Observation of earlier two-to-three dimensional structural transition in gold cluster anions by isoelectronic substitution: Mau\u3csub\u3e\u3ci\u3en\u3c/i\u3e\u3csup\u3e- \u3c/sup\u3e\u3c/sub\u3e (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 (MAun −, n=8–11; M=Ag,Cu) using photoelectron spectroscopy and density functional calculations. Photoelectron spectra of MAun − are found to be similar to those of the bare gold clusters Aun+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 MAu10 − from Au11 −, suggesting a major structural change in the doped clusters. Our calculations confirm that MAu8 − − possesses the same structure as Au9 − with Ag or Cu simply replacing one Au atom in its C2v 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 Au10 − we identify three coexisting low-lying planar isomers along with the D3h global minimum. The coexistence of so many low-lying isomers for the small-sized gold cluster Au10 − is quite unprecedented. Similar planar structures and isomeric forms are observed for the doped MAu9 − clusters. Although the global minimum of Au11 − is planar, our calculations suggest that only simulated spectra of 3D structures agree with the observed spectra for MAu10 −. For MAu11 −, only a 3D isomer is observed, in contrast to Au12 − 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

Observation of earlier two-to-three dimensional structural transition in gold cluster anions by isoelectronic substitution: Mau\u3csub\u3e\u3ci\u3en\u3c/i\u3e\u3csup\u3e- \u3c/sup\u3e\u3c/sub\u3e (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 (MAun −, n=8–11; M=Ag,Cu) using photoelectron spectroscopy and density functional calculations. Photoelectron spectra of MAun − are found to be similar to those of the bare gold clusters Aun+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 MAu10 − from Au11 −, suggesting a major structural change in the doped clusters. Our calculations confirm that MAu8 − − possesses the same structure as Au9 − with Ag or Cu simply replacing one Au atom in its C2v 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 Au10 − we identify three coexisting low-lying planar isomers along with the D3h global minimum. The coexistence of so many low-lying isomers for the small-sized gold cluster Au10 − is quite unprecedented. Similar planar structures and isomeric forms are observed for the doped MAu9 − clusters. Although the global minimum of Au11 − is planar, our calculations suggest that only simulated spectra of 3D structures agree with the observed spectra for MAu10 −. For MAu11 −, only a 3D isomer is observed, in contrast to Au12 − 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

Eigenvector Centrality Distribution for Characterization of Protein Allosteric Pathways

Determining the principal energy pathways for allosteric communication in biomolecules, that occur as a result of thermal motion, remains challenging due to the intrinsic complexity of the systems involved. Graph theory provides an approach for making sense of such complexity, where allosteric proteins can be represented as networks of amino acids. In this work, we establish the eigenvector centrality metric in terms of the mutual information, as a mean of elucidating the allosteric mechanism that regulates the enzymatic activity of proteins. Moreover, we propose a strategy to characterize the range of the physical interactions that underlie the allosteric process. In particular, the well known enzyme, imidazol glycerol phosphate synthase (IGPS), is utilized to test the proposed methodology. The eigenvector centrality measurement successfully describes the allosteric pathways of IGPS, and allows to pinpoint key amino acids in terms of their relevance in the momentum transfer process. The resulting insight can be utilized for refining the control of IGPS activity, widening the scope for its engineering. Furthermore, we propose a new centrality metric quantifying the relevance of the surroundings of each residue. In addition, the proposed technique is validated against experimental solution NMR measurements yielding fully consistent results. Overall, the methodologies proposed in the present work constitute a powerful and cost effective strategy to gain insight on the allosteric mechanism of proteins

Probing the structures of gold–aluminum alloy clusters Au\u3csub\u3e\u3ci\u3ex\u3c/i\u3e\u3c/sub\u3eAl\u3csub\u3e\u3ci\u3ey\u3c/i\u3e\u3c/sub\u3e\u3csup\u3e−\u3c/sup\u3e: A joint experimental and theoretical study

Besides the size and structure, compositions can also dramatically affect the properties of alloy nanoclusters. Due to the added degrees of freedom, determination of the global minimum structures for multi-component nanoclusters poses even greater challenges, both experimentally and theoretically. Here we report a systematic and joint experimental/theoretical study of a series of gold–aluminum alloy clusters, AuxAly−(x + y = 7,8), with various compositions (x = 1–3; y = 4–7). Well-resolved photoelectron spectra have been obtained for these clusters at different photon energies. Basin-hopping global searches, coupled with density functional theory calculations, are used to identify low-lying structures of the bimetallic clusters. By comparing computed electronic densities of states of the low-lying isomers with the experimental photoelectron spectra, the global minima are determined. It is found that for y ≥ 6 there is a strong tendency to form the magic-number square bi-pyramid motif of Al6−in the AuxAly−clusters, suggesting that the Al–Al interaction dominates the Au–Au interaction in the mixed clusters. A closely related trend is that for x \u3e 1, the gold atoms tend to be separated by Al atoms unless only the magic-number Al6−square bi-pyramid motif is present, suggesting that in the small-sized mixed clusters, Al and Au components do not completely mix with one another. Overall, the Al component appears to play a more dominant role due to the high robustness of the magic-number Al6−square bi-pyramid motif, whereas the Au component tends to be either “adsorbed” onto the Al6−square bi-pyramid motif if y ≥ 6, or stays away from one another if x \u3c y \u3c 6. Includes supplemental files

ƴ-Selective directed catalytic asymmetric hydroboration of 1,1-disubstituted alkenes

Directed catalytic asymmetric hydroborations of 1,1-disubstituted alkenes afford ƴ -dioxaborato amides and esters in high enantiomeric purity (90–95% ee)

Structure determination and small molecule binding studies of novel gold nanoparticles

Determining geometric and electronic structures of gold nanoparticles are of fundamental interest in order to understand their chemistry and for potential applications. In the bulk form gold is well known for its chemical inertness, which makes it extremely valuable for applications in electronics, dentistry, jewelry and art. However, recently it has been documented that at nano-scale, gold particles containing only a few atoms can exhibit catalytic behavior towards various kinds of substrates. These properties are highly size dependent and therefore understanding molecular level geometries of these particles are of pivotal importance. The main focus of this dissertation is therefore going to be towards the elucidation of isomeric structures of novel gold clusters using a combined experimental and theoretical approach and subsequent modeling to study how gold clusters binds with small molecules like Carbon monoxide and Oxygen. Many existing methods to study clusters involve a delicate combination of theory and experimental approaches. Our method of combining photoelectron spectroscopy and density functional theory has been extremely successful in predicting and analyzing the geometries of pure and doped novel gold clusters which was hitherto unknown. This method has also enabled us to unravel new geometries of gold nano-clusters when they bind to small molecules and therefore help predict catalytic activities and illustrate reaction pathways

Tuning the electronic properties of the golden buckyball by endohedral doping: \u3ci\u3eM\u3c/i\u3e@Au\u3csub\u3e16\u3c/sub\u3e\u3csup\u3e−\u3c/sup\u3e (\u3ci\u3eM\u3c/i\u3e=Ag,Zn, In)

The golden Au16- 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, MAu16- (M =Ag,Zn, In), are investigated by photoelectron spectroscopy and theoretical calculations. It is observed that the characteristic spectral features of 16-, reflecting its near tetrahedral (Td) symmetry, are retained in the photoelectron spectra of MAu16-, suggesting endohedral structures with little distortion from the parent Au16- cage for the doped clusters. Density functional calculations show that the endohedral structures of MAu16- with Td 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 Au16-, but simply donate its valence electrons to the parent Au16- cage, resulting in a closed-shell 18-electron system for Ag@ Au16-, a 19-electron system for Zn@ Au16- with a large energy gap, and a 20-electron system for In@ Au16-. The current work shows that the electronic properties of the golden buckyball can be systematically tuned through doping

Isomer identification and resolution in small gold clusters

A variety of experimental techniques are used to resolve energetically close isomers of Au7 − and Au8 − 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, O2-titration, and isoelectronic atom substitution by Cu and Ag. A new isomer structure is found for Au7 −, which consists of a triangular Au6 unit with a dangling Au atom. Isomer-specific photoelectron spectra of Au8 − are obtained from O2-titration experiment. The global minimum and low-lying structures of Au7 −, Au8 −, and MAun − (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, O2-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

B27\u3csup\u3e−\u3c/sup\u3e: 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