Lai-Sheng Wang

Synthesis of the H-cluster framework of iron-only hydrogenase

The metal-sulphur active sites of hydrogenases catalyse hydrogen evolution or uptake at rapid rates. Understanding the structure and function of these active sites—through mechanistic studies of hydrogenases, synthetic assemblies and in silico models—will help guide the design of new materials for hydrogen production or uptake. Here we report the assembly of the iron-sulphur framework of the active site of iron-only hydrogenase (the H-cluster), and show that it functions as an electrocatalyst for proton reduction. Through linking of a di-iron subsite to a {4Fe4S} cluster, we achieve the first synthesis of a metallosulphur cluster core involved in small-molecule catalysis. In addition to advancing our understanding of the natural biological system, the availability of an active, free-standing analogue of the H-cluster may enable us to develop useful electrocatalytic materials for application in, for example, reversible hydrogen fuel cells. (Platinum is currently the preferred electrocatalyst for such applications, but is expensive, limited in availability and, in the long term, unsustainable.)

Observation of an all-boron fullerene

After the discovery of fullerene-C60, it took almost two decades for the possibility of boron-based fullerene structures to be considered. So far, there has been no experimental evidence for these nanostructures, in spite of the progress made in theoretical investigations of their structure and bonding. Here we report the observation, by photoelectron spectroscopy, of an all-boron fullerene-like cage cluster at B40(-) with an extremely low electron-binding energy. Theoretical calculations show that this arises from a cage structure with a large energy gap, but that a quasi-planar isomer of B40(-) with two adjacent hexagonal holes is slightly more stable than the fullerene structure. In contrast, for neutral B40 the fullerene-like cage is calculated to be the most stable structure. The surface of the all-boron fullerene, bonded uniformly via delocalized σ and π bonds, is not perfectly smooth and exhibits unusual heptagonal faces, in contrast to C60 fullerene.

A concentric planar doubly π-aromatic B19− cluster

Atomic clusters often show unique, size-dependent properties and have become a fertile ground for the discovery of novel molecular structures and chemical bonding. Here we report an investigation of the B₁₉⁻ cluster, which shows chemical bonding reminiscent of that in [10]annulene (C₁₀H₁₀) and [6]circulene (C₂₄H₁₂). Photoelectron spectroscopy reveals a relatively simple spectrum for B₁₉⁻, with a high electron-binding energy. Theoretical calculations show that the global minimum of B₁₉⁻ is a nearly circular planar structure with a central B₆ pentagonal unit bonded to an outer B₁₃ ring. Chemical bonding analyses reveal that the B₁₉⁻ cluster possesses a unique double π-aromaticity in two concentric π-systems, with two π-electrons delocalized over the central pentagonal B₆ unit and another ten π-electrons responsible for the π-bonding between the central pentagonal unit and the outer ring. Such peculiar chemical bonding does not exist in organic compounds; it can only be found in atomic clusters.

Planar hexagonal B 36 as a potential basis for extended single-atom layer boron sheets

Boron is carbons neighbour in the periodic table and has similar valence orbitals. However, boron cannot form graphene-like structures with a honeycomb hexagonal framework because of its electron deficiency. Computational studies suggest that extended boron sheets with partially filled hexagonal holes are stable; however, there has been no experimental evidence for such atom-thin boron nanostructures. Here, we show experimentally and theoretically that B36 is a highly stable quasiplanar boron cluster with a central hexagonal hole, providing the first experimental evidence that single-atom layer boron sheets with hexagonal vacancies are potentially viable. Photoelectron spectroscopy of B36(-) reveals a relatively simple spectrum, suggesting a symmetric cluster. Global minimum searches for B36(-) lead to a quasiplanar structure with a central hexagonal hole. Neutral B36 is the smallest boron cluster to have sixfold symmetry and a perfect hexagonal vacancy, and it can be viewed as a potential basis for extended two-dimensional boron sheets.

Photoelectron spectroscopy of size‐selected transition metal clusters: Fe−n, n=3–24

A higher resolution magnetic bottle photoelectron spectrometer for the study of the electronic structure of size‐selected metal clusters is presented. The initial study on Fe−n (n=3–24) is reported at a photon energy of 3.49 eV. The photoelectron spectra of these clusters exhibit sharp features throughout the size range. The spectra for Fe−3–8 show large size dependence with many resolved features. The spectra for Fe−9–15 exhibit some similarity with each other, all with a rather sharp feature near the threshold. An abrupt spectral change occurs at Fe−16, then again at Fe−19 and Fe−23. These photoelectron spectral changes coincide remarkably with changes of the cluster reactivity with H2. Extended Huckel molecular orbital (EHMO) calculations are performed for all the clusters to aid the spectral interpretations. The calculations yield surprisingly good agreement with the experiment for clusters beyond Fe9 when body‐centered cubic (bcc) structures are assumed for Fe9–15 and a similarly close‐packed structu...

Threshold photodetachment of cold C−60

Abstract Threshold photodetachment of cold C − 60 has been investigated with a tunable dye laser from 2.55 to 2.80 eV. The cold C − 60 was produced through laser vaporization of a purified C 60 target with a specially designed nozzle. The threshold behavior of the detachment cross section as a function of energy was found to be well described by Wigners threshold law plus a dominating polarization term. The electron affinity of C 60 was measured to be 2.650±0.050 eV. Two unique effects, which are characteristic of bulk materials, were observed in this nano-sized molecule, i.e. the thermionic emission and the inelastic electron—“phonon” scattering effects, with the former strongly dependent on the laser fluences and the later on both the internal temperature of C − 60 and the photoelectron kinetic energy.

Structure of the NaxClx+1− (x=1–4) clusters via ab initio genetic algorithm and photoelectron spectroscopy

The application of the ab initio genetic algorithm with an embedded gradient has been carried out for the elucidation of global minimum structures of a series of anionic sodium chloride clusters, Na(x)Cl(x+1) (-) (x=1-4), produced in the gas phase using electrospray ionization and studied by photoelectron spectroscopy. These are all superhalogen species with extremely high electron binding energies. The vertical electron detachment energies for Na(x)Cl(x+1) (-) were measured to be 5.6, 6.46, 6.3, and 7.0 eV, for x=1-4, respectively. Our ab initio gradient embedded genetic algorithm program detected the linear global minima for NaCl(2) (-) and Na(2)Cl(3) (-) and three-dimensional structures for the larger species. Na(3)Cl(4) (-) was found to have C(3v) symmetry, which can be viewed as a Na(4)Cl(4) cube missing a corner Na(+) cation, whereas Na(4)Cl(5) (-) was found to have C(4v) symmetry, close to a 3x3 planar structure. Excellent agreement between the theoretically calculated and the experimental spectra was observed, confirming the obtained structures and demonstrating the power of the developed genetic algorithm technique.

First experimental photoelectron spectra of superhalogens and their theoretical interpretations

Photoelectron spectra of the MX2− (M=Li, Na; X=Cl, Br, I) superhalogen anions have been obtained for the first time. The first vertical detachment energies (VDEs) were measured to be 5.92±0.04 (LiCl2−), 5.86±0.06 (NaCl2−), 5.42±0.03 (LiBr2−), 5.36±0.06 (NaBr2−), 4.88±0.03 (LiI2−), and 4.84±0.06 eV (NaI2−), which are all well above the 3.61 eV electron detachment energy of Cl−, the highest among atomic anions. Experimental photoelectron spectra have been assigned on the basis of ab initio outer valence Green function (OVGF) calculations. The corresponding theoretical first VDEs were found to be 5.90 (LiCl2−), 5.81 (NaCl2−), 5.48 (LiBr2−), 5.43 (NaBr2−), 4.57 (LiI2−), and 4.50 eV (NaI2−), in excellent agreement with the experimental values. Photodetachment from the top four valence molecular orbitals (2σg22σu21πu41πg4) of MX2− was observed. Analysis of the polestrength showed that all electron detachment channels in this study can be described as primarily one-electron processes.

Development of a low-temperature photoelectron spectroscopy instrument using an electrospray ion source and a cryogenically controlled ion trap

The ability to control ion temperatures is critical for gas phase spectroscopy and has been a challenge in chemical physics. A low-temperature photoelectron spectroscopy instrument has been developed for the investigation of complex anions in the gas phase, including multiply charged anions, solvated species, and biological molecules. The new apparatus consists of an electrospray ionization source, a three dimensional (3D) Paul trap for ion accumulation and cooling, a time-of-flight mass spectrometer, and a magnetic-bottle photoelectron analyzer. A key feature of the new instrument is the capability to cool and tune ion temperatures from 10 to 350 K in the 3D Paul trap, which is attached to the cold head of a closed cycle helium refrigerator. Ion cooling is accomplished in the Paul trap via collisions with a background gas and has been demonstrated by observation of complete elimination of vibrational hot bands in photoelectron spectra of various anions ranging from small molecules to complex species. Further evidence of ion cooling is shown by the observation of H2-physisorbed anions at low temperatures. Cold anions result in better resolved photoelectron spectra due to the elimination of vibrational hot bands and yield more accurate energetic and spectroscopic information. Temperature-dependent studies are made possible for weakly bonded molecular and solvated clusters, allowing thermodynamic information to be obtained.

Fullerene triplet state production and decay: R2PI probes of C60 and C70 in a supersonic beam

Abstract Lifetimes of the lowest triplet state of the two most stable fullerenes, C 60 and C 70 , were measured in a supersonic beam by two-color resonant two-photon ionization. When prepared by intersystem crossing from the singlet manifold, excited at 4.03 eV, these triplet states were found to have lifetimes of 42 and 41 μs, respectively. The energies of these triplet states (1.7 and 1.6 eV, respectively) were measured by photoelectron spectroscopy of the corresponding negative ions.