A three-dimensional π-conjugated polyradicaloid molecular cage c -Ph14, consisting of three Chichibabin’s hydrocarbon motifs connected by two benzene-1,3,5-triyl bridgeheads, was synthesized. ...Compared with its linear model compound l -Ph4, the prism-like c -Ph14 has a more rigid structure, which shows significant impact on the molecular dynamics, stability, and electronic properties. A higher rotation energy barrier for the quinoidal biphenyl units was determined in c -Ph14 (15.64 kcal/mol) than that of l -Ph4 (11.40 kcal/mol) according to variable-temperature NMR measurements, leading to improved stability, a smaller diradical character, and an increased singlet–triplet energy gap. The pressure-dependent Raman spectroscopic studies on the rigid cage c -Ph14 revealed a quinoidal-to-aromatic transformation along the biphenyl bridges. In addition, the ellipsoidal cavity in the cage allowed selective encapsulation of fullerene C70 over C60, with an associate constant of about 1.43 × 104 M–1. Moreover, c -Ph14 and l -Ph4 exhibited similar redox behavior and their cationic species ( c -Ph14 6+ and l -Ph4 2+ ) were obtained by chemical oxidation, and the structures were identified by X-ray crystallographic analysis. The biphenyl unit showed a twisted conformation in l -Ph4 2+ and remained coplanarity in c -Ph14 6+ . Notably, molecules of c -Ph14 6+ form a one-dimensional columnar structure via close π–π stacking between the bridgeheads.
We show that the hydrogen in metal superhydride compounds can adopt two distinct statesatomic and molecular. At low pressures, the maximum number of atomic hydrogens is typically equal to the ...valency of the cation; additional hydrogens pair to form molecules with electronic states far below the Fermi energy causing low-symmetry structures with large unit cells. At high pressures, molecules become unstable, and all hydrogens become atomic. This study uses density functional theory, adopting BaH4 as a reference compound, which is compared with other stoichiometries and other cations. Increased temperature and zero-point motion also favor high-symmetry atomic states, and picosecond-timescale breaking and remaking of the bond permutations via intermediate H3 – units.
nCycloparaphenylenes behave as molecular templates of “perfectly chemically defined” single‐wall carbon nanotubes. These nCPP molecules have electronic, mechanical, and chemical properties in size ...correspondence with their giant congeners. Under mechanical stress, they form charge‐transfer salts, or complexes with fullerene, by one‐electron concave–convex electron transfer.
nCycloparaphenylenes behave as molecular templates of “perfectly chemically defined” single‐wall carbon nanotubes. These nCPP molecules have electronic, mechanical, and chemical properties in size correspondence with their giant congeners. Under mechanical stress, they form charge‐transfer salts, or complexes with fullerene, by one‐electron concave–convex electron transfer.
Synthesis of Weaire–Phelan Barium Polyhydride Peña-Alvarez, Miriam; Binns, Jack; Martinez-Canales, Miguel ...
The journal of physical chemistry letters,
05/2021, Letnik:
12, Številka:
20
Journal Article
Recenzirano
Odprti dostop
By combining pressures up to 50 GPa and temperatures of 1200 K, we synthesize the novel barium hydride, Ba8H46, stable down to 27 GPa. We use Raman spectroscopy, X-ray diffraction, and ...first-principles calculations to determine that this compound adopts a highly symmetric P m 3̅ n structure with an unusual 5 3 4 :1 hydrogen-to-barium ratio. This singular stoichiometry corresponds to the well-defined type-I clathrate geometry. This clathrate consists of a Weaire–Phelan hydrogen structure with the barium atoms forming a topologically close-packed phase. In particular, the structure is formed by H20 and H24 clathrate cages showing substantially weakened H–H interactions. Density functional theory (DFT) demonstrates that cubic P m 3̅ n Ba8H46 requires dynamical effects to stabilize the H20 and H24 clathrate cages.
High pressures in the 0–10 GPa range cause molecules to deform in unusual ways. A series of precisely defined carbon nanohoops consisting of n para-linked phenyl groups, n-cycloparaphenylenes ...(nCPPs, n = 7, 8, 9, 10, and 12) were studied in this pressure range using Raman spectroscopy and density functional theory (DFT) and compared with more rigid smaller 5- and 6CPPs and with the longer carbon nanotubes. The presented analysis sheds light on the different responses to pressure depending on the nanohoop size. Surprisingly, the pressure coefficients, the rate of the Raman shifts as a function of pressure, change at a particular pressure which is characteristic of each nCPP. We identified this pressure as the beginning of ovalization of the nanohoops in analogy to carbon nanotubes. This pressure induced ovalization is reversible in the range of pressure studied for nCPPs with n = 7, 9, 10, and 12. In the case of 8CPP, we find a metastable conformation at 8 GPa with significantly changed dihedral angles of adjacent phenyls. This high pressure molecular phase of 8CPP provides an example for a new mechanism of irreversibility involving different conformations upon high pressure treatment. Modeling provided atomic level insights into the changes of conformations and the development of aromatic vs quinonoid structures as a function of pressure.
Using optical spectroscopy, X-ray diffraction, and electrical transport measurements, we have studied the pressure-induced metallization in BaH2 and Ba8H46. Our combined measurements suggest a ...structural phase transition from BaH2-II to BaH2-III accompanied by band gap closure and transformation to a metallic state at 57 GPa. The metallization is confirmed by resistance measurements as a function of the pressure and temperature. We also confirm that, with further hydrogenation, BaH2 forms the previously observed Weaire–Phelan Ba8H46, synthesized at 45 GPa and 1200 K. In this compound, metallization pressure is shifted to 85 GPa. Through a comparison of the properties of these two compounds, a question is raised about the importance of the hydrogen content in the electronic properties of hydride systems.
Transition-metal nitrides have applications in a range of technological fields. Recent experiments have shown that new nitrogen-bearing compounds can be accessed through a combination of high ...temperatures and pressures, revealing a richer chemistry than was previously assumed. Here, we show that at pressures above 50 GPa and temperatures greater than 1500 K elemental copper reacts with nitrogen, forming copper diazenide (CuN2). Through a combination of synchrotron X-ray diffraction and first-principles calculations we have explored the stability and electronic structure of CuN2. We find that the novel compound remains stable down to 25 GPa before decomposing to its constituent elements. Electronic structure calculations show that CuN2 is metallic and exhibits partially filled N2 antibonding orbitals, leading to an ambiguous electronic structure between Cu+/Cu2+. This leads to weak Cu–N bonds and the lowest bulk modulus observed for any transition-metal nitride.
The reduced and oxidized states of an open-shell diindenob,ianthracene (DIAn) derivative have been investigated by experimental and theoretical techniques. As a result of moderate biradical ...character and the ability of cyclopenta-fused scaffolds to stabilize both positive and negative charges, DIAn exhibits rich redox chemistry with four observable and isolable charged states. Structural and electronic properties of the DIAn system are brought to light by UV–vis–NIR and Raman spectroelectrochemical measurements. Aromatization of the diindeno-fused anthracene core upon successive single-electron injections is revealed through single-crystal X-ray diffraction of radical anion and dianion salts. We present a rare case where the pseudoaromatic/quinoidal ground state of a neutral biradical polycyclic hydrocarbon leads to a stable cascade of five redox states. Our detailed investigation of the transformation of molecular structure along all four redox events provides a clearer understanding of the nature of charge carriers in ambipolar organic field-effect transistors.
Conducting polymers can be synthesized by irreversible diradical monomer polymerization. A reversible version of this reaction consisting of the formation/dissociation of σ‐dimers and σ‐polymers from ...a stable quinonoidal diradical precursor is described. The reaction reversibility is made by a quinonoidal molecule which changes its structure to an aromatic species by forming weak and long intermolecular C−C single bonds. The reaction provokes a giant chromic effect of about 2.5 eV. The two opposite but complementary quinonoidal and aromatic tautomers provide the Janus faces of the reactants and products which produces the observed chromic effect. A reaction mechanism is proposed to explain the variety of final products starting with structurally very similar reactants. These reversible reactions, covering an unusual regime of weak covalent supramolecular bonding, yield products which might be envisaged as novel molecular and polymeric soft matter phases.
Long and weak C−C bonds are formed from a quinonoidal diradical molecule and lead to either double σ‐bonded cyclophane dimers or σ‐bonded polymers. The highly strained character of the C−C bonds makes these reactions reversible and the quinonoidal–aromatic transformation results in a large change in the optical properties and thus large chromic effects.
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