Rhitankar Pal

Unraveling the Mechanisms of O2 Activation by Size-Selected Gold Clusters: Transition from Superoxo to Peroxo Chemisorption

The activation of dioxygen is a key step in CO oxidation catalyzed by gold nanoparticles. It is known that small gold cluster anions with even-numbered atoms can molecularly chemisorb O(2) via one-electron transfer from Au(n)(-) to O(2), whereas clusters with odd-numbered atoms are inert toward O(2). Here we report spectroscopic evidence of two modes of O(2) activation by the small even-sized Au(n)(-) clusters: superoxo and peroxo chemisorption. Photoelectron spectroscopy of O(2)Au(8)(-) revealed two distinct isomers, which can be converted from one to the other depending on the reaction time. Ab initio calculations show that there are two close-lying molecular O(2)-chemisorbed isomers for O(2)Au(8)(-): the lower energy isomer involves a peroxo-type binding of O(2) onto Au(8)(-), while the superoxo chemisorption is a slightly higher energy isomer. The computed detachment transitions of the superoxo and peroxo species are in good agreement with the experimental observation. The current work shows that there is a superoxo to peroxo chemisorption transition of O(2) on gold clusters at Au(8)(-): O(2)Au(n)(-) (n = 2, 4, 6) involves superoxo binding and n = 10, 12, 14, 18 involves peroxo binding, whereas the superoxo binding re-emerges at n = 20 due to the high symmetry tetrahedral structure of Au(20), which has a very low electron affinity. Hence, the two-dimensional (2D) Au(8)(-) is the smallest anionic gold nanoparticle that prefers peroxo binding with O(2). At Au(12)(-), although both 2D and 3D isomers coexist in the cluster beam, the 3D isomer prefers the peroxo binding with O(2).

Structural evolution of doped gold clusters: MAux- (M = Si, Ge, Sn; x = 5-8)

We report a joint experimental and theoretical study on the structures of a series of gold clusters doped with a group-14 atom: MAu(x)(-) (M = Si, Ge, Sn; x = 5-8). Well-resolved photoelectron spectra were obtained and compared to calculations at several levels of theory to identify the low-lying structures of MAu(5-8)(-). We found that the structure of SiAu(5)(-) is dominated by the tetrahedrally coordinated Si motif, which can be viewed as built from the tetrahedral SiAu(4)(-) by an extra Au atom bonded to a terminal gold atom. However, SiAu(6)(-) and SiAu(7)(-) have quasi-planar structures, similar to those of GeAu(6)(-)/SnAu(6)(-) and GeAu(7)(-)/SnAu(7)(-), respectively. SiAu(8)(-) again has a tetrahedrally coordinated Si structure, which displays a structural motif of a dangling Au-Si unit sitting on a gold cluster surface, resembling that of the larger Si-doped gold cluster SiAu(16)(-). For M = Ge, Sn, our results show that the major isomers of GeAu(5-8)(-) have structures similar to those of the corresponding SnAu(5-8)(-) clusters, and they can be viewed as grown from the previously suggested square-pyramidal GeAu(4)(-) and SnAu(4)(-), respectively. Population of minor isomers was observed for SnAu(5)(-), GeAu(6)(-), SnAu(6)(-), and GeAu(8)(-). The 3D to quasi-2D to 3D structural evolution for SiAu(5)(-) to SiAu(8)(-) and the structural convergence for MAu(x)(-) (M = Si, Ge, Sn) at x = 6, 7 manifest competitions between the tendency of forming molecule-like structures around the group-14 dopant (optimizing M-Au interactions) and the strong tendency of forming planar structures for small gold anion clusters (optimizing Au-Au interactions).

Observation of earlier two-to-three dimensional structural transition in gold cluster anions by isoelectronic substitution: Mau n - (n=8–11; M=Ag,Cu)

The effects of isoelectronic substitution on the electronic and structural properties of gold clusters are investigated in the critical size range of the two-dimensional (2D)-three-dimensional (3D) structural transition (MAu(n)(-), n=8-11; M=Ag,Cu) using photoelectron spectroscopy and density functional calculations. Photoelectron spectra of MAu(n)(-) are found to be similar to those of the bare gold clusters Au(n+1)(-), indicating that substitution of a Au atom by a Ag or Cu atom does not significantly alter the geometric and electronic structures of the clusters. The only exception occurs at n=10, where very different spectra are observed for MAu(10)(-) from Au(11)(-), suggesting a major structural change in the doped clusters. Our calculations confirm that MAu(8)(-) possesses the same structure as Au(9)(-) with Ag or Cu simply replacing one Au atom in its C(2v) planar global minimum structure. Two close-lying substitution isomers are observed, one involves the replacement of a center Au atom and another one involves an edge site. For Au(10)(-) we identify three coexisting low-lying planar isomers along with the D(3h) global minimum. The coexistence of so many low-lying isomers for the small-sized gold cluster Au(10)(-) is quite unprecedented. Similar planar structures and isomeric forms are observed for the doped MAu(9)(-) clusters. Although the global minimum of Au(11)(-) is planar, our calculations suggest that only simulated spectra of 3D structures agree with the observed spectra for MAu(10)(-). For MAu(11)(-), only a 3D isomer is observed, in contrast to Au(12)(-) which is the critical size for the 2D-3D structural transition with both the 2D and 3D isomers coexisting. The current work shows that structural perturbations due to even isoelectronic substitution of a single Au atom shift the 2D to 3D structural transition of gold clusters to a smaller size.

Isomer identification and resolution in small gold clusters

A variety of experimental techniques are used to resolve energetically close isomers of Au(7)(-) and Au(8)(-) by combining photoelectron spectroscopy and ab initio calculations. Two structurally distinct isomers are confirmed to exist in the cluster beam for both clusters. Populations of the different isomers in the cluster beam are tuned using Ar-tagging, O(2)-titration, and isoelectronic atom substitution by Cu and Ag. A new isomer structure is found for Au(7)(-), which consists of a triangular Au(6) unit with a dangling Au atom. Isomer-specific photoelectron spectra of Au(8)(-) are obtained from O(2)-titration experiment. The global minimum and low-lying structures of Au(7)(-), Au(8)(-), and MAu(n)(-) (n=6,7; M=Ag,Cu) are obtained through basin-hopping global minimum searches. The results demonstrate that the combination of well-designed photoelectron spectroscopy experiments (including Ar-tagging, O(2)-titration, and isoelectronic substitution) and ab initio calculation is not only powerful for obtaining the electronic and atomic structures of size-selected clusters, but also valuable in resolving structurally and energetically close isomers of nanoclusters.

S0-State Model of the Oxygen-Evolving Complex of Photosystem II

The S0 → S1 transition of the oxygen-evolving complex (OEC) of photosystem II is one of the least understood steps in the Kok cycle of water splitting. We introduce a quantum mechanics/molecular mechanics (QM/MM) model of the S0 state that is consistent with extended X-ray absorption fine structure spectroscopy and X-ray diffraction data. In conjunction with the QM/MM model of the S1 state, we address the proton-coupled electron-transfer (PCET) process that occurs during the S0 → S1 transition, where oxidation of a Mn center and deprotonation of a μ-oxo bridge lead to a significant rearrangement in the OEC. A hydrogen bonding network, linking the D1-D61 residue to a Mn-bound water molecule, is proposed to facilitate the PCET mechanism.

Tuning the electronic properties of the golden buckyball by endohedral doping: M@Au16−(M=Ag,Zn,In)

The golden Au(16)(-) cage is doped systematically with an external atom of different valence electrons: Ag, Zn, and In. The electronic and structural properties of the doped clusters, MAu(16)(-) (M = Ag,Zn,In), are investigated by photoelectron spectroscopy and theoretical calculations. It is observed that the characteristic spectral features of Au(16)(-), reflecting its near tetrahedral (T(d)) symmetry, are retained in the photoelectron spectra of MAu(16)(-), suggesting endohedral structures with little distortion from the parent Au(16)(-) cage for the doped clusters. Density functional calculations show that the endohedral structures of M@Au(16)(-) with T(d) symmetry are low-lying structures, which give simulated photoelectron spectra in good agreement with the experiment. It is found that the dopant atom does not significantly perturb the electronic and atomic structures of Au(16)(-), but simply donate its valence electrons to the parent Au(16)(-) cage, resulting in a closed-shell 18-electron system for Ag@Au(16)(-), a 19-electron system for Zn@Au(16)(-) with a large energy gap, and a 20-electron system for In@Au(16)(-). The current work shows that the electronic properties of the golden buckyball can be systematically tuned through doping.

A photoelectron spectroscopy and ab initio study of the structures and chemical bonding of the B25− cluster

Photoelectron spectroscopy and ab initio calculations are used to investigate the structures and chemical bonding of the B25(-) cluster. Global minimum searches reveal a dense potential energy landscape with 13 quasi-planar structures within 10 kcal/mol at the CCSD(T)/6-311+G(d) level of theory. Three quasi-planar isomers (I, II, and III) are lowest in energy and nearly degenerate at the CCSD(T) level of theory, with II and III being 0.8 and 0.9 kcal/mol higher, respectively, whereas at two density functional levels of theory isomer III is the lowest in energy (8.4 kcal/mol more stable than I at PBE0/6-311+G(2df) level). Comparison with experimental photoelectron spectroscopic data shows isomer II to be the major contributor while isomers I and III cannot be ruled out as minor contributors to the observed spectrum. Theoretical analyses reveal similar chemical bonding in I and II, both involving peripheral 2c-2e B-B σ-bonding and delocalized interior σ- and π-bonding. Isomer III has an interesting elongated ribbon-like structure with a π-bonding pattern analogous to those of dibenzopentalene. The high density of low-lying isomers indicates the complexity of the medium-sized boron clusters; the method dependency of predicting relative energies of the low-lying structures for B25(-) suggests the importance of comparison with experiment in determining the global minima of boron clusters at this size range. The appearance of many low-lying quasi-planar structures containing a hexagonal hole in B25(-) suggests the importance of this structural feature in maintaining planarity of larger boron clusters.

Structure evolution of gold cluster anions between the planar and cage structures by isoelectronic substitution: Aun− (n = 13–15) and MAun− (n = 12–14; M = Ag, Cu)

The structural and electronic effects of isoelectronic substitution by Ag and Cu atoms on gold cluster anions in the size range between 13 and 15 atoms are studied using a combination of photoelectron spectroscopy and first-principles density functional calculations. The most stable structures of the doped clusters are compared with those of the undoped Au clusters in the same size range. The joint experimental and theoretical study reveals a new C(3v) symmetric isomer for Au(13)(-), which is present in the experiment, but has hitherto not been recognized. The global minima of Au(14)(-) and Au(15)(-) are resolved on the basis of comparison between experiment and newly computed photoelectron spectra that include spin-orbit effects. The coexistence of two isomers for Au(15)(-) is firmly established with convincing experimental evidence and theoretical calculations. The overall effect of the isoelectronic substitution is minor on the structures relative to those of the undoped clusters, except that the dopant atoms tend to lower the symmetries of the doped clusters.

Probing the electronic and structural properties of doped aluminum clusters: MAl12- (M=Li, Cu, and Au).

Photoelectron spectroscopy (PES) is combined with theoretical calculations to investigate the electronic and atomic structures of three doped aluminum clusters, MAl12- (M=Li, Cu, and Au). Well-resolved PES spectra have been obtained at two detachment photon energies, 266 nm (4.661 eV) and 193 nm (6.424 eV). Basin-hopping global optimization method in combination with density-functional theory calculations has been used for the structural searches. Good agreement between the measured PES spectra and theoretical simulations helps to identify the global minimum structures. It is found that LiAl12- (C(5nu)) can be viewed as replacing a surface Al atom by Li on an icosahedral Al13-, whereas Cu prefers the central site to form the encapsulated D3d-Cu@Al12-. For AuAl12- (C1), Au also prefers the central site, but severely distorts the Al12 cage due to its large size.

B27−: Appearance of the smallest planar boron cluster containing a hexagonal vacancy

Photoelectron spectroscopy and ab initio calculations have been carried out to probe the structures and chemical bonding of the B27 (-) cluster. Comparison between the experimental spectrum and the theoretical results reveals a two-dimensional (2D) global minimum with a triangular lattice containing a tetragonal defect (I) and two low-lying 2D isomers (II and III), each with a hexagonal vacancy. All three 2D isomers have 16 peripheral boron atoms and 11 inner boron atoms. Isomer I is shown to be mainly responsible for the observed photoelectron spectrum with isomers II and III as minor contributors. Chemical bonding analyses of these three isomers show that they all feature 16 localized peripheral B-B σ-bonds. Additionally, isomer I possesses 16 delocalized σ bonds and nine delocalized π bonds, while isomers II and III each contain 17 delocalized σ bonds and eight delocalized π bonds. It is found that the hexagonal vacancy is associated generally with an increase of delocalized σ bonds at the expense of delocalized π bonds in 2D boron clusters. The hexagonal vacancy, characteristic of borophenes, is found to be a general structural feature for mid-sized boron clusters. The current study shows that B27 (-) is the first boron cluster, where a hexagonal vacancy appears among the low-lying isomers accessible experimentally.