Search query: J. Am. Chem. Soc. 2015, 136,  8859.

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abstract: Motivated by recent experimental synthesis of a semiconducting metal organic graphene analogue (J. Am. Chem. Soc. 2014, 136, 8859) i.e. Ni3(2,3,6,7,10,11 hexaiminotriphenylene)2 [Ni3(HITP)2] a new Kagome lattice, Cu3(HITP)2, is designed by substituting the coordination of Ni by Cu. Such substitution results in interesting changes in electronic properties of the M3(HITP)2 bulk and two dimensional (2D) sheets. In Ni3(HITP)2, each Ni atom adopts the dsp(2) hybridization, forming a perfect 2D conjugation, whereas in Cu3(HITP)2, each Cu atom adopts the sp(3) hybridization, resulting in a distorted 2D sheet. The M3(HITP)2 bulks, assembled from M3(HITP)2 sheets via both strong p p interaction and weak metal metal interaction, are metallic. However, the 2D Ni3(HITP)2 sheet is a semiconductor with a narrow band gap whereas the 2D Cu3(HITP)2 sheet is a metal. Remarkably, both the 2D M3(HITP)2 Kagome lattices possess Dirac bands in the vicinity of the Fermi level. Additional ab initio molecular dynamics simulations show that both sheets exhibit high thermal stability at elevated temperatures. Our theoretical study offers new insights into tunability of electronic properties for the 2D metal organic frameworks (MOFs)
author_names: Shuang Chen

Jun Dai

Xiao Cheng Zeng
title: Metal organic Kagome lattices M3(2,3,6,7,10,11 hexaiminotriphenylene)2 (M Ni and Cu) from semiconducting to metallic by metal substitution.
venue: Physical chemistry chemical physics PCCP
year: 2015

abstract: Relative stability of geometric magic number gold nanoclusters with high point group symmetry (Ih) D(5h) O(h) and size up to 3.5 nm, as well as structures obtained by global optimization using an empirical potential, is investigated using density functional theory (DFT) calculations. Among high symmetry nanoclusters, our calculations suggest that from Au(147) to Au(923) the stability follows the order Ih D(5h) Oh. However, at the largest size of Au(923) the computed cohesive energy differences among high symmetry I(h) D(5h) and O(h) isomers are less than 4 meV/atom (at PBE level of theory) suggesting the larger high symmetry clusters are similar in stability. This conclusion supports a recent experimental demonstration of controlling morphologies of high symmetry Au(923) clusters Plant, S. R. Cao, L. Palmer, R. E. J. Am. Chem. Soc. 2014, 136, 7559) Moreover, at and beyond the size of Au(549) the face centered cubic (FCC) based structure appears to be slightly more stable than the Ih structure with comparable size, consistent with experimental observations. Also, for the Au clusters with the size below or near Au(561) reconstructed icosahedral and decahedral clusters with lower symmetry are slightly more stable than the corresponding high symmetry isomers. Catalytic activities of both high symmetry and reconstructed I(h) Au(147) and both Ih Au(309) clusters are examined. CO adsorption on Au(309) exhibits less sensitivity on the edge and vertex sites compared to Au(147) whereas the CO/O2 coadsorption is still energetically favorable on both gold nanoclusters. Computed activation barriers for CO oxidation are typically around 0.2 eV, suggesting that the gold nanoclusters of 2 nm in size are highly effective catalysts for CO oxidation.
author_names: Hui Li

Lei Li

Andreas Pedersen

Yi Gao

Navneet Singh Khetrapal

Hannes Jonsson

Xiao Cheng Zeng
title: Magic number gold nanoclusters with diameters from 1 to 3.5 nm: relative stability and catalytic activity for CO oxidation.
venue: Nano letters
year: 2015

abstract: NMR signal amplification by reversible exchange (SABRE) is a NMR hyperpolarization technique that enables nuclear spin polarization enhancement of molecules via concurrent chemical exchange of a target substrate and parahydrogen (the source of spin order) on an iridium catalyst. Recently, we demonstrated that conducting SABRE in microtesla fields provided by a magnetic shield enables up to 10% 15N polarization (Theis, T. et al. J. Am. Chem. Soc.2015, 137, 1404) Hyperpolarization on 15N (and heteronuclei in general) may be advantageous because of the long lived nature of the hyperpolarization on 15N relative to the short lived hyperpolarization of protons conventionally hyperpolarized by SABRE, in addition to wider chemical shift dispersion and absence of background signal. Here we show that these unprecedented polarization levels enable 15N magnetic resonance imaging. We also present a theoretical model for the hyperpolarization transfer to heteronuclei, and detail key parameters that should be optimized for efficient 15N hyperpolarization. The effects of parahydrogen pressure, flow rate, sample temperature, catalyst to substrate ratio, relaxation time (T1) and reversible oxygen quenching are studied on a test system of 15N pyridine in methanol d4. Moreover, we demonstrate the first proof of principle 13C hyperpolarization using this method. This simple hyperpolarization scheme only requires access to parahydrogen and a magnetic shield, and it provides large enough signal gains to enable one of the first 15N images (2 x 2 mm2 resolution) Importantly, this method enables hyperpolarization of molecular sites with NMR T1 relaxation times suitable for biomedical imaging and spectroscopy.
author_names: Milton L Truong

Thomas Theis

Aaron M Coffey

Roman V Shchepin

Kevin W Waddell

Fan Shi

Boyd M Goodson

Warren S Warren

Eduard Y Chekmenev
title: 15N Hyperpolarization by Reversible Exchange Using SABRE SHEATH
venue: The journal of physical chemistry. C, Nanomaterials and interfaces
year: 2015

abstract: Anisotropic gold nanostructures prepared by the seed method in the presence of Ag ions have been used to study their photostability to low power UV irradiation (254 nm) at room temperature. It has been observed that, whereas spheres are very stable to photoirradiation, rods and prisms suffer from photocorrosion and finally dissolve completely with the production of Au(III) ions. Interpretation of these differences is based on the presence of semiconductor like Ag clusters, adsorbed onto rods and prisms, able to photocorrode the Au nanoparticles, which are absent in the case of Au spheres. We further show direct evidence of the presence of Ag clusters in Au nanorods by XANES. These results confirm a previous hypothesis (J. Am. Chem. Soc. 2014, 136, 1182 1185) about the major influence of very stable small Ag clusters, not only on the anisotropic formation of nanostructures but also on their photostability.
author_names: Yasser Attia Attia

David Buceta

Felix G Requejo

Lisandro J Giovanetti

Manuel Arturo Lopez-Quintela
title: Photostability of gold nanoparticles with different shapes: the role of Ag clusters.
venue: Nanoscale
year: 2015

abstract: In a very recent article, atomically thin two dimensional MoS2xSe2(1 x) nanosheets have been synthesized with complete composition tunability using a temperature gradient assisted chemical vapor deposition technique [J. Am. Chem. Soc. 2014, 136, 3756] To have a better understanding of the composition dependent tunability of the properties of this class of materials we here perform first principles calculations on the detailed electronic structure of single layered transition metal dichalcogenides MoS2(1 x)Se2x and MoS2(1 x)Te2x. The positive value of mixing energy of both MoS2(1 x)Se2x and MoS2(1 x)Te2x 2D sheets at various composition confirms their formation at some energy cost. The analysis of the composition dependent band structure and the density of states of these 2D sheets reveals certain interesting features. The band gap variation of the MoS2(1 x)Se2x nanosheets is almost linear with composition while that of MoS2(1 x)Te2x deviates slightly from linearity. We have also calculated the optical absorption spectrum of these nanosheets as a function of composition and found that the optical transitions are mainly metal d d type spin forbidden transitions.
author_names: Biplab Rajbanshi

Sunandan Sarkar

Pranab Kumar Sarkar
title: The electronic and optical properties of MoS(2(1 x))Se(2x) and MoS(2(1 x))Te(2x) monolayers.
venue: Physical chemistry chemical physics PCCP
year: 2015

abstract: AbstractProline favors trans configured peptide bonds in native proteins. Although cis/trans configurations vary for non native and unstructured states, solvent also influences these preferences. Water induces the all cis right handed polyproline I (PPI) helix of polyproline to fold into the all trans left handed polyproline II (PPII) helix. Our recent work has shown that this occurs via a sequential mechanism involving six resolved intermediates [Shi, L. Holliday, A.E. Shi, H. Zhu, F. Ewing, M.A. Russell, D.H. Clemmer, D.E. Characterizing intermediates along the transition from PPI to PPII using ion mobility mass spectrometry. J. Am. Chem. Soc. 136, 12702 12711 (2014) Here, we use ion mobility mass spectrometry to make the first detailed thermodynamic measurements of the folding intermediates, which inform us about how and why this transition occurs. It appears that early intermediates are energetically favorable because of the hydration of the peptide backbone, whereas late intermediates are enthalpically unfavorable. However, folding continues, as the entropy of the system increases upon successive formation of each new structure. When PPII is immersed in 1 propanol, the PPII PPI transition occurs, but this reaction occurs through a very different mechanism. Early on, the PPII population splits onto multiple pathways that eventually converge through a late intermediate that continues on to the folded PPI helix. Nearly every step is endothermic. Folding results from a stepwise increase in the disorder of the system, allowing a wide scale search for a critical late intermediate. Overall, the data presented here allow us to establish the first experimentally determined energy surface for biopolymer folding as a function of solution environment. Graphical Abstract
author_names: Liuqing Shi

Alison E Holliday

Matthew S Glover

Michael A Ewing

David H Russell

David E Clemmer
title: Ion Mobility Mass Spectrometry Reveals the Energetics of Intermediates that Guide Polyproline Folding
venue: Journal of The American Society for Mass Spectrometry
year: 2015

abstract: Building on the pioneering work of Ho and DeGrado (J Am Chem Soc 1987, 109, 6751 6758) in the late 1980s, protein design approaches have revealed many fundamental features of protein structure and stability. We are now in the era that the early work presaged the design of new proteins with practical applications and uses. Here we briefly survey some past milestones in protein design, in addition to highlighting recent progress and future aspirations. (c) 2015 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 104: 334 350, 2015.
author_names: Lynne Regan

Diego Caballero

Michael R Hinrichsen

Alejandro Virrueta

Danielle M Williams

Corey S O'Hern
title: Protein design: Past, present, and future
venue: Biopolymers
year: 2015

abstract: Organic ionic plastic crystals (OIPCs) show strong potential as solid state electrolytes for lithium battery applications, demonstrating promising electrochemical performance and eliminating the need for a volatile and flammable liquid electrolyte. The ionic conductivity (s) in these systems has recently been shown to depend strongly on polycrystalline morphology, which is largely determined by the sample's thermal history. [K. Romanenko et al. J. Am. Chem. Soc. 2014, 136, 15638] Tailoring this morphology could lead to conductivities sufficiently high for battery applications, so a more complete understanding of how phenomena such as solid solid phase transitions can affect the sample morphology is of significant interest. Anisotropic relaxation of nuclear spin magnetisation provides a new MRI based approach for studies of polycrystalline materials at both a macroscopic and molecular level. In this contribution, morphology alterations induced by solid solid phase transitions in triisobutyl(methyl)phosphonium bis(fluorosulfonyl)imide (P1444FSI) and diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate (P1224PF6) are examined using magnetic resonance imaging (MRI) alongside nuclear magnetic resonance (NMR) spectroscopy, diffusion measurements and conductivity data. These observations are linked to molecular dynamics and structural behaviour crucial for the conductive properties of OIPCs. A distinct correlation is established between the conductivity at a given temperature, s(T) and the intensity of the narrow NMR signal that is attributed to a mobile fraction, fm(T) of ions in the OIPC. To explain these findings we propose an analogy with the well studied relationship between permeability (k) and void fraction (th) in porous media, with k(th) commonly quantified by a power law dependence that can also be employed to describe s(fm)
author_names: Konstantin Romanenko

Jennifer M Pringle

Luke A O'Dell

Maria Forsyth
title: New insights into the thermal behaviour of organic ionic plastic crystals: magnetic resonance imaging of polycrystalline morphology alterations induced by solid solid phase transitions.
venue: Physical chemistry chemical physics PCCP
year: 2015

abstract: W(CNAryl)6 complexes containing 2,6 diisopropylphenyl isocyanide (CNdipp) are powerful photoreductants with strongly emissive long lived excited states. These properties are enhanced upon appending another aryl ring, e.g. W(CNdippPh(OMe2))6; CNdippPh(OMe2) 4 (3,5 dimethoxyphenyl) 2,6 diisopropylphenylisocyanide (Sattler et al. J. Am. Chem. Soc. 2015, 137, 1198 1205) Electronic transitions and low lying excited states of these complexes were investigated by time dependent density functional theory (TDDFT) the lowest triplet state was characterized by time resolved infrared spectroscopy (TRIR) supported by density functional theory (DFT) The intense absorption band of W(CNdipp)6 at 460 nm and that of W(CNdippPh(OMe2))6 at 500 nm originate from transitions of mixed pp*(CN C)/MLCT(W Aryl) character, whereby W is depopulated by ca. 0.4 e( and the electron density changes are predominantly localized along two equatorial molecular axes. The red shift and intensity rise on going from W(CNdipp)6 to W(CNdippPh(OMe2))6 are attributable to more extensive delocalization of the MLCT component. The complexes also exhibit absorptions in the 300 320 nm region, owing to W CN MLCT transitions. Electronic absorptions in the spectrum of W(CNXy)6 (Xy 2,6 dimethylphenyl) a complex with orthogonal aryl orientation, have similar characteristics, although shifted to higher energies. The relaxed lowest W(CNAryl)6 triplet state combines pp* excitation of a trans pair of CN C moieties with MLCT (0.21 e( and ligand to ligand charge transfer (LLCT, 0.24 0.27 e( from the other four CNAryl ligands to the axial aryl and, less, to CN groups; the spin density is localized along a single Aryl NC W CN Aryl axis. Delocalization of excited electron density on outer aryl rings in W(CNdippPh(OMe2))6 likely promotes photoinduced electron transfer reactions to acceptor molecules. TRIR spectra show an intense broad bleach due to n(CN) a prominent transient upshifted by 60 65 cm( 1) and a weak down shifted feature due to antisymmetric CN stretch along the axis of high spin density. The TRIR spectral pattern remains unchanged on the femtosecond nanosecond time scale, indicating that intersystem crossing and electron density localization are ultrafast <100 fs)
author_names: Hana Kvapilova

Wesley Sattler

Aaron Sattler

Igor V Sazanovich

Ian P Clark

Michael Towrie

Harry B Gray

Stanislav Zalis

Antonin Vlcek
title: Electronic Excited States of Tungsten(0) Arylisocyanides.
venue: Inorganic chemistry
year: 2015

abstract: The nitrosylation of inorganic protein cofactors, specifically that of [Fe S] clusters to form iron nitrosyls, plays a number of important roles in biological systems. In some of these cases, it is expected that a repair process reverts the nitrosylated iron species to intact [Fe S] clusters. The repair of nitrosylated [2Fe 2S] cluster, primarily in the form of protein bound dinitrosyl iron complexes (DNICs) has been observed in vitro and in vivo, but the mechanism of this process remains uncertain. The present work expands upon a previous observation (Fitzpatrick et al. J. Am. Chem. Soc. 2014, 136, 7229) of the ability of mononitrosyl iron complexes (MNICs) to be converted into [2Fe 2S] clusters by the addition of nothing other than a cysteine analogue. Herein, each of the critical elementary steps in the cluster repair has been dissected to elucidate the roles of the cysteine analogue. Systematic variations of a cysteine analogue employed in the repair reaction suggest that (i) the bidentate coordination of a cysteine analogue to MNIC promotes NO release from iron, and (ii) deprotonation of the a carbon of the ferric bound cysteine analogue leads to the C S cleavage en route to the formation of [2Fe 2S] cluster. The [2Fe 2S] cluster bearing a cysteine analogue has also been synthesized from thiolate bridged iron dimers of the form [Fe2(m SR)2(SR)4](0/2 which implies that such species may be present as intermediates in the cluster repair. In addition to MNICs, mononuclear tetrathiolate ferric or ferrous species have been established as another form of iron from which [2Fe 2S] clusters can be generated without need for any other reagent but a cysteine analogue. The results of these experiments bring to light new chemistry of classic coordination complexes and provides further insight into the repair of NO modified [2Fe 2S] clusters.
author_names: J H Jun Fitzpatrick

Eunsuk Kim
title: New Synthetic Routes to Iron Sulfur Clusters: Deciphering the Repair Chemistry of [2Fe 2S] Clusters from Mononitrosyl Iron Complexes.
venue: Inorganic chemistry
year: 2015