Bond Length Contraction Research Articles

Photon-Modulated Bond Covalency of [Sm(II)(η9-C9H9)2

Lanthanides are widely assumed not to form covalent bonds due to the localized nature of their 4f valence electrons. This work demonstrates that the ionic bond of Sm(II) with cyclononatetraenyl (η9-C9H9-) in [Sm(η9-C9H9)2] can be modulated and becomes more covalent by photon-induced transfer of Sm 4f electrons to Sm 5d orbitals. This photon-induced change in bonding properties facilitates a subsequent reconfiguration of [Sm(η9-C9H9)2]. As a result, Sm-C bond length contraction is detected and the local Sm coordination environment exhibits more extensive disorder. Both Sm 4f and 5d electrons have increased participation in covalent Sm-ligand interactions. The Sm L3-edge valence band resonant inelastic X-ray scattering (VB-RIXS), high-resolution X-ray absorption near-edge structure (HR-XANES), and quantum chemical computations showcase a spectroscopic methodology for in-depth studies of bond covalency of lanthanide atoms.

Equatorial restriction of the photoinduced Jahn-Teller switch in Mn(iii)-cyclam complexes.

Ultrafast transient absorption spectra were recorded for solutions of [MnIII(cyclam)(H2O)(OTf)][OTf]2 (cyclam = 1,4,8,11-tetraazacyclotetradecane and OTf = trifluoromethanesulfonate) in water to explore the possibility to restrict the equatorial expansion following photoexcitation of the dxy ← dz2 electronic transition, often resulting in a switch from axial to equatorial Jahn-Teller distortion in MnIII complexes. Strong oscillations were observed in the excited state absorption signal and were attributed to an excited state wavepacket. The structural rigidity of the cyclam ligand causes a complex reaction coordinate with frequencies of 333, 368, 454 and 517 cm-1, and a significantly shorter compressed-state lifetime compared to other MnIII complexes with less restricted equatorial ligands. Complementary density functional theory quantum chemistry calculations indicate a switch from an axially elongated to a compressed structure in the first excited quintet state Q1, which is accompanied by a modulation of the axial tilt angle. Computed harmonic frequencies for the axial stretching mode (∼379 cm-1) and the equatorial expansions (∼410 and 503 cm-1) of the Q1 state agree well with the observed coherences and indicate that the axial bond length contraction is significantly larger than the equatorial expansion, which implies a successful restriction of the wavepacket motion. The weak oscillation observed around 517 cm-1 is assigned to a see-saw motion of the axial tilt (predicted ∼610 cm-1). The results provide insights into the structural perturbations to the molecular evolution along excited state potential energy surfaces of MnIII octahedral complexes and can be used to guide the synthesis of optically controlled MnIII-based single-molecule magnets.

High-pressure behavior of tetragonal barium carbodiimide, BaNCN

The high-pressure behavior of tetragonal barium carbodiimide (BaNCN) was investigated by in-situ high-pressure X-ray diffraction measurements and first-principles theoretical calculations. No phase transition or decomposition of tetragonal BaNCN was observed during compression to 6.0 GPa and subsequent decompression to ambient pressure. The bulk modulus estimated from the unit cell volume with respect to the applied pressure was 69(2) GPa, which is comparable to that estimated by first-principles calculations. The theoretical calculations indicate that contraction of the Ba-N bond length has a crucial effect on the volume change in response to pressure, whereas the C-N bonds in the NCN 2- anions remain robust against the change in pressure. • High-pressure behavior of tetragonal BaNCN was examined by synchrotron XRD and DFT calculations. • Tetragonal BaNCN exhibits a small bulk modulus of 69(2) GPa. • The large compressibility of the BaNCN originates from the contraction of Ba-N bond. • The C-N bond in NCN 2- anion is robust with respect to pressure.

Matrix-Isolation Infrared Spectra and Electronic Structure Calculations for Dinitrogen Complexes with Uranium Trioxide Molecules UO3(η1-NN)1-4.

Investigations of the interactions of uranium trioxide (UO3) with other species are expected to provide a new perspective on its reaction and bonding behaviors. Herein, we present a combined matrix-isolation infrared spectroscopy and theoretical study of the geometries, vibrational frequencies, electronic structures, and bonding patterns for a series of dinitrogen (N2) complexes with UO3 moieties UO3(η1-NN)1-4. The complexes are prepared by reactions of laser-ablated uranium atoms with O2/N2 mixtures or laser-ablated UO3 molecules with N2 in solid argon. UO3(η1-NN)1-4 are classified as "nonclassical" metal-N2 complexes with increased Δν(N2) values according to the experimental observations and the computed blue-shifts of N-N stretching frequencies and N-N bond length contractions. Electronic structure analysis suggests that UO3(η1-NN)1-4 are σ-only complexes with a total lack of π-back-donation. The energy decomposition analysis combined with natural orbitals for chemical valence calculations reveal that the bonding between the UO3 moiety and N2 ligands in UO3(η1-NN)1-4 arises from the roughly equal electrostatic attractions and orbital mixings. The inspection of orbital interactions from pairwise contributions indicates that the strongest orbital stabilization comes from the σ-donations of the 4σ*- and 5σ-based ligand molecular orbitals (MOs) into the hybrid 7s/6dx2-y2 MO of the U center. The electron polarization induced by electrostatic effects in the Ninner ← Nouter direction provides complementary contributions to the orbital stabilization in UO3(η1-NN)1-4. In addition, the reactions of UO3 with N2 ligands and the origination of the nonclassical behavior in UO3(η1-NN)1-4 are discussed.

Effects of Mono-Vacancies of Oxygen and Manganese on the Properties of the MnO2/Graphene Heterostructure.

The effects of the monovacancies of oxygen (VO) and manganese (VMn) on the structural and electronic properties of the 1T–MnO2/graphene heterostructure are investigated, within the framework of density functional theory (DFT). We found that the values of the formation energy for the heterostructure without and with vacancies of VO and VMn were −20.99 , −32.11, and −20.81 , respectively. The negative values of the formation energy indicate that the three heterostructures are energetically stable and that they could be grown in the experiment (exothermic processes). Additionally, it was found that the presence of monovacancies of VO and VMn in the heterostructure induce: (a) a slight decrease in the interlayer separation distance in the 1T–MnO2/graphene heterostructure of ~0.13% and ~1.41%, respectively, and (b) a contraction of the (Mn−O) bond length of the neighboring atoms of the VO and VMn monovacancies of ~2.34% and ~6.83%, respectively. Calculations of the Bader charge for the heterostructure without and with VO and VMn monovacancies show that these monovacancies induce significant changes in the charge of the first-neighbor atoms of the VO and VMn vacancies, generating chemically active sites (locales) that could favor the adsorption of external atoms and molecules. From the analysis of the density of state and the structure of the bands, we found that the graphene conserves the Dirac cone in the heterostructure with or without vacancies, while the 1T–MnO2 monolayer in the heterostructures without and with VO monovacancies exhibits half-metallic and magnetic behavior. These properties mainly come from the hybridization of the 3d–Mn and 2p–O states. In both cases, the heterostructure possesses a magnetic moment of 3.00 μβ/Mn. From this behavior, it can be inferred the heterostructures with and without VO monovacancies could be used in spintronics.

Reexamination of the ground-state Born-Oppenheimer Yb2 potential

The precision of the photoassociation spectroscopy of Yb dimer in degenerate gases is enough to improve the constraints on the new short-range gravity-like forces if the theoretical knowledge of the Born-Oppenheimer interatomic potential and non-Born-Oppenheimer interactions is refined [M. Borkowski et al. Sci. Rep. A {\bf 9}, 14807 (2019)]. The ground-state interaction potential of ytterbium dimer is investigated at the eXact 2-component core-correlated CCSD(T) level of {\it ab initio} theory in the complete basis set limit with extensive augmentation by diffuse functions. For the small basis set the comparison is made with the four-component relativistic finite-nuclei CCSD(T) calculations to identify the contraction of the dimer bond length as the main unrecoverable consequence of the scalar-relativistic approximation. Empirical constraint on the number of bound vibrational energy levels of the $^{174}$Yb$_2$ dimer is accounted for by representing the global {\it ab initio}-based Born-Oppenheimer potential with the model semianalytical function containing the scale and shift parameters. The results support the previous evaluation of the Yb dimer potentials from the photoassociation spectroscopy data and provide an accurate and flexible reference for future refinement of the constraints on the short-range gravity-like forces by ultracold atomic spectroscopy.

Deformation Potentials: Towards a Systematic Way beyond the Atomic Fragment Approach in Orbital-Free Density Functional Theory.

This work presents a method to move beyond the recently introduced atomic fragment approximation. Like the bare atomic fragment approach, the new method is an ab initio, parameter-free, orbital-free implementation of density functional theory based on the bifunctional formalism that treats the potential and the electron density as two separate variables, and provides access to the Kohn–Sham Pauli kinetic energy for an appropriately chosen Pauli potential. In the present ansatz, the molecular Pauli potential is approximated by the sum of the bare atomic fragment approach, and a so-called deformation potential that takes the interaction between the atoms into account. It is shown that this model can reproduce the bond-length contraction due to multiple bonding within the list of second-row homonuclear dimers. The present model only relies on the electron densities of the participating atoms, which themselves are represented by a simple monopole expansion. Thus, the bond-length contraction can be rationalized without referring to the angular quantum numbers of the participating atoms.

Theoretical investigation on defect engineering for tuning photocatalytic activities in isostructural ScxmCuynOz−1 (m = Ca, n = Ni)

A novel bifunctional photocatalyst for overall water splitting and catalyst separations was realized through defect engineering approach. For the first time a systematic investigation of the photocatalytic activity in isostructural oxide is presented. The analysis are based on the optical absorption, band edge potential, energy band gap, and electrons-holes effective masses. The hole-doped Sc2Cu2O5 compound with Ca@Sc site @25%, show high optical absorption in the visible range and small effective masses efficient to suppress backward oxidation reaction. The computed band edge potentials for both valence and conduction band of the Ni@Cu site are deep enough for these compounds to be use as cocatalyst for either of redox reactions. High absorption under visible light irradiation, and increased spin moment was observed in oxygen vacancy configurations at the Cu coordinated defect sites, which is also characterized by the nearest bond length contraction in Cu-O sites after geometry relaxation.

Uncovering the structural, electronic and vibrational properties of atomically precise PdmCun clusters and their interaction with CO2 molecule.

In this work, we address the structural stability, electronic properties and effect of metal-metal interaction on Raman spectra of icosahedral (Ih) PdmCun (m+n=13) clusters using first principles calculations based on dispersion-corrected density functional theory (DFT-D2). Initially, we investigated the relative stability of Ih PdmCun clusters over monometallic Ih Pd13 and Cu13 clusters by calculating the average binding energy per atom, mixing energy, second order energy difference and average bond length. The Ih Pd5Cu8 is the most stable bimetallic cluster with the 2.88eV, -0.218eV and 0.678eV average binding energy per atom, mixing energy and second order energy difference, respectively. The main goals of the present study are to figure out the chemical enhancement, modulation in electronic properties and Pd-Cu bond length in Ih PdmCun clusters after systematic doping of Cu-atom. Further, to examine the doping effect of Cu atom in Pd cluster, we have also analysed the Raman spectra of Ih PdmCun clusters. In case of Ih Cu13 cluster, the contraction of Cu-Cu bond length as compared to its bulk form resulted in a significant blue-shift of characteristic Raman peak (212cm-1) of Ih Pd13 cluster. Finally, the interaction mechanism of the CO2 gas molecule over Pd-Cu alloy clusters have also been studied to understand the effect of composition on reactivity of CO2 gas molecule.

High-Pressure Band-Gap Engineering and Metallization in the Perovskite Derivative Cs3 Sb2 I9.

Among photovoltaic materials, the antimony-based, perovskite-like structure Cs3 Sb2 I9 stands out owing to its low toxicity and air stability. Here, changes in the optoelectronic properties and crystal structure of the lead-free perovskite derivative Cs3 Sb2 I9 are reported, caused by pressure-induced lattice compression. At 20.0 GPa, Cs3 Sb2 I9 with a wide band gap (2.34 eV) successfully broke through the Shockley-Queisser limit (1.34 eV), accompanied by clear piezochromism from orange-yellow to opaque black. Additionally, Cs3 Sb2 I9 experienced completely reversible amorphization at 20.0 GPa. These optical changes could be attributed to atomic-orbital overlap enhancement caused by contraction of the Sb-I bond length and diminution of the Sb-I bond angle. In addition, Cs3 Sb2 I9 underwent a transition from semiconductor to conductor upon compression and obtained metallic properties at 44.3 GPa, indicating new electronic properties. The obtained results may further broaden the research prospects of halide perovskite materials in the field of photovoltaics.

From the Terrace Contraction to the Hexameric Sulfur Phase in the Au(100) Surface: A Combined Density Functional Theory and Scanning Tunneling Microscopy Study

The lifting of the Au(100) quasi-hexagonal reconstruction by atoms or molecules leaves a rough surface with islands, terraces, and vacancies where different patterns of adsorbates can appear. Although the roughness of the (1 × 1)-Au(100) surface has been well characterized, to our knowledge, there are no theoretical studies considering its effect on the formation of adsorbate patterns, for instance, in the case of sulfur adsorption. In this study, we have combined DFT calculations and STM measurements to explore the structural effects induced on the terrace when the sulfur adsorption takes place on the island and vacancy. Models predict a decrease of the Au–Au bond length in the terrace if the terrace has an appropriate size when the simultaneous adsorption of the sulfur structure (2×2) occurs on the island and vacancy. According to our predictions, this process of gold bond length contraction on the terrace can affect the formation of the sulfur structure on it. STM images reveal regions where the (2×2) ...

Atomistic modeling of the infrared response of fullerenes under hydrostatic pressure

The anharmonic infrared spectrum of individual C60 and C70 fullerenes under hydrostatic pressure was theoretically computed by means of atomistic simulations. Using a tight-binding model for the fullerenes and a simple particle-based pressure-transmitting fluid, the structural and vibrational properties were determined at room temperature and up to 20 GPa. All properties generally exhibit relative variations that are linear with increasing pressure, but whose magnitude can be comparable to pure thermal effects. The bond length contraction usually agrees with existing results, and for C70 our approach manages to reproduce the occasionally negative pressure coefficient found for some low-frequency modes in experiments.

The chemistry and structure of calcium (alumino) silicate hydrate: A study by XANES, ptychographic imaging, and wide- and small-angle scattering

Calcium (alumino)silicate hydrate (C-(A-)S-H) is the main binding phase in blended cement concrete. Understanding the chemistry and structure of C-(A-)S-H is essential to optimizing concrete properties such as compressive strength and durability; yet questions remain around the coordination environments of Ca and Al in its structure with various chemical compositions and equilibration temperatures. C-(A-)S-H with Ca/Si = 0.6–1.6, Al/Si = 0–0.1, and equilibrated at 7–80 °C is studied by nanoscale soft X-ray spectroscopy at the Ca L2,3- and Si K-edges. Highly distorted CaO7 complexes occur in the intralayer of C-(A-)S-H irrespective of Ca/Si, Al/Si, and temperature. Zeolitic Ca in the interlayer of C-(A-)S-H is highly distorted from an ideal octahedral coordination. Third aluminate hydrate is either not Ca-bearing or its Ca is structurally similar to C-(A-)S-H and does not resemble the Ca in AFm-phases. Increasing aluminosilicate chain polymerization in C-(A-)S-H shifts the Si K-edge to higher energies, implying Al uptake in the bridging and/or cross-linked sites, as well as a contraction of SiO bond lengths. C-(A-)S-H exhibits a foil-like morphology, with individual foils comprised of nano-sized platelets with comparable thickness regardless of Ca/Si or Al/Si at 7–50 °C. Coarser C-(A-)S-H foils occur at 80 °C and higher Al/Si ratios relative to lower temperatures and Al content.

The structural parameters, structural stability and bulk modulus in RE2Sn2O7by first-principles calculations

A systematic density functional theory study is performed to investigate the lattice parameters, the internal positional parameter [Formula: see text] and bond length of RE2Sn2O7(RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) pyrochlores. To analyze the structural stability in extreme conditions and verify whether bond strength varies inversely to bond length in rare-earth stannate pyrochlores, the structural parameter and bonding strength under hydrostatic pressure are studied. The tight connection between the different bond length contraction and the variation of 48[Formula: see text] oxygen positional parameter [Formula: see text] is also discovered. We calculated the bond length and the bulk modulus of RE2Sn2O7and found that the [Formula: see text]RE–O[Formula: see text] bond plays a predominant role in determining the bulk modulus. Meanwhile, the present calculations suggest that the “bimodal effect” also exists in rare-earth stannate pyrochlores.

An Origin for Lattice Expansion in PVP-Protected Small Pd Metal Nanoparticles

Abstract Noble metal nanoparticles always show bond length contraction with the decrease in particle size. PVP-protected small Pd metal nanoparticles (MNPs) were reported to have the unique characteristic of expanding Pd-Pd bond lengths with decreases in particle size. To investigate the origin of this phenomenon in more detail, this work examined Pd MNPs supported on SiO2 (Pd MNPs/SiO2) via extended X-ray absorption fine structure (EXAFS) analysis, where the SiO2 support was used to stabilize the MNPs to prevent unexpected aggregation at higher temperatures. EXAFS data showed that each sample had a face-centered cubic (fcc) structure and that the smallest particles had the longest Pd-Pd bonds. Temperature dependent EXAFS measurements also revealed significant static disorder in the bond-elongated Pd MNPs/SiO2 samples. The XAFS spectral features of these bond-elongated Pd MNPs on SiO2 are in good agreement with those for palladium carbide, and we conclude that the lattice expansion in Pd MNPs can be attributed to the formation of palladium carbide.

CATION ORDERING AT OCTAHEDRAL SITES IN SCHORL-DRAVITE SERIES TOURMALINES

Tourmalines with Al 3+ , Mg 2+ , Fe 3+ , Fe 2+ , Mn 2+ , V 3+ , and Cr 3+ cation disorder at the two octahedrally coordinated sites ( Y , Z ) have recently been described. Their complete structural and compositional description enabled a detailed statistical study of Mg 2+ , Al 3+ , and Fe 2+ disorder reactions. The sizes of the a and c unit-cell parameters are positively correlated with Y –O> and Z –O> bond lengths, respectively, and this enables inference of octahedral distortion. While total octahedral Al 3+ and Mg 2+ show weak negative correlations with a consistent with their relatively smaller ionic radii, Fe 2+ has a weak positive correlation with a because of its larger ionic radius. Negative Al 3+ and positive Mg 2+ correlations with c show that increased Mg 2+ content, rather than Al 3+ , is related to increased c and Z O 6 octahedral dilation. Fe 2+ negatively correlates with c , and this suggests that none or only a negligible Fe 2+ proportion can be incorporated at the Z site. All Y -site bond lengths except Y –O2 increase with the occupancy of Fe 2+ at the Y site and decrease with the occupancy of Mg at the Y site. Al 3+ has no significant effect except for Y –O2 bond-length contraction. In contrast, Al 3+ negatively correlates with all bonds involving the Z site, and Mg 2+ has the opposite effect. Fe 2+ shows negative correlation with Z –O bond lengths, which also suggests that Fe 2+ is not present at the Z site in any significant amount. Consequently, although Al-Mg disorder may be very common in Mg-bearing tourmalines but should never be over 2 apfu Mg at the Z site, disorder involving Z Fe 2+ seems unlikely. Complementary bond-topology graphs revealed that Z R 2+ induces increased bond valence and decreased Y –O6 bond length, with consequent Y –O2 bond-valence decrease and bond-length increase. This corresponds with variations in bond lengths dependent on site occupancy, especially since Y R 2+ requires the presence of R 3+ at neighboring Y sites. A topological study of selected structural arrangements reveals that Y R 2+ – Z R 3+ – Z R 3+ and Y R 3+ – Z R 3+ – Z R 2+ arrangements with W OH can exist together in bond-valence stability. In contrast, bond-valence requirements involving W O have more degrees of freedom. Consequently, although these bond-valence requirements may produce structures with Al-Mg disorder, the disorder most likely occurs in arrangements with W OH.

High-precision scanning transmission electron microscopy at coarse pixel sampling for reduced electron dose

Abstract Determining the precise atomic structure of materials’ surfaces, defects, and interfaces is important to help provide the connection between structure and important materials’ properties. Modern scanning transmission electron microscopy (STEM) techniques now allow for atomic resolution STEM images to have down to sub-picometer precision in locating positions of atoms, but these high-precision techniques generally require large electron doses, making them less useful for beam-sensitive materials. Here, we show that 1- to 2-pm image precision is possible by non-rigidly registering and averaging a high-angle dark field image series of a 5- to 6-nm Au nanoparticle even though a very coarsely sampled image and decreased exposure time was used to minimize the electron dose. These imaging conditions minimize the damage to the nanoparticle and capture the whole nanoparticle in the same image. The high-precision STEM image reveals bond length contraction around the entire nanoparticle surface, and no bond length variation along a twin boundary that separates the nanoparticle into two grains. Surface atoms at the edges and corners exhibit larger bond length contraction than atoms near the center of surface facets.

A comparative DFT study of the structural and electronic properties of nonpolar GaN surfaces

Abstract A comparative analysis of the geometry and the electronic characteristics of nonpolar GaN surfaces was carried out using density-functional theory (DFT) with different approximations for the exchange-correlation energy (LDA, PBE, PBEsol, RPBE, TPSS, revTPSS, and HSE). The obtained data show that the GaN( 1 0 1 ¯ 0 ) (m-plane) is more energetically stable than the GaN( 1 1 2 ¯ 0 ) (a-plane) surface. However, these surfaces have similar surface relaxation geometry, with a Ga-N surface bond-length contraction of around 6–7% and a Ga-N surface rotational angle in the range of 6–9°. Our results show that the use of different exchange-correlation functionals does not significantly change the surface energy and surface geometry. In addition, we found the presence of surface intra-gap states that reduce the band gap of the nonpolar GaN surface with respect to the bulk value, in agreement with recent photoelectron and surface optical spectroscopy experiments.

Role of gold nanoclusters supported on TiO2(110) model catalyst in CO oxidation reaction

It was reported previously that O adatoms adsorbed dissociatively on the five-fold Ti rows of rutile TiO2(110) made the surface O-rich and reacted with CO molecules to form CO2. An electronic charge transfer taking place from gold nanoclusters to the O-rich TiO2(110) support played a crucial role to enhance the catalytic activity [Mitsuhara et al., J. Chem. Phys. 136, 124303 (2012)]. In this study, the authors have further accumulated experimental data for the CO oxidation reaction enhanced by gold nanoclusters on the TiO2(110) surface. Based on the results obtained here and previously, the authors propose an “interface dipole model,” which explains the strong activity of Au nanoclusters supported on O-rich TiO2(110) in CO oxidation reaction. Simultaneously, the authors also discuss the cationic cluster model proposed by Wang and Hammer [Phys. Rev. Lett. 97, 136107 (2006)] and the d-band model predicted by Hammer and Nørskov [Adv. Catal. 45, 71 (2000)]. The latter is, in particular, widely accepted to explain the activities of heterogeneous catalysts. Contrary to the d-band model, our ab initio calculations demonstrate that the d-band center for Au nanoclusters moves apart from the Fermi level with decreasing the cluster size and this is due to contraction of the Au-Au bond length.

Measuring Surface Atom Bond Length Contraction in Au and Pt Nanoparticles Using High-Precision STEM Imaging

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.