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1.
Nature ; 583(7816): 396-399, 2020 07.
Artigo em Inglês | MEDLINE | ID: mdl-32669698

RESUMO

Curium is unique in the actinide series because its half-filled 5f 7 shell has lower energy than other 5f n configurations, rendering it both redox-inactive and resistant to forming chemical bonds that engage the 5f shell1-3. This is even more pronounced in gadolinium, curium's lanthanide analogue, owing to the contraction of the 4f orbitals with respect to the 5f orbitals4. However, at high pressures metallic curium undergoes a transition from localized to itinerant 5f electrons5. This transition is accompanied by a crystal structure dictated by the magnetic interactions between curium atoms5,6. Therefore, the question arises of whether the frontier metal orbitals in curium(III)-ligand interactions can also be modified by applying pressure, and thus be induced to form metal-ligand bonds with a degree of covalency. Here we report experimental and computational evidence for changes in the relative roles of the 5f/6d orbitals in curium-sulfur bonds in [Cm(pydtc)4]- (pydtc, pyrrolidinedithiocarbamate) at high pressures (up to 11 gigapascals). We compare these results to the spectra of [Nd(pydtc)4]- and of a Cm(III) mellitate that possesses only curium-oxygen bonds. Compared with the changes observed in the [Cm(pydtc)4]- spectra, we observe smaller changes in the f-f transitions in the [Nd(pydtc)4]- absorption spectrum and in the f-f emission spectrum of the Cm(III) mellitate upon pressurization, which are related to the smaller perturbation of the nature of their bonds. These results reveal that the metal orbital contributions to the curium-sulfur bonds are considerably enhanced at high pressures and that the 5f orbital involvement doubles between 0 and 11 gigapascal. Our work implies that covalency in actinides is complex even when dealing with the same ion, but it could guide the selection of ligands to study the effect of pressure on actinide compounds.

2.
Phys Chem Chem Phys ; 2020 May 13.
Artigo em Inglês | MEDLINE | ID: mdl-32400781

RESUMO

Exploration of photovoltaic materials has received enormous interest for a wide range of both fundamental and applied research. Therefore, in this work, we identify a CsSi compound with a Zintl phase as a promising candidate for photovoltaic material by using a global structure prediction method. Electronic structure calculations indicate that this phase possesses a quasi-direct band gap of 1.45 eV, suggesting that its optical properties could be superior to those of diamond-Si for capturing sunlight from the visible to the ultraviolet range. In addition, a novel silicon allotrope is obtained by removing Cs atoms from this CsSi compound. The superconducting critical temperature (Tc) of this phase was estimated to be of 9 K in terms of a substantial density of states at the Fermi level. Our findings represent a new promising CsSi material for photovoltaic applications, as well as a potential precursor of a superconducting silicon allotrope.

3.
Org Lett ; 22(3): 1003-1007, 2020 02 07.
Artigo em Inglês | MEDLINE | ID: mdl-31944777

RESUMO

Hybrid tetrapeptides sharing a backbone with a central α/ß-dipeptide segment flanked by aromatic γ-amino acid residues fold into the same hairpin conformation with an expanded ß-turn. This hairpin/ß-turn motif is general for accommodating different α- and ß-amino acid residues. Replacing glycine with other α-amino acid residues has an insignificant influence on or slightly decreases the stabilities of the folded conformations; substituting ß-alanine with other ß-amino acid residues enhances the stabilities of the folded structures.

4.
J Am Chem Soc ; 142(6): 2915-2924, 2020 02 12.
Artigo em Inglês | MEDLINE | ID: mdl-31895977

RESUMO

Supramolecular chemistry in aqueous media is an area with great fundamental and practical significance. To examine the role of multiple noncovalent interactions in controlled assembling and binding behavior in water, the self-association of five water-soluble hexakis(m-phenylene ethynylene) (m-PE) macrocycles, along with the molecular recognition behavior of the resultant assemblies, is investigated with UV-vis, fluorescence, CD, and NMR spectroscopy, mass spectrometry, and computational studies. In contrast to their different extents of self-aggregation in organic solvents, all five macrocycles remain aggregated in water at concentrations down to the micromolar (µM) range. CD spectroscopy reveals that 1-F6 and 1-H6, two macrocycles carrying chiral side chains and capable of H-bonded self-association, assemble into tubular stacks. The tubular stacks serve as supramolecular hosts in water, as exemplified by the interaction of macrocycles 1-H6 and 2-H6 and guests G1 through G4, each having a rod-like oligo(p-phenylene ethynylene) (p-PE) segment flanked by two hydrophilic chains. Fluorescence and 1H NMR spectroscopy revealed the formation of kinetically stable, discrete assemblies upon mixing 2-H6 and a guest. The binding stoichiometry, determined with fluorescence, 1H NMR, and ESI-MS, reveals that the discrete assemblies are novel pseudorotaxanes, each containing a pair of identical guest molecules encased by a tubular stack. The two guest molecules define the number of macrocyclic molecules that comprise the host, which curbs the "infinite" stack growth, resulting in a tubular stack with a cylindrical pore tailoring the length of the p-PE segment of the bound guests. Each complex is stabilized by the action of multiple noncovalent forces including aromatic stacking, side-chain H-bonding, and van der Waals interactions. Thus, the interplay of multiple noncovalent forces aligns the molecules of macrocycles 1 and 2 into tubular stacks with cylindrical inner pores that, upon binding rod-like guests, lead to tight, discrete, and well-ordered tubular assemblies that are unprecedented in water.

5.
Nanoscale ; 12(1): 347-355, 2020 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-31825450

RESUMO

Graphene and carbon nanotubes (CNT) are the representatives of two-dimensional (2D) and one-dimensional (1D) forms of carbon, both exhibiting unique geometric structures and peculiar physical and chemical properties. Herein, we propose a family or series of 2D carbon-based highly anisotropic Dirac materials by weaving together an array of CNTs by direct C-C bonds or by graphene ribbons. By employing first-principles calculations, we demonstrate that these nano-makisus are thermally and dynamically stable and possess unique electronic properties. These 2D carbon allotropes are all metals and some nano-makisus show largely anisotropic Dirac cones, causing very different transport properties for the Dirac fermions along different directions. The Fermi velocities in the kx direction could be ∼170 times higher than those in the ky direction, which is the strongest anisotropy among 2D carbon allotropes to the best of our knowledge. This intriguing feature of the electronic structure has only been observed in heavy element materials with strong spin-orbit coupling. These results indicate that carbon based materials may have much broader applications in future nanoelectronics.

6.
J Am Chem Soc ; 141(36): 14239-14248, 2019 Sep 11.
Artigo em Inglês | MEDLINE | ID: mdl-31381306

RESUMO

Multilevel protein structures typically involve polypeptides of sufficient lengths. Here we report the folding and assembly of seven short tetrapeptides sharing the same types of α-, ß-, and aromatic γ-amino acid residues. These are two sets of hybrid peptides, with three members in one set and four in the other, having complementary hydrogen-bonding sequences that were hypothesized to pair into linear H-bonded duplexes. However, instead of undergoing the anticipated pairing, the initially examined three oligomers, 1 and 2a or 2b, differing only in their central αß hybrid dipeptide sequence, do not associate with each other and exhibit distinctly different folding behavior. Experiments based on NMR and mass spectrometry, along with computational studies and systematic inference, reveal that oligomer 1 folds into an expanded ß-turn containing an unusual hybrid α/ß-amino acid sequence composed of glycine and ß-alanine, two α- and ß-amino acid residues that are conformationally most flexible, and peptides 2a and 2b adopt a noncanonical, extended helical conformation and dimerize into double helices undergoing rapid conformational exchange or helix inversion. The different central dipeptide sequences, αß vs ßα, result in drastically different intramolecular H-bonding patterns that are responsible for the observed folding behavior of 1 and 2. The revealed turn and double helix have few natural or synthetic counterparts, and provide novel and unique folding prototypes based on which chiral α- and ß-amino acids are incorporated. The resultant derivatives 1a, 1b, 2c, and 2d follow the same folding and assembling behavior and demonstrate the generality of this system with the formation of expanded ß-turns and double helices with enhanced folding stabilities, hampered helix inversion, as well as defined and dominant helical sense. This work has demonstrated the unique capability of synthetic foldamers in generating structures with fascinating folding and assembling behavior. The revealed systems offer ample opportunity for further structural optimization and applications.

7.
J Chem Phys ; 150(5): 050901, 2019 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-30736697

RESUMO

The theoretical exploration of the phase diagrams of binary hydrides under pressure using ab initio crystal structure prediction techniques coupled with first principles calculations has led to the in silico discovery of numerous novel superconducting materials. This Perspective focuses on the alkaline earth and rare earth polyhydrides whose superconducting critical temperature, Tc, was predicted to be above the boiling point of liquid nitrogen. After providing a brief overview of the computational protocol used to predict the structures of stable and metastable hydrides under pressure, we outline the equations that can be employed to estimate Tc. The systems with a high Tc can be classified according to the motifs found in their hydrogenic lattices. The highest Tcs are found for cages that are reminiscent of clathrates and the lowest for systems that contain atomic and molecular hydrogen. A wide variety of hydrogenic motifs including 1- and 2-dimensional lattices, as well as H10 δ- molecular units comprising fused H5 δ- pentagons, are present in phases with intermediate Tcs. Some of these phases are predicted to be superconducting at room temperature. Some may have recently been synthesized in diamond anvil cells.

8.
Langmuir ; 34(38): 11335-11346, 2018 09 25.
Artigo em Inglês | MEDLINE | ID: mdl-30157638

RESUMO

The adsorption and subsequent electrooxidative polymerization of ferriprotoporphyrin IX chloride (hemin; FePPCl) was investigated on highly ordered pyrolytic graphite, glassy carbon, and polycrystalline Pt electrodes using electrochemical atomic force microscopy, first-principles calculations, and cyclic voltammetry. Hemin was shown to readily adsorb to all three surfaces; however, it was more continuous over the carbon surfaces compared to the Pt surface. This disparity in adsorption appears to be a major contributing factor to differences observed between the electrodes following hemin electropolymerization. Despite differences in roughness and morphology, hemin polymerized as a continuous layer over each electrode surface. Periodic density functional theory calculations were used to model FePP (without Cl) on both the Pt(111) and graphite surfaces using the vdW-DF-optPBE functional to account for the dispersion interactions. Our calculations suggest that the FePP molecule chemisorbs to the Pt surface while at the same time exhibiting intramolecular hydrogen bonding between the carboxylic acid groups, which are extended away from the surface. In contrast to FePP-Pt chemisorption, FePP was found to physisorb to graphite. The preferred spin state upon adsorption was found to be S = 2 on Pt(111), whereas on graphite, the high and intermediate spin states were nearly isoenergetic. Additionally, gas-phase calculations suggest that much of the surface roughness observed microscopically for the polymerized porphyrin layer may originate from the nonparallel stacking of porphyrin molecules, which interact with each other by forming four intermolecular hydrogen bonds and through dispersion interactions between the stacked porphyrin rings. Regardless of polymer thickness, the underlying electrode appears to be able to participate in at least some redox processes. This was observed for the hemin-polymerized Pt electrode using the 2H+/H2 redox couple and was suspected to be due to some Pt surface atoms not being specifically coordinated to the hemin molecules and therefore available to react with H+ that was small enough to diffuse through the polymer layer.

9.
Nat Commun ; 9(1): 951, 2018 03 05.
Artigo em Inglês | MEDLINE | ID: mdl-29507302

RESUMO

Until very recently, helium had remained the last naturally occurring element that was known not to form stable solid compounds. Here we propose and demonstrate that there is a general driving force for helium to react with ionic compounds that contain an unequal number of cations and anions. The corresponding reaction products are stabilized not by local chemical bonds but by long-range Coulomb interactions that are significantly modified by the insertion of helium atoms, especially under high pressure. This mechanism also explains the recently discovered reactivity of He and Na under pressure. Our work reveals that helium has the propensity to react with a broad range of ionic compounds at pressures as low as 30 GPa. Since most of the Earth's minerals contain unequal numbers of positively and negatively charged atoms, our work suggests that large quantities of He might be stored in the Earth's lower mantle.

10.
Adv Mater ; 29(39)2017 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-28846811

RESUMO

The Fe(II) spin crossover complex [Fe{H2 B(pz)2 }2 (bipy)] (pz = pyrazol-1-yl, bipy = 2,2'-bipyridine) can be locked in a largely low-spin-state configuration over a temperature range that includes temperatures well above the thermal spin crossover temperature of 160 K. This locking of the spin state is achieved for nanometer thin films of this complex in two distinct ways: through substrate interactions with dielectric substrates such as SiO2 and Al2 O3 , or in powder samples by mixing with the strongly dipolar zwitterionic p-benzoquinonemonoimine C6 H2 (-⋯ NH2 )2 (-⋯ O)2 . Remarkably, it is found in both cases that incident X-ray fluences then restore the [Fe{H2 B(pz)2 }2 (bipy)] moiety to an electronic state characteristic of the high spin state at temperatures of 200 K to above room temperature; that is, well above the spin crossover transition temperature for the pristine powder, and well above the temperatures characteristic of light- or X-ray-induced excited-spin-state trapping. Heating slightly above room temperature allows the initial locked state to be restored. These findings, supported by theory, show how the spin crossover transition can be manipulated reversibly around room temperature by appropriate design of the electrostatic and chemical environment.

11.
Chem Commun (Camb) ; 53(60): 8463-8466, 2017 Jul 25.
Artigo em Inglês | MEDLINE | ID: mdl-28702538

RESUMO

Atomically precise chevron graphene nanoribbons (GNRs) have been synthesized on Cu(111) substrates by the surface-assisted coupling of 6,11-dibromo-1,2,3,4-tetraphenyltriphenylene (C42Br2H26) and thermal cyclodehydrogenation of the resulting polymer. The GNRs form on Cu(111) epitaxially along the 〈112〉 crystallographic directions, which was found to be in agreement with the computational results, and at lower temperatures than on Au(111). This work demonstrates that the substrate plays an important role in the on-surface synthesis of GNRs and can result in new assembly modes of GNR structures.

12.
J Phys Chem Lett ; 8(15): 3457-3465, 2017 Aug 03.
Artigo em Inglês | MEDLINE | ID: mdl-28691486

RESUMO

We report synchrotron X-ray diffraction, photoconductivity, and photoluminescence investigations of methylammonium-lead-bromide (MAPbBr3) under various stress conditions, supported by density-functional-theory (DFT) calculations. The properties of MAPbBr3 show substantial dependence on the hydrostatic conditions. While nonhydrostatic compression of MAPbBr3 leads to amorphization above 2.4 GPa, under quasi-hydrostatic (Ar) and hydrostatic (He) pressure, the sample remains in crystalline phases. A sequence of phase transitions between two cubic phases and orthorhombic Pnma phase is observed when using Ar, or no pressure-transmitting-medium (PTM). In helium-PTM only transitions between the two cubic structures and a new isostructural phase transition with a large volume collapse to a third cubic-phase at 2.7 GPa was observed. The photoluminescence measurements indicate a pressure-induced band gap-narrowing in the cubic phase I, and a blue-shift in the orthorhombic structure. DFT calculations illustrate that the dynamics of the organic molecules and the inorganic lattice, coupled via the N-H···Br hydrogen-bonding interactions, affect the Pb-Br distance and the bandgap evolution under pressure.

13.
Org Lett ; 19(10): 2666-2669, 2017 05 19.
Artigo em Inglês | MEDLINE | ID: mdl-28474528

RESUMO

Aromatic oligoureas composed of meta-linked residues bearing phenolic ether side chains are synthesized. The basic N,N'-diarylurea units adopt a trans,trans intramolecularly H-bonded conformation that is further strengthened by additional intermolecular H-bonding. Such basic units, in combination with meta-linked benzene residues, result in stably folded helical oligoureas in the highly polar DMF with up to four turns and with a small cylindrical inner pore that would be difficult to acquire.

14.
Angew Chem Int Ed Engl ; 56(34): 10192-10195, 2017 08 14.
Artigo em Inglês | MEDLINE | ID: mdl-28474853

RESUMO

At 80 GPa, phases with the PH2 stoichiometry, which are composed of simple cubic like phosphorus layers capped with hydrogen atoms and layers of H2 molecules, are predicted to be important species contributing to the recently observed superconductivity in compressed phosphine. The electron-phonon coupling in these phases results from the motions of the phosphorus atoms and the hydrogen atoms bound to them. The role of the mobile H2 layers is to decrease the Coulomb repulsion between the negatively charged hydrogen atoms capping the phosphorus layers. An insulating PH5 phase, the structure and bonding of which is reminiscent of diborane, is also predicted to be metastable at this pressure.

15.
ACS Nano ; 11(3): 2486-2493, 2017 03 28.
Artigo em Inglês | MEDLINE | ID: mdl-28165713

RESUMO

A strategy to synthesize a 2D graphenic but ternary monolayer containing atoms of carbon, nitrogen, and boron, h-BCN, is presented. The synthesis utilizes bis-BN cyclohexane, B2N2C2H12, as a precursor molecule and relies on thermally induced dehydrogenation of the precursor molecules and the formation of an epitaxial monolayer on Ir(111) through covalent bond formation. The lattice mismatch between the film and substrate causes a strain-driven periodic buckling of the film. The structure of the film and its corrugated morphology is discussed based on comprehensive data from molecular-resolved scanning tunneling microscopy imaging, X-ray photoelectron spectroscopy, low-energy electron diffraction, and density functional theory. First-principles calculations further predict a direct electronic band gap that is intermediate between gapless graphene and insulating h-BN.

16.
Phys Chem Chem Phys ; 18(34): 24106-18, 2016 Sep 14.
Artigo em Inglês | MEDLINE | ID: mdl-27526292

RESUMO

Calculations of NMR shielding tensors and nuclear quadrupole coupling (NQC) tensors at the Kohn-Sham density functional level are used to simulate (27)Al magic-angle spinning (MAS) NMR spectra of the important olefin polymerization co-catalyst methylaluminoxane (MAO) at 77, 298, 398, and 498 K and spectrometer magnetic field inductions B ranging from 14.1 to 23.5 T. The calculations utilize the temperature (T) dependent distribution of species present in MAO determined recently by Zurek and coworkers from first-principles theory [Macromolecules, 2014, 47, 8556]. The NMR calculations suggest that variable-T and variable-B NMR measurements are able to quantify the ratio of free versus bound trimethyl-aluminum (TMA) in MAO via characteristic spectral features assigned to 3-coordinate and 4-coordinate Al sites in MAO as well as spectral features arising from free TMA or its dimer. The T-dependent distribution of species causes other characteristic features in the NMR spectra to appear/disappear that can be associated with different aluminum environments such as square vs. hexagonal faces in cage and tubular structures. The simulated spectra at 298 K and 19.6 T are in reasonably good agreement with the experimental solid-state NMR (SSNMR) spectra obtained previously for MAO gel. The promise and limitations of solid-state NMR to unravel the enigma surrounding the structure(s) of MAO are discussed.

17.
Inorg Chem ; 55(17): 8371-80, 2016 Sep 06.
Artigo em Inglês | MEDLINE | ID: mdl-27513717

RESUMO

The synthesis, electronic structure, and characterization via single-crystal X-ray diffraction, nuclear magnetic resonance (NMR) spectroscopy, and magnetic susceptibility of (Me4N)2PuCl6 are reported. NMR measurements were performed to both search for the direct (239)Pu resonance and to obtain local magnetic and electronic information at the Cl site through (35)Cl and (37)Cl spectra. No signature of (239)Pu NMR was observed. The temperature dependence of the Cl spectra was simulated by diagonalizing the Zeeman and quadrupolar Hamiltonians for (35)Cl, (37)Cl, and (14)N isotopes. Electronic structure calculations predict a magnetic Γ5 triplet ground state of Pu(IV) in the crystalline electric field of the undistorted PuCl6 octahedron. A tetragonal distortion would result in a very small splitting (∼20 cm(-1)) of the triplet ground state into a nonmagnetic singlet and a doublet state. The Cl shifts have an inflection point at T ≈ 15 K, differing from the bulk susceptibility, indicating a nonmagnetic crystal field ground state. The Cl spin-lattice relaxation time is constant to T = 15 K, below which it rapidly increases, also supporting the nonmagnetic crystal field ground state.

18.
J Phys Chem Lett ; 7(3): 435-40, 2016 Feb 04.
Artigo em Inglês | MEDLINE | ID: mdl-26750982

RESUMO

The synthesis of 2D H-bonded cocrystals from the room-temperature ferroelectric organics croconic acid (CA) and 3-hydroxyphenalenone (3-HPLN) is demonstrated through self-assembly on a substrate under ultrahigh vacuum. 2D cocrystal polymorphs of varied stoichiometry were identified with scanning tunneling microscopy, and one of the observed structural building blocks consists of two CA and two 3-HPLN molecules. Computational analysis with density functional theory confirmed that the experimental (CA)2(3-HPLN)2 tetramers are lower in energy than single-component structures due to the ability of the tetramers to pack efficiently in two dimensions, the promotion of favorable electrostatic interactions between tetramers, and the optimal number of intermolecular hydrogen bonds. The structures investigated, especially the experimentally found tetrameric building blocks, are not polar. However, it is demonstrated computationally that cocrystallization can, in principle, result in heterogeneous structures with dipole moments that exceed those of homogeneous structures and that 2D structures with select stoichiometries could favor metastable polar structures.

19.
J Am Chem Soc ; 138(6): 1884-92, 2016 Feb 17.
Artigo em Inglês | MEDLINE | ID: mdl-26777416

RESUMO

Evolutionary algorithms (EAs) coupled with density functional theory (DFT) calculations have been used to predict the most stable hydrides of phosphorus (PHn, n = 1-6) at 100, 150, and 200 GPa. At these pressures phosphine is unstable with respect to decomposition into the elemental phases, as well as PH2 and H2. Three metallic PH2 phases were found to be dynamically stable and superconducting between 100 and 200 GPa. One of these contains five formula units in the primitive cell and has C2/m symmetry (5FU-C2/m). It comprises 1D periodic PH3-PH-PH2-PH-PH3 oligomers. Two structurally related phases consisting of phosphorus atoms that are octahedrally coordinated by four phosphorus atoms in the equatorial positions and two hydrogen atoms in the axial positions (I4/mmm and 2FU-C2/m) were the most stable phases between ∼160-200 GPa. Their superconducting critical temperatures (Tc) were computed as 70 and 76 K, respectively, via the Allen-Dynes modified McMillan formula and using a value of 0.1 for the Coulomb pseudopotential, µ*. Our results suggest that the superconductivity recently observed by Drozdov, Eremets, and Troyan when phosphine was subject to pressures of 207 GPa in a diamond anvil cell may result from these, and other, decomposition products of phosphine.

20.
Phys Chem Chem Phys ; 17(18): 12265-72, 2015 May 14.
Artigo em Inglês | MEDLINE | ID: mdl-25891957

RESUMO

Evolutionary structure searches predict two hitherto unknown phases of KCl that are the most stable in the pressure regime of 200-600 GPa. I41/amd-KCl, which has the lowest enthalpy between ∼200-350 GPa, can be thought of as being composed of two three-connected nets. This structure can be compared with that of the Cs-IV electride (Cs(+)e(-)): the potassium ions assume the positions of the cesium ions, and the chloride ions are found roughly in the regions of the valence electrons. Above ∼350 GPa a Pnma phase, which is isotypic with phases of CsH and CsI that are stable under pressure, becomes preferred. Just as in Pnma-CsI, the atoms in Pnma-KCl assume an hcp-like lattice; these alkali halides resemble the high-pressure structures of the isoelectronic noble gas solids Xe and Ar, respectively. The equation of state of KCl is extended to 600 GPa, enabling the use of this alkali halide as a pressure guage in ultra-high pressure static compression experiments. KCl is predicted to remain insulating to at least 420 GPa.

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