X-ray absorption spectroscopy (XAS) is influential in studying the probed element's structural information. X-ray absorption near edge structure (XANES) spectra reflect the electronic structure, while extended X-ray absorption fine structure (EXAFS) spectra provide local atomic structural information such as coordination number, bond length, and structural disorder. Thus, in-situ XAS is promising and widely used for monitoring electronic structure and atomic configuration changes of SACs during real-time working conditions. We have prepared Ni, Cu single atoms on nitrogen-doped carbon (NiCu@NC-900), which exhibit improved catalytic ability compared with Ni SACs and Cu SACs. Nuclear magnetic resonance results show only ethanol as a liquid product during CO2 reduction reaction (CO2RR). To explore the origin of improved catalytic performance of SACs toward CO2RR, we are going to monitor changes of SACs under open-circuit voltage (OCV) and working conditions by Operando XAS.[1] Li, Z., Ji, S., Liu, Y., Cao, X., Tian, S., Chen, Y., Niu, Z., Li, Y. Chem. Rev. 120, 2 (2020).[2] Ji, S., Chen, Y., Wang, X., Zhang, Z., Wang, D., Li, Y. Chem. Rev. 120, 21 (2020).[3] Speck, F. D., Paul, M. T. Y., Ruiz-Zepeda, F., Gatalo, M., Kim, H., Kwon, H. C., Mayrhofer, K. J. J., Choi, M., Choi, C. H., Hodnik, N., Cherevko, S. J. Am. Chem. Soc. 142, 36 (2020).

XTA (X-ray transient absorption) spectroscopy has been used to probe the transient structure during interfacial electron transfer between photo-sensitizer and semiconductor nanocrystals. Two sample systems have been studied, Ru(dcbpy)2(NCS)2 and Os(dcbpy)2(CN)2 (dcbpy = bis(4,4′-dicarboxy-2,2′-bipyridyl) adsorbed to TiO2 nanoparticle surface. The photoexcited Ruthenium or Osmium polypyridyl photosensitizers inject electrons to conduction band of TiO2 nanoparticle, resulting in interfacial charge-transfer state. Both the electronic configuration and molecular structure changes of Ruthenium and Osmium complexes in the charge-separated state have been observed.

Electrochemical energy storage is an enabling technology for personal and industrial electronics, adoption of intermittent renewable energy, and the electrification of transportation. From a fundamental solid-state chemistry perspective, and in the context of batteries, it is interesting to explore new mixed ionic–electronic conductors that can withstand large changes in composition and electronic configuration over ∼1000 charge– discharge cycles to function as electrode materials and to explore new pure ion conductors with extremely low electronic conductivities that could function as solid electrolytes or interfacial coatings. Understanding the mechanisms that facilitate ion and/or electron transport or induce material degradation are the keys to discovering and engineering the next generation of battery materials.Relatively few unique crystal structures underpin most battery materials. We are particularly interested in novel complex oxides that might offer new insights into structure–property relationships or even new performance characteristics. This talk will focus on recent examples from our lab: (i) defects, electrochemistry, and metal–metal bonding in NaNb7O18 and NaNb13O33 framework structures (Wadsley–Roth derivatives) with tunnel-blocking defects, and (ii) new lithium-rich “layered” structures, Li3MO4 (M = Nb, Ta) synthesized via instantaneous ion exchange in a molten salt flux. Both families of materials are characterized with an ‘NMR crystallography’ approach that combines X-ray and neutron diffraction with DFT-supported solid-state NMR spectroscopy.

The 2D NMR of ND3 molecules in the intercalation complexes TaS2(ND3) and TiS2(ND3) has been studied at temperatures from 5–300 K by wideline techniques. The magnitude and temperature dependence of the observed electric quadrupolar coupling constant (e2qQ/h)eff provides information about the configuration of the intercalated ND3 molecules and their dynamics. The similarity of (e2qQ/h)eff to previously reported values for solid ND3 suggests that intercalation entails only small changes in molecular geometry or electronic configuration. The molecules exhibit several types of motion, including spinning about their threefold (C3) symmetry axis and a complex reorientation of that axis. The rates of these motions are reduced as the temperature is lowered, with reorientation falling to ∼106 s−1 near 140 K. C3 spinning persists at rates greater than 106 s−1 even at temperatures of 5 K. The quadrupole interaction does not vanish at high temperatures, ∼300 K, showing that reorientation of the C3 axis is not isotropic and the form of motion is deduced from the high temperature value of the coupling constant. A central component of the spectrum is observed at low temperatures and attributed to ND3 molecules residing near molecular vacancies in the layers. The present results are compared in detail with 1H observations in the ND3 analogues.

The photophysical properties of fluorescent Hammett acidity indicator derived from 3,4,5,6-tetrahydrobis(pyrido[3,2-g]indolo)[2,3-a:3',2'-j]acridine (1a), 6-bis(pyrido[3,2-g]indol-2'-yl)pyridine (1b) and their analogues have been investigated in sulfuric acid solutions by means of absorption, fluorimetry, relaxation dynamics and computational approach. These new indicators undergo a reversible protonation process in the Hammett acidity range of H0 < 0, accompanied by a drastic increase of the bright blue-green (1a) or yellow (1b) fluorescence intensity upon increasing the acidity. For 1a in H2 SO4 , the emission yield increases as large as 200 folds from pH = -0.41 to the Hammett acidity range of -5.17, the results of which are rationalized by a much increase of the steric hindrance upon third protonation toward the central pyridinic site, together with their accompanied changes of electronic configuration from charge transfer to a delocalized ππ* character in the lowest lying excited state. The combination of 1a and 1b renders a wide and linear range of H0 measurement from -1.2 to -5.1 detected by highly intensive fluorescence.

The tetramanganese–calcium cluster of the oxygen‐evolving complex of photosystem II adopts electronically and magnetically distinct but interconvertible valence isomeric forms in its first light‐driven oxidized catalytic state, S2. This bistability is implicated in gating the final catalytic states preceding O−O bond formation, but it is unknown how the biological system enables its emergence and controls its effect. Here we show that the Mn4CaO5 cluster in the resting (dark‐stable) S1 state adopts orientational Jahn–Teller isomeric forms arising from a directional change in electronic configuration of the “dangler” MnIII ion. The isomers are consistent with available structural data and explain previously unresolved electron paramagnetic resonance spectroscopic observations on the S1 state. This unique isomerism in the resting state is shown to be the electronic origin of valence isomerism in the S2 state, establishing a functional role of orientational Jahn–Teller isomerism unprecedented in biological or artificial catalysis.

Studies on the La 2− xSr xNiO 4 (0 ⩽ x ⩽ 1) system

In order to study the electronic configuration changes arising from substitution of weak Li‐O bonds by the stronger Mg‐O bonds in the perovskite‐type planes of the K2NiF4 structure, the novel title oxides are prepared in four successive steps: 1. calcination of a stoichiometric mixture of the corresponding nitrates; 2. annealing at 950 °C in an oxygen stream (4 d); 3. 750 °C, 1.5 kbar O2 pressure, 48 h; 4. 800 °C, 1.5 kbar O2, 6 h. A second order spin‐transition is detected with increasing temp. by magnetic measurements and XRD.

The electronic structure and transport properties of In₂₄M₈O₄₈ (M = Ge(4+), Sn(4+), Ti(4+), and Zr(4+)) have been studied by using the full-potential linearized augmented plane-wave method and the semiclassical Boltzmann theory, respectively. It is found that the magnitude of powerfactor with respect to relation time follows the order of In₂₄Sn₈O₄₈ > In₂₄Zr₈O₄₈ > In₂₄Ge₈O₄₈ > In₂₄Ti₈O₄₈. The largest powerfactor is 2.7 × 10¹² W/K² ms for In₂₄Sn₈O₄₈ at 60 K, which is nearly thirty times larger than those of conventional n-type thermoelectric materials. The origin of the different thermoelectric behavior for these compounds is discussed from the electronic structure level. It is found that, at low temperature, the dopant strongly affect the bands near the Fermi level, which consequently leads to their different thermoelectric properties. The electronic configuration and the difference in atomic number between the dopant and the host atom also play an important role on the thermoelectric properties of In₂₄M₈O₄₈. Our calculations give a valuable insight on how to enhance the thermoelectric performance of In₃₂O₄₈.

Monte Carlo methods for using the ground state variational principle to find bounds on the ground state energy by minimizing Et = (ψt|H|ψt)/(ψt|ψt) are plagued by the intrinsic randomness of the Monte Carlo estimate of Et. These do not give Et, but rather Ēt, ± σt, with Ēt the Monte Carlo estimate of Et and σt the estimate for the standard deviation. When ψt is an cigenfunction of H, it is possible to find σt = 0. It is also possible to calculate differences such as (σt − σt′) over the same points so that their difference is much more accurate than would seem possible from the knowledge of σt and σt′, alone. This allows us to vary ψt so as to minimize σt, or Ēt + nσt, rather than Ēt alone. The method has been tested on the ground state of a single lithium atom. A trial wave function, ψt. involving a very general form and 33 constants was found for which Et = −14.898 ± 0.003 Ry which is lower than the Hartree-Fock energy bound of −14.865 Ry (C. Clementi, IBM J. Res. Develop. (Suppl.) 9, 2 (1965)) and above the true energy of −14.956 Ry (S. Larsson, Phys. Rev. 169, 49 (1968)). The small Monte Carlo error relative to the cohesive energy of solid lithium, −0.512 Ry (V. Heine, in The Physics of Metals, J. Ziman, Ed. (Cambridge Univ. Press, England, 1969)), along with the inherent flexibility of the method implies that it should be possible to extend the work to treat collections of M nuclei and 3M electrons. The nuclear- electronic probability distribution for atomic lithium was also evaluated. A comparison with that for the Hartree-Fock wave function shows small differences in accord with those expected from the inclusion of electron- electron parts in the trial wave functions considered here which are theoretically present but are omitted in the Hartrce Fock approximation.

X-ray absorption, resonant X-ray emission, and X-ray photoelectron spectroscopical methods have been applied for the study of the electronic structure of defective lithium cobaltites LixCoO2 (0.6≤x≤1.0). Resonant O Kα X-ray emission spectra of LiCoO2 showed localized excitonic states due to a dd transition between occupied and unoccupied Co 3d states. On the base of measurements of Co 3s X-ray photoelectron, Co 2p, and O 1s X-ray absorption spectra, it was established that in defective cobaltites the electronic holes are localized mainly in O 2p states. An evidence of phase separation in LixCoO2 has been found. It was shown that the semiconductor-to-metal transition in LixCoO2 (x<0.76) at about 160 K is not accompanied by changes in the Co 3d electronic configuration which remains 3d6.