Anirban Bhattacharjee

Copper molybdenum sulfide: a new efficient electrocatalyst for hydrogen production from water

A new inorganic solid state electrocatalyst for the hydrogen evolution reaction (HER) is reported. Highly crystalline layered ternary sulfide copper-molybdenum-sulfide (Cu2MoS4) was prepared by a simple precipitation method from CuI and [MoS4]2− precursors. In aqueous solution and over a wide pH range (pH 0 to 7), this Cu2MoS4 showed very good catalytic activity for HER with an overvoltage requirement of only ca. 135 mV and an apparent exchange current density of 0.040 mA cm−2 (Tafel slope of ca. 95 mV per decade was found irrespective of the pH value). This Cu2MoS4 catalyst was found to be stable during electrocatalytic hydrogen generation. Therefore, it represents an attractive alternative to platinum.

Combined experimental-theoretical characterization of the hydrido-cobaloxime [HCo(dmgH)2(PnBu3)].

A combined theoretical and experimental approach has been employed to characterize the hydrido-cobaloxime [HCo(dmgH)(2)(PnBu(3))] compound. This complex was originally investigated by Schrauzer et al. [Schrauzer et al., J. Am. Chem. Soc. 1971, 93,1505] and has since been referred to as a key, stable analogue of the hydride intermediate involved in hydrogen evolution catalyzed by cobaloxime compounds [Artero, V. et al. Angew. Chem., Int. Ed. 2011, 50, 7238-7266]. We employed quantum chemical calculations, using density functional theory and correlated RI-SCS-MP2 methods, to characterize the structural and electronic properties of the compound and observed important differences between the calculated (1)H NMR spectrum and that reported in the original study by Schrauzer and Holland. To calibrate the theoretical model, the stable hydrido tetraamine cobalt(III) complex [HCo(tmen)(2)(OH(2))](2+) (tmen = 2,3-dimethyl-butane-2,3-diamine) [Rahman, A. F. M. M. et al. Chem. Commun. 2003, 2748-2749] was subjected to a similar analysis, and, in this case, the calculated results agreed well with those obtained experimentally. As a follow-up to the computational work, the title hydrido-cobaloxime compound was synthesized and recharacterized experimentally, together with the Co(I) derivative, giving results that were in agreement with the theoretical predictions.

A Computational Study of the Mechanism of Hydrogen Evolution by Cobalt(Diimine‐Dioxime) Catalysts

Cobalt(diimine-dioxime) complexes catalyze hydrogen evolution with low overpotentials and remarkable stability. In this study, DFT calculations were used to investigate their catalytic mechanism, to demonstrate that the initial active state was a Co(I) complex and that H2 was evolved in a heterolytic manner through the protonation of a Co(II)-hydride intermediate. In addition, these catalysts were shown to adjust their electrocatalytic potential for hydrogen evolution to the pH value of the solution and such a property was assigned to the presence of a H(+)-exchange site on the oxime bridge. It was possible to establish that protonation of the bridge was directly involved in the H2-evolution mechanism through proton-coupled electron-transfer steps. A consistent mechanistic scheme is proposed that fits the experimentally determined electrocatalytic and electrochemical potentials of cobalt(diimine-dioxime) complexes and reproduces the observed positive shift of the electrocatalytic potential with increasing acidity of the proton source.

Structure and Dynamics of the U4+ Ion in Aqueous Solution: An ab Initio Quantum Mechanical Charge Field Molecular Dynamics Study

The structure and dynamics of the stable four-times positively charged uranium(IV) cation in aqueous solution have been investigated by ab initio quantum mechanical charge field (QMCF) molecular dynamics (MD) simulation at the Hartree-Fock double-zeta quantum mechanical level. The QMCF-MD approach enables investigations with the accuracy of a quantum mechanics/molecular mechanics approach without the need for the construction of solute-solvent potentials. Angular distribution functions; radial distribution functions; coordination numbers of the first, second, and third shell (9, 19, and 44, respectively); coordination number distribution functions; tilt- and Theta-angle distribution functions; as well as local density corrected triangle distribution functions have been employed for the evaluation of the hydrated ions structure. Special attention was paid to the determination of the geometry of the first hydration layer, and the results were compared to experimental large-angle X-ray scattering and extended X-ray absorption fine structure data. The solvent dynamics around the ion were also investigated using mean ligand residence times and related data and, resulting from the unavailability of any experimental data, were compared to ions with similar properties.

Local density corrected three-body distribution functions for probing local structure reorganization in liquids

Three-body distribution functions are calculated for metal ions in an aqueous medium in order to investigate and characterise solvent structure reorganization. Based on the existing formulation of three body correlation function, a local density correction is introduced to enable a comparison of different sub-regions within a solvate as well as different systems, thus taking into account the varying density arising from the influence of the solute.

Revisiting the hydration of Pb(II): a QMCF MD approach.

A quantum mechanical charge field (QMCF) molecular dynamics (MD) study of Pb(II) in an aqueous medium was carried out in order to gain insight into its solvation behavior, for both structural and dynamic aspects. Applying the advanced methodology and different basis sets, some new aspects concerning the solvation of Pb(II) have been revealed. One of the most interesting outcomes of the current simulation is the variation of first shell coordination number from 7 to 9 in the Pb(H2O)n(2+) complex with Pb(H2O)8(2+) as a major species. Moreover, a far more dynamic and labile hydration shell was found compared to previous QM/MM MD simulation with only the first hydration shell treated by quantum mechanics, which reported a very rigid first hydration shell with a fixed coordination number of 9. The current simulation results are in much better agreement with the properties reported from the recent thermodynamic studies than the previous QM/MM MD study.

Structural and dynamic aspects of hydration of HAsO4(-2): an ab initio QMCF MD simulation.

An ab initio quantum mechanical charge field simulation has been carried out in order to obtain molecular level insight into the hydration behavior of HAsO4(-2), one of the major biologically active components of As(V) oxoanion in neutral to slightly alkaline aqueous medium. Moreover, a geometrical definition of hydrogen bonding has been used to probe and characterize both solute-solvent and solvent-solvent hydrogen bonding present in the system. The asymmetry of the anion induced by the protonation of one of the oxygens of the arsenate anion causes rather irregular hydration structure. The nonprotonated oxygen atoms preferably form relatively stable hydrogen bonds with two to three water molecules in their vicinity, while the protonated oxygen forms one or two hydrogen bonds, weaker than water-water hydrogen bonds. The two types of As-O distances obtained from the simulation (1.68 and 1.78 A for the protonated and nonprotonated oxygens, respectively) are in good agreement with the experimental data. The two types of As-O stretching frequencies obtained from the simulation (855 and 660 cm(-1) reproduce well the experimental ATR-FTIR results (859 and 680-700 cm(-1)).

Theoretical modeling of low-energy electronic absorption bands in reduced cobaloximes

The reduced Co(I) states of cobaloximes are powerful nucleophiles that play an important role in the hydrogen-evolving catalytic activity of these species. In this work we analyze the low-energy electronic absorption bands of two cobaloxime systems experimentally and use a variety of density functional theory and molecular orbital ab initio quantum chemical approaches. Overall we find a reasonable qualitative understanding of the electronic excitation spectra of these compounds but show that obtaining quantitative results remains a challenging task.

Electronic structure and hydration of tetramine cobalt hydride complexes.

In this work we have studied two hydridotetraminecobalt(III) complexes using a mixture of computational techniques. These species were chosen as simple and computationally tractable models of the Co(III)-hydrido compounds that are known to be important intermediates in the catalytic cycles of hydrogen evolution mediated by the cobaloxime complexes. We have performed both static density functional theory (DFT) calculations of the complexes in implicit solvent and adaptive hybrid DFT/molecular mechanical (MM) molecular dynamics (MD) simulations in explicit solvent and compared our results to the experimental structural and spectral data that are available for one of the compounds. A principal aim of the study has been to provide a benchmark for future work on cobaloxime and other hydrogen-evolving catalysts using adaptive DFT/MM MD methods.

Homolytic Cleavage of FeS Bonds in Rubredoxin under Mechanical Stress

The biosynthesis, stability, and folding pathways of the iron– sulfur proteins can be probed by analyzing protein unfolding and Fe S bond dissociation. Some of the simplest iron–sulfur proteins belong to the rubredoxin family, which participate in electron transfer processes in bacteria and archea. Rubredoxins lack inorganic sulfide and have only one Fe center coordinated by the side-chains of four cysteine residues (Figure 1a and b). Recent rubredoxin unfolding experiments, performed by protein engineering and singlemolecule atomic-force microscopy, indicated that their Fe S bonds had an unexpectedly low stability under mechanical stress. It was suggested that the activation process for bond dissociation could occur by heterolytic fission, as observed for disulfide bridges, or homolytic cleavage, as hypothesized for the rupture of C Si bonds. Given that Fe complexes can have near-degenerate levels with different total spin, an understanding of the reactivity of iron–sulfur clusters requires that their spin states and spin crossovers be characterized during the reaction. Herein we have adopted a multiscale modeling approach employing quantum chemical (QC), molecular mechanical (MM), and hybrid QC/MM potentials, to address these questions. Full details of the models and programs used are given in the Supporting Information. We started by studying the mechanical unfolding of rubredoxin using an MM potential with an implicit model of solvent. A standard biomolecular force field was employed, except for the Fe S bonds of the iron–sulfur complex which were represented by specially parameterized Morse potentials that permitted bond dissociation. The starting structures for all simulations were derived from those of the oxidized rubredoxin from Pyrococcus furiosus (protein databank (PDB) codes 1BRF and 1CAA). Unfolding was emulated by performing molecular dynamics (MD) simulations of the protein with an added harmonic potential that pulled apart the Nand C-termini at a constant speed. Although we imposed no bias on the order of dissociation of the Fe S bonds, we observed that the Fe S(Cys5) bond ruptured first in most of our simulations. Results of one of these simulations carried out with a pulling speed of 10 nm ns 1 are shown in Figure 2. It is clear from the Ca RMSDs that rubredoxin unfolds during the trajectory. The first contacts disrupted are interchain hydrogen bonds in the anti-parallel b-strand formed between the Nand C-termini. Other local polar contacts that stabilize the secondary structure are approximately maintained throughout the simulation, but fluctuations of the hydrophobic contacts and hydrogen bonds in the protein s core result in progressive disorganization of the globular fold. In the last part of the trajectory, the hydrogen bonds that hold together the Nterminal anti-parallel b-strand are broken. Large fluctuations are observed for both the Fe–Sg bond distances ( 0.5 ) and Sg-Fe-Sg valence angles ( 208) along the trajectory, until the Fe Sg(Cys5) bond breaks after about 1 ns of simulation (Supporting Information Figure S1). A snapshot of one of the unfolded rubredoxin structures obtained just before Fe Sg bond disruption is shown in Figure 1c. The local symmetry around the iron–sulfur center is broken upon mechanical unfolding with the iron tetrahedral coordination (pseudo-Td) changing to C2. This step is characterized by changes in the Sg-Fe-Sg valence angles from approximately 1098 in folded rubredoxin to 90–1258 in the Figure 1. Rubredoxin structures: a) schematic representation of the folded protein; b) close-up of the iron–sulfur cluster from the folded protein; c) schematic representation of the protein from one of the pulling simulations just before Fe S bond rupture: and d) close-up of the iron–sulfur cluster from the structure shown in (c). In (b) and (d) important hydrogen bonds are indicated by dashed lines and Fe orange, S yellow, C green, N blue.