Regioselective Fluorohydrin Synthesis from Allylsilanes and Evidence for a Silicon-Fluorine Gauche Effect.

Allylsilanes can be regioselectively transformed into the corresponding 3-silylfluorohydrin in good yield using a sequence of epoxidation followed by treatment with HF·Et3N with or without isolation of the intermediate epoxide. Various silicon-substitutions are tolerated, resulting in a range of 2-fluoro-3-silylpropan-1-ol products from this method. Whereas other fluorohydrin syntheses by epoxide opening using HF·Et3N generally require more forcing conditions (e.g., higher reaction temperature), opening of allylsilane-derived epoxides with this reagent occurs at room temperature. We attribute this rate acceleration along with the observed regioselectivity to a ß-silyl effect that stabilizes a proposed cationic intermediate. The use of enantioenriched epoxides indicates that both SN1- and SN2-type mechanisms may be operable depending on substitution at silicon. Conformational analysis by NMR and theory along with a crystal structure obtained by X-ray diffraction points to a preference for silicon and fluorine to be proximal to one another in the products, perhaps favored due to electrostatic interactions.

Energetic Stability and Band-Edge Orbitals of Layered Inorganic Perovskite Compounds for Solar Energy Applications.

Halide and oxide perovskite semiconductors (e.g., CsPbI3 and SrTiO3) have been widely studied for solar energy conversion applications. The optoelectronic properties and performance of these compounds can be tuned through the growth of layered perovskite superstructures. While oxides are quite varied in the compositions and geometries taken up by layered perovskites, halides have proven much more limited. In this paper, we use density functional theory calculations and chemical intuition to explore why this is the case. We show that, in general, the thermodynamic stability or instability of layered perovskite superstructures depends on the interplay of their ionic and covalent character and, just as importantly, on the features of other competing phases.

Thermal unequilibrium of strained black CsPbI3 thin films.

The high-temperature, all-inorganic CsPbI3 perovskite black phase is metastable relative to its yellow, nonperovskite phase at room temperature. Because only the black phase is optically active, this represents an impediment for the use of CsPbI3 in optoelectronic devices. We report the use of substrate clamping and biaxial strain to render black-phase CsPbI3 thin films stable at room temperature. We used synchrotron-based, grazing incidence, wide-angle x-ray scattering to track the introduction of crystal distortions and strain-driven texture formation within black CsPbI3 thin films when they were cooled after annealing at 330°C. The thermal stability of black CsPbI3 thin films is vastly improved by the strained interface, a response verified by ab initio thermodynamic modeling.

Calibrating the Extended Hückel Method to Quantitatively Screen the Electronic Properties of Materials.

The extended Hückel (eH) tight-binding method has historically been prized for its computational ease and intuitive chemical clarity. However, its lack of quantitative predictiveness has prevented the eH method from being used as a tool for rapidly screening materials for desired electronic properties. In this work, we demonstrate that when eH input parameters are calibrated using density functional theory (DFT) calculations of carefully chosen sets of simple crystals, the eH parameters retain most of their quantitative accuracy when transferred to more complex, structurally related phases. Using solar-energy-relevant semiconductors and insulators in the Sr-Ti-O family as a case study, we show that calibrated eH parameters can match the features of DFT band structures within about two tenths of an eV, at a tiny fraction of the computational cost of DFT.

Design Principles for the Atomic and Electronic Structure of Halide Perovskite Photovoltaic Materials: Insights from Computation.

In the current decade, perovskite solar cell research has emerged as a remarkably active, promising, and rapidly developing field. Alongside breakthroughs in synthesis and device engineering, halide perovskite photovoltaic materials have been the subject of predictive and explanatory computational work. In this Minireview, we focus on a subset of this computation: density functional theory (DFT)-based work highlighting the ways in which the electronic structure and band gap of this class of materials can be tuned via changes in atomic structure. We distill this body of computational literature into a set of underlying design principles for the band gap engineering of these materials, and rationalize these principles from the viewpoint of band-edge orbital character. We hope that this perspective provides guidance and insight toward the rational design and continued improvement of perovskite photovoltaics.

Square planar Cu(I) stabilized by a pyridinediimine ligand.

A set of distorted square planar Cu(I) complexes were synthesized and characterized utilizing the sterically encumbering pyridinediimine ligand, (iPr)PDI (where (iPr)PDI = 2,6-(2,6-(i)Pr2C6H3N=CMe)2C5H3N). The oxidation state of the Cu center(s) were elucidated to be Cu(I) with a neutral PDI ligand system based on structural, spectroscopic, and computational data.

Probing the Protonation State and the Redox-Active Sites of Pendant Base Iron(II) and Zinc(II) Pyridinediimine Complexes.

Utilizing the pyridinediimine ligand [(2,6-(i)PrC6H3)N═CMe)(N((i)Pr)2C2H4)N═CMe)C5H3N] (didpa), the zinc(II) and iron(II) complexes Zn(didpa)Cl2 (1), Fe(didpa)Cl2 (2), [Zn(Hdidpa)Cl2][PF6] (3), [Fe(Hdidpa)Cl2][PF6] (4), Zn(didpa)Br2 (5), and [Zn(Hdidpa)Br2][PF6] (6), Fe(didpa)(CO)2 (7), and [Fe(Hdidpa)(CO)2][PF6] (8) were synthesized and characterized. These complexes allowed for the study of the secondary coordination sphere pendant base and the redox-activity of the didpa ligand scaffold. The protonated didpa ligand is capable of forming metal halogen hydrogen bonds (MHHBs) in complexes 3, 4, and 6. The solution behavior of the MHHBs was probed via pKa measurements and (1)H NMR titrations of 3 and 6 with solvents of varying H-bond accepting strength. The H-bond strength in 3 and 6 was calculated in silico to be 5.9 and 4.9 kcal/mol, respectively. The relationship between the protonation state and the ligand-based redox activity was probed utilizing 7 and 8, where the reduction potential of the didpa scaffold was found to shift by 105 mV upon protonation of the reduced ligand in Fe(didpa)(CO)2.

Understanding Trends in CO2 Adsorption in Metal-Organic Frameworks with Open-Metal Sites.

Using van der Waals-corrected density functional theory and a local chemical bond analysis, we study and explain trends in the binding between CO2 and open-metal coordination sites within a series of two metal-organic frameworks (MOFs), BTT, and MOF-74 for Ca, Mg, and nine divalent transition-metal cations. We find that Ti and V result in the largest CO2 binding energies and show that for these cations the CO2 binding energies for both structure types are twice the value expected based on pure electrostatics. We associate this behavior with the specific electronic configuration of the divalent cations and symmetry of the metal coordination site upon CO2 binding, which result in empty antibonding orbitals between CO2 and the metal cation. We demonstrate that a chemical bond analysis and electrostatic considerations can be used to predict trends of CO2 binding affinities to MOFs with transition-metal cations.

Zigzag inversion domain boundaries in indium zinc oxide-based nanowires: structure and formation.

Existing models for the crystal structure of indium zinc oxide (IZO) and indium iron zinc oxide (IFZO) conflict with electron microscopy data. We propose a model based on imaging and spectroscopy of IZO and IFZO nanowires and verify it using density functional theory. The model features a {121 [symbol: see text]} "zigzag" layer, which is an inversion domain boundary containing 5-coordinate indium and/or iron atoms. Higher [symbol: see text] values are observed for greater proportion of iron. We suggest a mechanism of formation in which the basal inclusion and the zigzag diffuse inward together from the surface of the nanowire.

Band gap and edge engineering via ferroic distortion and anisotropic strain: the case of SrTiO(3).

The effects of ferroic distortion and biaxial strain on the band gap and band edges of SrTiO(3) are calculated by using density functional theory and many-body perturbation theory. Anisotropic strains are shown to reduce the gap by breaking degeneracies at the band edges. Ferroic distortions are shown to widen the gap by allowing new band edge orbital mixings. Compressive biaxial strains raise band edge energies, while tensile strains lower them. To reduce the SrTiO(3) gap, one must lower the symmetry from cubic while suppressing ferroic distortions. Our calculations indicate that, for engineered orientation of the growth direction along [111], the SrTiO(3) gap can be controllably and considerably reduced at room temperature.

Laves phases, gamma-brass, and 2x2x2 superstructures: a new class of quasicrystal approximants and the suggestion of a new quasicrystal.

Of the most common cubic intermetallic structure types, several (MgCu(2), Cu(5)Zn(8), Ti(2)Ni, and alpha-Mn) have superstructures with unusual symmetry properties. These superstructures (Be(5)Au, Li(21)Si(5), Sm(11)Cd(45), and Mg(44)Ir(7)) have the unusual property of pairs of perpendicular pseudo fivefold axes, most apparent in their X-ray diffraction patterns. The current work shows that an 8D to 3D projection method cleanly describes most (and in one case, all) of the atomic positions in the four superstructures mentioned above. This type of projection, which maps the E(8) lattice (a mathematically simple 8D crystal) into 3D space, combines the desired higher dimensional point group's perpendicular fivefold rotations with 3D translational symmetry-exactly what we see in the experimental crystal structures. The projection method successfully accounts for all heavy atom positions in the four superstructures, and at least 60-70 % of the light atom positions. The results suggest that all of these structures, previously known to be connected only by qualitative similarities in their atomic "clusters", are approximants of a single, as-yet unknown, class of quasicrystal.

The mystery of perpendicular fivefold axes and the fourth dimension in intermetallic structures.

The structures of eight related known intermetallic structure types are the impetus to this paper: Li21Si5, Mg44Rh7, Zn13(Fe,Ni)2, Mg6Pd, Na6Tl, Zn91Ir11, Li13Na29Ba19, and Al69Ta39. All belong to the F43m space group, have roughly 400 atoms in their cubic unit cells, are built up at least partially from the gamma-brass structure, and exhibit pseudo-tenfold symmetric diffraction patterns. These pseudo-tenfold axes lie in the {110} directions, and thus present a paradox. The {110} set is comprised of three pairs of perpendicular directions. Yet no 3D point group contains a single pair of perpendicular fivefold axes (by Friedel's Law, a fivefold axis leads to a tenfold diffraction pattern). The current work seeks to resolve this paradox. Its resolution is based on the largest of all 4D Platonic solids, the 600-cell. We first review the 600-cell, building an intuition discussing 4D polyhedroids (4D polytopes). We then show that the positions of common atoms in the F43m structures lie close to the positions of vertices in a 3D projection of the 600-cell. For this purpose, we develop a projection method that we call intermediate projection. The introduction of the 600-cell resolves the above paradox. This 4D Platonic solid contains numerous orthogonal fivefold rotations. The six fivefold directions that are best preserved after projection prove to lie along the {110} directions of the F43m structures. Finally, this paper shows that at certain ideal projected cluster sizes related to one another by the golden mean (tau=(1+ radical 5)/2), constructive interference leading to tenfold diffraction patterns is optimized. It is these optimal values that predominate in actual F43m structures. Explicit comparison of experimental cluster sizes and theoretically derived cluster sizes shows a clear correspondence, both for isolated and crystalline pairs of projected 600-cells.

A quantum mechanically guided view of Mg44Rh7.

We present a new geometric description of Mg(44)Rh(7), a compound with 408 atoms in its cubic unit cell. Using both experimental site preferences and LDA-DFT-calibrated extended Hückel (eH) calculations as guides, we highlight the structural units within Mg(44)Rh(7) that reflect the electron-richness or electron-poorness of each crystallographic site. The units that best account for these site preferences and electron populations are 34- and 25-atom fragments of the Ti(2)Ni structure, rather than the variety of clusters often used to describe complicated intermetallic and ionic structures. These Ti(2)Ni pieces, located using a systematic search algorithm, fit together in a beautifully intricate network. An examination of this network reveals some surprising geometric features of Mg(44)Rh(7), including a fractal-like arrangement of similar atomic formations on different length scales, geometrically connected to an approximate fivefold symmetry.