Boron could be the next element after carbon capable of forming 2D-materials similar to graphene. Theoretical calculations predict that the most stable planar all-boron structure is the so-called α-sheet. The mysterious structure of the α-sheet with peculiar distribution of filled and empty hexagons is rationalized in terms of chemical bonding. We show that the hexagon holes serve as scavengers of extra electrons from the filled hexagons. This work could advance rational design of all-boron nanomaterials.
Tetracoordinate planar carbon (that is, carbon atoms coordinated by four other atoms in a square-planar arrangement), first proposed by Hoffmann et al. [1] over thirty years ago, was recently observed in the vibrationally averaged D 4h [Al 4 C] À , [2] C 2v Al 3 XC, [Al 3 XC] À (X = Si,Ge), [3,4] and [CAl 4 ] 2À .[5] Tetracoordinate planar (TP) Si and Ge centers were also discovered in MAl 4 and [MAl 4 ] À .[6] The TP bonding character of Group 14 elements, contrary to the conventional concept of tetrahedral C, Si, and Ge, is supported by excellent agreements between the measured photoelectron spectra and ab initio vertical detachment energies (VDE) of these molecules in the TP structures.Here we extend the range of TP centers to include B, N, and O and the ligands from p-block elements to d-block transition metals (Cu and Ni). On the basis of ab initio optimization results, we present the first theoretical evidence that, in the form of the X-centered hydrometals M 4 H 4 X, these first-row nonmetals X (X = B, C, N, O) are tetracoordinated by four transition-metal ligands M (M = Cu, Ni) in perfect squares. Hydrocopper Cu 4 H 4 and hydronickel Ni 4 H 4 are found to be suitable for hosting these tetracoordinate planar nonmetals (TPNs) with the high symmetry of D 4h . To the best of our knowledge, neither experimental nor theoretical data on these unusual molecules are available to date. Our work was stimulated by the proposal of aromatic hydrocopper Cu 4 H 4 at the DFT level. [7] Various initial structures obtained at the B3LYP/Lanl2dz level were optimized at the B3LYP/6-311 + G(3df,p) level, and imaginary frequencies were checked at the same level. The DFT binding energies of 23.1 eV for D 4h Ni 4 H 4 C and 18.7 eV for C 4v Cu 4 H 4 C demonstrate the stability of these complexes with respect to dissociation. The B3LYP geometries were further refined with the second-order Møller-Plesset (MP2) procedure. We calculated the NMR shielding tensors using the gauge-independent atomic orbital (GIAO) procedure [8,9] at the B3LYP/6-311 + G(3df,p) level, and VDEs, ionization potentials (IPs), and electron affinities (EAs) utilizing the restricted outer valence Green function method (ROVGF) [10,11] with a smaller basis of 6-31 + G(d,p). The optimized MP2 bond parameters, normal vibrational frequencies, and electronic properties of these systems are provided in the Supporting Information. All the calculations were performed with the Gaussian 03 program. À and Cu 4 H 4 C, on the other hand, were found to be transition states with imaginary frequencies of 266i and 135i cm À1 , respectively, and lie 0.962 and 0.124 eV higher in energy than the corresponding tetracoordinate pyramidal C 4v structures. These imaginary frequencies correspond to the A 2u vibrational mode, in which the central atom X moves up and down along the fourfold axis. Distortion of the D 4h structure in the A 2u mode leads to the C 4v global minimum, in which the X atom lies about 0.
During photoelectron spectroscopy experiments, the spectra of B(11)O(-) and B(10)Au(-) clusters are found to exhibit similar patterns except for a systematic spectral shift of ∼0.5 eV, hinting that they possess similar geometric structures. The electron affinities are measured to be 4.02 ± 0.04 eV for B(11)O and 3.55 ± 0.02 eV for B(10)Au. DFT calculations at the B3LYP level show that B(11)O(-) and B(10)Au(-) adopt similar C(1) ((1)A) ground states, which are based on the quasiplanar B(10) cluster interacting with a BO unit and Au, respectively. The B(11)O(-) and B(10)Au(-) clusters are thus valent isoelectronic because both BO and Au can be viewed as monovalent units, forming highly covalent B-BO and B-Au bonds analogous to the B-H bond in B(10)H(-). For B(10)Au(-), we also find a highly symmetric D(10h) ((1)A(1g)) planar molecular wheel as a minimum on the potential energy surface. However, it is 45 kcal/mol above the ground state at the B3LYP level and not viable for experimental observation. Natural bond orbital analyses reveal interesting covalent versus ionic B-Au bonding in the C(1) B(10)Au(-) and D(10h) B(10)Au(-) structures, respectively, providing insight for the design of D(nh) MB(n) molecular wheels.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.