Closed-shell Electronic Structures Research Articles

Regulation of quantum spin conversions in a single molecular radical.

Free radicals, generally formed through the cleavage of covalent electron-pair bonds, play an important role in diverse fields ranging from synthetic chemistry to spintronics and nonlinear optics. However, the characterization and regulation of the radical state at a single-molecule level face formidable challenges. Here we present the detection and sophisticated tuning of the open-shell character of individual diradicals with a donor-acceptor structure via a sensitive single-molecule electrical approach. The radical is sandwiched between nanogapped graphene electrodes via covalent amide bonds to construct stable graphene-molecule-graphene single-molecule junctions. We measure the electrical conductance as a function of temperature and track the evolution of the closed-shell and open-shell electronic structures in real time, the open-shell triplet state being stabilized with increasing temperature. Furthermore, we tune the spin states by external stimuli, such as electrical and magnetic fields, and extract thermodynamic and kinetic parameters of the transition between closed-shell and open-shell states. Our findings provide insights into the evolution of single-molecule radicals under external stimuli, which may proof instrumental for the development of functional quantum spin-based molecular devices.

Chemical Doping of Organic and Coordination Polymers for Thermoelectric and Spintronic Applications: A Theoretical Understanding.

ConspectusThe controlled doping of organic semiconductors (OSCs) is crucial not only for improving the performance of electronic and optoelectronic devices but also for enabling efficient thermoelectric conversion and spintronic applications. The mechanism of doping for OSCs is fundamentally different from that of their inorganic counterparts. In particular, the interplay between dopants and host materials is complicated considering the low dielectric constant, strong lattice-charge interaction, and flexible nature of materials. Recent experimental breakthroughs in the molecular design of dopants and the precise doping with high spatial resolution call for more profound understandings as to how the dopant interacts with the charge introduced to OSCs and how the admixture of dopants alters the electronic properties of host materials before one can exploit controllable doping to realize desired functionalities.By employing state-of-the-art computational tools, we revealed the effects of doping in representative and emerging organic and coordination polymers aiming toward thermoelectric and spintronic applications. We showed that dopants and hosts should be taken as an integrated system, and the type of charge-transfer interaction between them is the key for spin polarization. First, we found doping-induced modifications to the electronic band in a potassium-doped coordination polymer, an n-type thermoelectric material. The charge localization due to the Coulomb interaction between the completely ionized dopant and the injected charge on the polymer backbone and also the polaron band formation at low doping levels are responsible for the nonmonotonic temperature dependence of the conductivity and Seebeck coefficient observed in recent experiments. The mechanistic insights gained from these results have provided important guidelines on how to control the doping level and working temperature to achieve a high thermoelectric conversion efficiency. Next, we demonstrated that the ionized dopants scatter charge carriers via screened Coulomb interactions, and it may become a dominant scattering mechanism in doped polymers. After incorporating the ionized dopant scattering mechanism in PEDOT:Tos, a p-type thermoelectric polymer, we were able to reproduce the measured Seebeck coefficient-electrical conductivity relationship spanning a wide range of doping levels, highlighting the importance of ionized dopant scattering in charge transport.In the two cases described above, charge injection is enabled by integral charge transfer between the dopant and host polymers. In a third example, we showed that a novel type of stacked two-dimensional polymer, conjugated covalent organic frameworks (COFs) with closed-shell electronic structures, can be spin polarized by iodine doping via fractional charge transfer even at high doping levels. We then manifested that magnetization can be attained in nonmagnetic materials lacking metal d electrons and further designed two new COFs with tunable spintronic structure and magnetic interactions after the iodine doping. These findings have suggested a practical route to enable spin polarization in nonradical materials by chemical doping via orbital hybridization, which holds great promise for flexible spintronic applications.

Overcrowded Ethylene-Bridged Nanohoop Dimers: Regioselective Synthesis, Multiconfigurational Electronic States, and Global Hückel/Möbius Aromaticity.

The design and preparation of molecular systems with multiple geometric and electronic configurations are the cornerstones for multifunctional materials with stimuli-responsive behaviors. We describe here the regioselective and facile synthesis of two types of overcrowded ethylene-bridged nanohoop dimers, with folded and twisted geometric structures as well as closed-shell, diradical and dication electronic structures. The strained nanohoop structures have a profound effect on the overall molecular and electronic configurations, which resulted in the destabilized diradical state. X-ray crystallographic analysis revealed the folded molecular geometry for the neutral species and twisted geometry for the dication species. The unique molecular dynamics, optical properties, and dynamic redox properties were disclosed in the solution phase by spectroscopic and electrochemical methods. Furthermore, the global Hückel and Möbius aromaticity were revealed by a combination of experimental and theoretical approaches. Our studies shed light on the design of nanohoop-incorporated multiconfigurational materials with unique topologies and functions.

Evolution of the electronic structure in open-shell donor-acceptor organic semiconductors

Most organic semiconductors have closed-shell electronic structures, however, studies have revealed open-shell character emanating from design paradigms such as narrowing the bandgap and controlling the quinoidal-aromatic resonance of the π-system. A fundamental challenge is understanding and identifying the molecular and electronic basis for the transition from a closed- to open-shell electronic structure and connecting the physicochemical properties with (opto)electronic functionality. Here, we report donor-acceptor organic semiconductors comprised of diketopyrrolopyrrole and naphthobisthiadiazole acceptors and various electron-rich donors commonly utilized in constructing high-performance organic semiconductors. Nuclear magnetic resonance, electron spin resonance, magnetic susceptibility measurements, single-crystal X-ray studies, and computational investigations connect the bandgap, π-extension, structural, and electronic features with the emergence of various degrees of diradical character. This work systematically demonstrates the widespread diradical character in the classical donor-acceptor organic semiconductors and provides distinctive insights into their ground state structure-property relationship.

Exploring the Limits of Dative Boratrane Bonding: Iron as a Strong Lewis Base in Low-Valent Non-Heme Iron-Nitrosyl Complexes.

We previously reported the synthesis and preliminary characterization of a unique series of low-spin (ls) {FeNO}8-10 complexes supported by an ambiphilic trisphosphineborane ligand, [Fe(TPB)(NO)]+/0/-. Herein, we use advanced spectroscopic techniques and density functional theory (DFT) calculations to extract detailed information as to how the bonding changes across the redox series. We find that, in spite of the highly reduced nature of these complexes, they feature an NO+ ligand throughout with strong Fe-NO π-backbonding and essentially closed-shell electronic structures of their FeNO units. This is enabled by an Fe-B interaction that is present throughout the series. In particular, the most reduced [Fe(TPB)(NO)]- complex, an example of a ls-{FeNO}10 species, features a true reverse dative Fe → B bond where the Fe center acts as a strong Lewis-base. Hence, this complex is in fact electronically similar to the ls-{FeNO}8 system, with two additional electrons "stored" on site in an Fe-B single bond. The outlier in this series is the ls-{FeNO}9 complex, due to spin polarization (quantified by pulse EPR spectroscopy), which weakens the Fe-NO bond. These data are further contextualized by comparison with a related N2 complex, [Fe(TPB)(N2)]-, which is a key intermediate in Fe(TPB)-catalyzed N2 fixation. Our present study finds that the Fe → B interaction is key for storing the electrons needed to achieve a highly reduced state in these systems, and highlights the pitfalls associated with using geometric parameters to try to evaluate reverse dative interactions, a finding with broader implications to the study of transition metal complexes with boratrane and related ligands.

Prediction of the Reactivity of Argon with Xenon under High Pressures.

Pressure significantly modifies the microscopic interactions in the condense phase, leading to new patterns of bonding and unconventional chemistry. Using unbiased structure searching techniques combined with first-principles calculations, we demonstrate the reaction of argon with xenon at a pressure as low as 1.1 GPa, producing a novel van der Waals compound XeAr2. This compound is a wide-gap insulator and crystallizes in a MgCu2-type Laves phase structure. The calculations of phonon spectra and formation enthalpy indicate that XeAr2 would be stable without any phase transition or decomposition at least up to 500 GPa.

Structures and stability of closo-hydroborate dianions BnHn2– (n = 5-12) from the PSM model

In this theoretical study, the structures and energetic properties of several closohydroborate dianions BnHn2– (n = 5-12) were systematically investigated employing the B3LYP functional in conjunction with the aug-cc-pVTZ basis set. Global equilibrium geometries were determined, and the growth mechanism is established. Several thermodynamic parameters including the one-step fragmentation energy, the second-order difference in energy, and the HOMO– LUMO energy gap were also computed to evaluate their stability pattern. Computed results show that among investigated species B6H62– and B12H122– are exceptionally stable with closed-shell electronic structures. Their valence electrons generate magic numbers which could be understood by using the phenomenological shell model. The B6H62– with 26 itinerant electrons has an octahedral form ground state (Oh symmetry) and a closed electronic configuration, i.e. 1S2/1P6/2S2/1D10/2P6. Similarly, the anion B12H122– with 50 mobile electrons, which favors an icosahedron (Ih symmetry), also has a closed 1S2/1P6/2S2/1D10/2P6/1F14/2D10 electron shell. Therefore, these systems bear the highly symmetric conformations and constitute the exceedingly stable members of the series examined.

Structural diradical character.

A reliable first-principles description of singlet diradical character is essential for predicting nonlinear optical and magnetic properties of molecules. As diradical and closed-shell electronic structures differ in their distribution of single, double, triple, and aromatic bonds, modeling electronic diradical character requires accurate bond-length patterns, in addition to accurate absolute bond lengths. We therefore introduce structural diradical character, which we suggest as an additional measure for comparing first-principles calculations with experimental data. We employ this measure to identify suitable exchange-correlation functionals for predicting the bond length patterns and electronic diradical character of a biscobaltocene with the potential for photoswitchable nonlinear optical activity. Out of four popular approximate exchange-correlation functionals with different exact-exchange admixtures (BP86, TPSS, B3LYP, TPSSh), the two hybrid functionals TPSSh and B3LYP perform best for diradical bond length patterns, with TPSSh being best for the organometallic validation systems and B3LYP for the organic ones (for which the D3 dispersion correction was included). Still, none of the functionals is suitable for correctly describing relative bond lengths across the range of molecules studied, so that none can be recommended for predictive studies of (potential) diradicals without reservation. © 2018 Wiley Periodicals, Inc.

Effects of composition, crystal structure, and surface orientation on band alignment of divalent metal oxides: A first-principles study

Using first-principles calculations, this paper systematically investigates the ionization potentials (IPs) and electron affinities (EAs) of nonpolar surfaces of binary divalent metal oxides with formally closed-shell electronic structures, namely, BeO, MgO, CaO, SrO, BaO, ZnO, HgO, SnO, and PbO in relevant crystal structures. An emphasis is put on the understanding of the effects of chemical composition, crystal structure, and surface orientation on the surface band positions. Slab models for nonpolar surfaces are automatically generated using a proposed algorithm that provides a set of unique nonpolar surface orientations. A non-self-consistent dielectric-dependent hybrid functional approach is employed that is shown to provide a significant improvement in the band-gap, IP, and EA evaluation over standard density functional theory calculations using the generalized gradient approximation. The valence band maximum, O $2s$ band center, and O-site local potential versus the vacuum level are examined for prototypical rocksalt, wurtzite, and zincblende surfaces of selected systems without atomic relaxation. All of these quantities are found to qualitatively follow the tendency in the Madelung potential at the O site, where decreasing interatomic distance results in deeper energy levels. The valence band maxima versus the vacuum level or the IPs after atomic relaxation also show a similar trend even when other cation species and crystal structures are included in the discussion, while there is much less correlation for EAs. An overall trend indicates that lattice volume reflecting cation species is a determinant factor to the IPs of the divalent metal oxides investigated in this paper although there are surface-specific dipole contributions that cause the surface orientation dependence of the IPs.

Study on carbon dioxide reduction with water over metal oxide photocatalysts

Various metal oxides with 0.1 wt% Ag loaded as a cocatalyst were prepared by an impregnation method and examined their photocatalytic activity for CO2 reduction with water. Among all the prepared Ag‐loaded metal oxides, Ga2O3, ZrO2, Y2O3, MgO, and La2O3 showed activities for CO and H2 productions under ultraviolet light irradiation. Thus, metal oxides involving metal cations with closed shell electronic structures such as d0, d10, and s0 had the potential for CO2 reduction with water. In situ Fourier transform infrared measurement revealed that the photocatalytic activity and selectivity for CO production are controlled by the amount and chemical states of CO2 adsorbed on the catalyst surface and by the surface basicity, as summarized as follows: Ag/ZrO2 enhanced H2 production rather than CO production due to very little CO2 adsorption. Ag/Ga2O3 exhibited the highest activity for CO production, because adsorbed monodentate bicarbonate was effectively converted to bidentate formate being the reaction intermediates for CO production owing to its weak surface basicity. Ag/La2O3, Ag/Y2O3, and Ag/MgO having both weak and strong basic sites adsorbed larger amount of carbonate species including their ions and suppressed H2 production. However, the adsorbed carbonate species were hardly converted to the bidentate formate.

Methane activation by gold-doped titanium oxide cluster anions with closed-shell electronic structures.

The reactivity of closed-shell gas phase cluster anions AuTi3O7- and AuTi3O8- with methane under thermal collision conditions was studied by mass spectrometric experiments and quantum chemical calculations. Methane activation was observed with the formation of AuCH3 in both cases, while the formation of formaldehyde was also identified in the reaction system of AuTi3O8-. The cooperative effect of the separated Au+ and O2- ions on the clusters induces the cleavage of the first C-H bond of methane. Further activation of the second C-H bond by a peroxide ion O22- leads to the formation of formaldehyde. This study shows that closed-shell species on metal oxides can be reactive enough to facilitate thermal H-CH3 bond cleavage and the subsequent conversion.

Activation and Transformation of Ethane by Au2 VO3(+) Clusters with Closed-Shell Electronic Structures.

The study of chemical reactions between gold-containing heteronuclear oxide clusters and small molecules can provide molecular level mechanisms to understand the excellent activity of gold supported by metal oxides. While the promotion role of gold in alkane transformation was identified in the clusters with atomic oxygen radicals (O(-.)), the role of gold in the systems without O(-.) is not clear. By employing mass spectrometry and quantum chemistry calculations, the reactivity of Au2 VO3(+) clusters with closed-shell electronic structures toward ethane was explored. Both the dehydrogenation and ethene elimination channels were identified. It is gold rather than oxygen species initiating the C-H activation. The Au-Au dimer formed during the reactions plays important roles in ethane transformation. The reactivity comparison between Au2 VO3(+) and bare Au2(+) demonstrates that Au2 VO3(+) not only retains the property of bare Au2(+) that transforming ethane to dihydrogen, but also exhibits new functions in converting ethane to ethene, which reveals the importance of the composite system. This study provides a further understanding of the reactivity of metal oxide supported gold in alkane activation and transformation.

Endohedrally doping the gold cage [formula omitted] with an trivalent atom B, Al, Ga, and In: Density functional studies

The relativistic density functional calculations explore that M@Au16- (M=B, Al, Ga, and In) with M at the cage center, named as M@Au16--center, should be the most stable. The M@Au16--center (M=B, Al, Ga, and In) clusters have closed-shell electronic structures and large energy gaps. All of these properties are characteristic of a magic cluster and can be well understood by the jellium model. Therefore, we strongly suggest M@Au16--center (M=B, Al, Ga, and In) are magic clusters and promising as building blocks in developing cluster-assembled materials. The energy levels and the wavefunctions of frontier orbitals explore that the HOMOs of M@Au16--center should be partly occupied by the M atom, while the LUMOs of them should be mostly from the Au16- bodies. The difference charge densities and the natural bonding orbital charge analyses imply the Au–M bonds have both the ionic and covalent characters. Finally, the mean static linear polarizabilities and first-order hyperpolarizabilities of M@Au16--center are larger than those of Au16-, while their anisotropies of the polarizability tensors are smaller than those of Au16-. We rationalize the nonlinear properties by studying the low-energy optical absorption band obtained by employing time-dependent density functional theory.

Time-Dependent Density Functional Theory Studies of Optical Properties of Ag Nanoparticles: Octahedra, Truncated Octahedra, and Icosahedra

The effect of the size and shape of silver nanoparticles on their optical absorption properties is theoretically investigated to understand the plasmonic properties of these systems. Time-dependent density functional theory (TDDFT) calculations are employed to calculate the optical absorption spectra for a series of silver clusters (Agn, n = 6–85) in various charge states whose structures are octahedral, truncated octahedral, and icosahedral. Octahedral Agn clusters with n = 6, 19, 44, 85, truncated octahedral Agn clusters with n = 13, 38, 55, 79, and icosahedral Agn clusters with n = 13, 43, 55 are calculated. Charged systems are considered to obtain closed shell electronic structures. These calculations are performed with the ADF code with the BP86/DZ level of theory in the optimizations and the SAOP functional and LB94 functional in the excitation calculations. A sharp excitation peak originates from a mixture of orbital transitions, and a broad excitation arises from multiple excited states in octahedral, truncated octahedral, and icosahedral Agn clusters. We predict that the absorption peak maximum red shifts as the cluster becomes larger and blue shifts as the shape of clusters changed from octahedral to icosahedral.

The role of phonon- and photon-coupled interactions in electron pairing in solid state materials

The essential role of the phonon- and photon-coupled interactions in the electron pairing in the macroscopic sized conventional superconductivity is investigated by comparing the conventional superconductivity in the macroscopic sized materials with the nondissipative diamagnetic currents in the negatively charged small sized molecules such as cyclopentadiene monoanion (5an−) and cyclobutadiene dianion (4an2−). The nondissipative diamagnetic currents in the negatively charged small sized molecules such as 5an− and 4an2− cannot be explained by the Bardeen–Cooper–Schrieffer (BCS) theory because attractive phonon-coupled interactions between two electrons (V6π,N,BCS,phonon,e–e) are zero and direct electron–electron repulsion (V6π,N,BCS,repulsive,e–e) becomes very large. However, these phenomena in 5an− and 4an2− can be well explained by the photon-coupled interactions between all nuclei and electrons. The phonon-coupled interactions (Vphonon,e–e) play an essential role in the forming of the closed-shell electronic structures with finite valence–conduction band gaps, in which two electrons occupying the same orbitals with opposite momentum and spins become stable, and the photon-coupled interactions (Vmacro,photon,N,j,σ) play an essential role in attractive interactions between these two electrons. Because of the photon-coupled interactions as well as the phonon-coupled interactions, electron pairs can be formed in the macroscopic sized materials, and the conventional superconducting states can appear below the superconducting critical temperature (Tc). Our molecular perspective suggested in this article is compared with the BCS theory, and the reason why the conventional superconductivity has been excellently explained by the BCS theory is also discussed.

Highly stable and symmetric boron caged B@Co12@B80 core-shell cluster

The geometry, stability, and electronic properties of B@Co12@Bn and Co13@Bn clusters with a wide range n=55 up to 92 are studied by ab initio calculations. We find that B@Co12@B80 and Co13@B80 with closed B80 shell are two stable magic clusters with nearly perfect icosahedral symmetry, and B@Co12@B80 is more stable than Co13@B80 energetically. The strong core-shell bonding yields a very large energy gain of ∼30 eV. This high stability is attributed to the favorable closed-shell atomic and electronic structures. The B@Co12@B80 exhibits a large highest occupied and lowest unoccupied energy gap (0.96 eV) that is close to the value for isolated B80 fullerene.

Structural and electronic properties of neutral clusters In 12X (X = C, Si, Ge, and Sn) and their anions from first principles

The structural and electronic properties of In 12 X (X = C, Si, Ge, Sn) and their anions have been investigated using the density functional theory at BLYP and B3LYP levels. The equilibrium geometries of In 12 X (X = Si, Ge, and Sn) and their anions favor the pseudo D 5 h structures, while those of In 12 C and its anion tend to form C s ones. The calculated adiabatic electron affinities of In 12 Si and In 12 Ge are 2.01 and 2.04 eV, respectively, in good agreement with the available experimental data. The magnetic moments of the ground state In 12 X are all zero, indicating that they have closed shell electronic structures. Thus, indium behaves as a trivalent atom in the clusters.

Photocatalytic water splitting by RuO2-loaded metal oxides and nitrides with d0- and d10 -related electronic configurations

This article reviews photocatalysis for water splitting by various kinds of metal oxides and nitrides such as ferroelectric metal oxides, different kinds of titanates with d0 (Ti4+) electronic configuration as a core metal ion, and various typical metal oxides with d10 (In3+, Ga3+, Ge4+, Sn4+, Sb5+) configuration, and d10 (Ga3+) metal nitride, together with d10s2 (Pb2+) and d0f0 (Ce4+) metal oxides. Ferroelectric metal oxides with single domain structure showed anomalous photovoltaic effects that controlled the behavior of photoexcited electrons. Various metal oxides involving ionic alkaline metal/alkaline earth metal ions showed good correlation between photocatalytic activity and the distortion of octahedral XO6/tetrahedral XO4 (X = core metal ion). When the metal oxides involved covalent metal ions such as Zn2+ and Pb2+, the activity was invoked even in distortion-free structures: this is due to strong electronic effects that affect the band structure. The activity of d10 metal oxides and nitrides is associated with conduction bands of hybridized sp orbitals with large dispersion that are able to generate photoexcited electrons with large mobility. A feature of the photocatalytically active metal oxides and nitrides discovered so far, which is their closed shell electronic structures is discussed.

The evolution of the structure and electronic properties of the fullerene derivatives C60（CF3）n(n=2, 4, 6, 10): A density functional calculation

The generalized gradient approximation （GGA） based on density functional theory （DFT） is used to analyze the evolution of the structure and electronic properties of the fullerene derivatives C60（CF3）n （n=2, 4, 6, 10）. It is found that among the three respective possible stable isomers of C60（CF3）4 and C60（CF3）6, the structure C60（CF3）4 with ppp addition and C60（CF3）6 with pppmp addition are the most stable structures, respectively. By analyzing the structures of C60（CF3）2, C60（CF3）41，C60（CF3）61, and C60（CF3）10, it was found that both the average C—C bond length and the CC60—CCF3 bond length increase with the increase of the CF3 number. It was found from the electronic properties of four compounds that the heat of reaction of the compounds almost linearly increases with the number of the CF3, and its maximum appears at n=6, thus, C60（CF3）6 should be the most easily synthesized derivative. It is known from the Mulliken charge that the interaction between CF3 and the cage and the electron transference from CF3 to the cage increase with the number of the CF3 However, the net spins of the compounds are all zero, indicative of their closedshell electronic structures. Finally, the frontier orbital analysis shows that the electron detaching and attaching both occur at carbon sites of the cage.

Diamagnetic currents in the closed-shell electronic structures in sp 3-type hydrocarbons

The mechanism of the occurrence of diamagnetic currents in sp 3-type electronic states in alkanes and pure diamonds is suggested, from analogy with the nondissipative diamagnetic currents in closed-shell electronic structures in aromatic hydrocarbons such as polyacenes and annulenes with microscopic sizes. It is found that most of all electrons can form electron pairs originating from attractive Coulomb interactions between two electrons with opposite momenta and spins occupying the same orbitals in pure diamonds with macroscopic sizes at 298 K. In principle, diamagnetic currents in alkanes and pure diamonds can be expected to occur by such electron pairing. The diamagnetic conducting transition temperature T c neutral N sp 3 values for macroscopic sized diamonds with large valence–conduction band gaps are estimated to be much larger than the superconducting transition temperatures T c,BCS values for the conventional superconductors with small valence–conduction band gaps. Even very small ratio of impurities or dopants would destroy possible nondissipative diamagnetic current states in the diamonds with macroscopic sizes. Therefore, very pure diamonds exhibiting nondissipative diamagnetic currents are very difficult to be produced up to now, and diamonds with impurities would become insulator or semiconductor. The technology which enables us to produce very pure diamonds in which no impurities exist is awaited to investigate whether such pure diamonds become nondissipative diamagnetic current states or not at 298 K.