Robustness of the chiral-icosahedral golden shell I-Au60 in multi-shell structures.

Motivated by the recent theoretical discovery [S.-M. Mullins et al., Nat. Commun. 9, 3352 (2018)] of a surprisingly contracted 60-atom hollow shell of chiral-icosahedral symmetry (I-Au60) of remarkable rigidity and electronegativity, we have explored, via first-principles density functional theory calculations, its physico-chemical interactions with internal and external shells, enabling conclusions regarding its robustness and identifying composite forms in which an identifiable I-Au60 structure may be realized as a product of natural or laboratory processes. The dimensions and rigidity of I-Au60 suggest a templating approach; e.g., an Ih-C60 fullerene fits nicely within its interior, as a nested cage. In this work, we have focused on its susceptibility, i.e., the extent to which the unique structural and electronic properties of I-Au60 are modified by incorporation into selected multi-shell structures. Our results confirm that the I-Au60 shell is robustly maintained and protected in various bilayer structures: Ih-C60@I-Au60, Ih-Au32@I-Au60 2+, Au60(MgCp)12, and their silver analogs. A detailed analysis of the structural and electronic properties of the selected I-Au60 shell-based nanostructures is presented. We found that the I-Au60 shell structure is quite well retained in several robust forms. In all cases, the I-symmetry is preserved, and the I-Au60 shell is slightly deformed only in the case of the Ih-C60@I-Au60 system. This analysis serves to stimulate and provide guidance toward the identification and isolation of various I-Au60 shell-based nanostructures, with much potential for future applications. We conclude with a critical comparative discussion of these systems and of the implications for continuing theoretical and experimental investigations.

Evidence for Octupole Correlations in Multiple Chiral Doublet Bands.

Two pairs of positive-and negative-parity doublet bands together with eight strong electric dipole transitions linking their yrast positive- and negative-parity bands have been identified in ^{78}Br. They are interpreted as multiple chiral doublet bands with octupole correlations, which is supported by the microscopic multidimensionally-constrained covariant density functional theory and triaxial particle rotor model calculations. This observation reports the first example of chiral geometry in octupole soft nuclei.

Nonzero quadrupole moments of candidate tetrahedral bands.

Negative-parity bands in the vicinity of 156Gd and 160Yb have been suggested as candidates for the rotation of tetrahedral nuclei. We report the observation of the odd and even-spin members of the lowest energy negative-parity bands in 160Yb and 154Gd. The properties of these bands are similar to the proposed tetrahedral band of 156Gd and its even-spin partner. Band-mixing calculations are performed and absolute and relative quadrupole moments deduced for 160Yb and 154Gd. The values are inconsistent with zero, as required for tetrahedral shape, and the bands are interpreted as octupole vibrational bands. The failure to observe the in-band E2 transitions of the bands at low spins can be understood using the measured B(E1) and B(E2) values.

Chiral symmetry breaking yields the I-Au60 perfect golden shell of singular rigidity.

The combination of profound chirality and high symmetry on the nm-scale is unusual and would open exciting avenues, both fundamental and applied. Here we show how the unique electronic structure and bonding of quasi-2D gold makes this possible. We report a chiral symmetry breaking, i.e., the spontaneous formation of a chiral-icosahedral shell (I-Au60) from achiral (Ih) precursor forms, accompanied by a contraction in the Au-Au bonding and hence the radius of this perfect golden sphere, in which all 60 sites are chemically equivalent. This structure, which resembles the most complex of semi-regular (Archimedean) polyhedra (34.5*), may be viewed as an optimal solution to the topological problem: how to close a 60-vertex 2D (triangular) net in 3D. The singular rigidity of the I-Au60 manifests in uniquely discrete structural, vibrational, electronic, and optical signatures, which we report herein as a guide to its experimental detection and ultimately its isolation in material forms.

Reactivity of a titanium dinitrogen complex supported by guanidinate ligands: investigation of solution behavior and a novel rearrangement of guanidinate ligands.

The titanium dinitrogen complex, [[(Me(2)N)C(N(i)Pr)(2)]( 2)Ti](2)(N(2)) (2), was synthesized by reduction of the dichloride precursor, [(Me(2)N)C(N(i)Pr)(2)](2)TiCl(2) (1). The dinitrogen complex reacts with phenyl azide to yield the titanium imido complex, [(Me(2)N)C(N(i)Pr)(2)](2)TiNPh (3). The fluxional behavior of the guanidinate ligands in compounds 1-3 was investigated using variable temperature and two-dimensional NMR techniques; guanidinate ligand rotation and racemization reactions were observed. Rearrangement of the guanidinate ligand to an asymmetrical bonding mode utilizing the dimethylamino and amide-nitrogen atoms is observed in the bridging oxo and sulfido derivatives (4 and 5). These compounds are formed by the reactions of 2 with pyridine N-oxide and propylene sulfide, respectively. The ligand rearrangement was observed to be reversible for the bridging sulfido complex 5; the structure of this compound is sensitive to temperature and solvent. The solid-state and solution structures of compounds 1-5 are discussed.