Sailaja Krishnamurty

N2 activation on Al metal clusters: catalyzing role of BN-doped graphene support

The successful sustenance of life demands an ambient abiotic process for N2 activation and dissociation. The Bosch-Haber process remains the only abiotic and synthetic means for N2 activation and its fixation. Metal nanoclusters have been recently reported for activating molecular nitrogen. Interestingly, the metal clusters explored so far for N2 activation are free clusters and, hence, are practically not applicable by experimental chemists. Using density functional theory (DFT) based methodology, we propose a potential catalytic system for di-nitrogen activation, viz. supported Al clusters. Al clusters supported on BN doped graphene sheets are seen to activate N2 molecule with a red shift in the N-N stretching frequency up to 874 cm-1 with activation barriers as low as 1.14 eV.

Computational investigation on the catalytic activity of Rh6 and Rh4Ru2 clusters towards methanol activation

Catalysis of molecular activation of small molecules through scission of strong chemical bonds is one of the major challenges faced by chemists. More specifically, activation of the strong C–H and O–H bonds of various alcohols, especially methanol, is one of the various important intermediate steps of key organic reactions. Our present work explores a suitable metal cluster catalyst towards methanol dissociation. In particular, we have examined the effect of ruthenium doping (Rh:Ruxa0=xa02:1) on the catalytic activity of Rh6 cluster towards methanol dissociation. Density functional theory-based calculations illustrate two competitive pathways for methanol dissociation, which are via O–H and C–H bond breaking. Both the pathways are found to be energetically favourable in the presence of bimetallic and mono-metallic clusters. Importantly, energy barrier for O–H bond dissociation reduces considerably in doped cluster as compared to pure Rh6 cluster and is smaller than the values reported for a number of other small metallic clusters.

Tailoring the structure and electronic properties of platinum and gold–platinum nanocatalysts towards enhanced O2 activation

Geometric structure plays a crucial role in enhancing the catalytic activity of a material towards reactions such as oxygen reduction, methanol oxidation etc. Pt and Pt based bimetallic alloys of different morphologies such as nanorods, core–shell etc. have been synthesized so far contributing to varying catalytic activity in fuel cells. However, the electronic factors contributing to enhanced catalytic activity of particular structure/shape/morphology have not been understood so far. In a first such computational study, we demonstrate this complex structure–property correlation on model six-atom Pt clusters in the context of O2 activation at the cathode of direct methanol fuel cells. Since recent studies have shown that Au–Pt bimetallic alloys exhibit superior catalytic activity towards oxygen reduction reaction with respect to other bimetallic clusters, we have identified 4 distinct six-atom Pt cluster morphologies, viz., trigonal planar, distorted octahedral, normal octahedral and double square planar, and doped them sequentially with gold atoms until a ratio of 1u2006:u20061 is obtained. Thus, clusters with varying conformations/shapes and bimetallic compositions are considered for potential O2 activation. The O–O bond stretching frequencies are calculated to evaluate the degree of bond activation in O2 molecules. Our study reveals that the Pt6 and AunPtm (n + m = 6) clusters with a double square planar shape exhibit superior catalytic activity towards O–O bond activation with respect to other six-atom conformations. Analysis of their electronic properties demonstrates that the amount of charge transferred by the cluster to the O2 molecule and the effective overlap between the d-orbitals of the metal atom and the pi-molecular orbitals of O2 can be directly correlated with their catalytic activities. In order to validate these conclusions, we have further extended such analysis to various Ptm (m = 3–13) conformations, wherein, once again the frontier molecular orbital composition is seen to play a predominant role in oxygen activation. Thus, the present study unveils the electronic properties determining the catalytic activity of a catalytic cluster towards O–O bond activation.

Stretching the threshold of reversible dynamics in silicon clusters: A case of carbon alloyed Si6.

Silicon clusters with 3-50 atoms undergo isomerization/reversible dynamics or structural deformation at significantly lower temperatures of 350 K-500 K. Through Born Oppenheimer Molecular Dynamical (BOMD) simulations, the current study demonstrates that carbon alloying enhances the thermal stability of a silicon cluster. The study is carried out on a Si6 cluster which has been recently reported to undergo reversible dynamical movements using aberration-corrected transmission electron microscopy. Present BOMD simulations validate the experimentally observed reversible atomic displacements (reversible dynamical movements) at finite temperatures which are seen to persist nearly up to 2000 K. Carbon alloying of Si6 is seen to stretch the threshold of reversible dynamics from 200 K to 600 K depending upon the alloying concentration of carbon in the cluster.

Understanding the molecular conformations of Na-dimyristoylphosphatidylglycerol (DMPG) using DFT-based method

The molecular conformations of phospholipids comprising a lipid bilayer determine the physico-chemical properties of the latter. In this study, we attempt to understand the various possible conformations available for an anionic lipid molecule dimyristoylphosphatidylglycerol (DMPG) with Na as its charge-compensating cation. The various possible molecular orientations available for lipid molecule are analysed using a density functional theory-based method. Our study reveals a rich conformational space with two different types of glycerol body orientations, more commonly known as rotamers. Interestingly, this is in agreement with the molecular conformations proposed earlier by NMR studies on lipid monomer solutions. We demonstrate that these conformations are an outcome of delicate balance of electrostatic and van der Waals forces along with intra-molecular hydrogen bonds achieved by a critical combination of torsion angles. Na+ ions are seen to interact predominantly with the oxygen atoms of the glycerol groups in tail and head along with that of phosphate oxygen atoms leading to a cage-like orientation of lipid molecule around the Na+. Following the conformational analysis, we attempt to evaluate the electronic properties of few low-lying conformations. This study shows that though the water molecules screen the Na–Olipid interactions, they do not dramatically modify the Na–Olipid bond distances. The lipid conformation retains the cage-like structure around the Na+ in the presence of water molecules. Some amount of charge transfer from the water molecules to Na ion is noted. The water molecules modify the phosphate-tail glycerol group interactions leading to a more stable Na–DMPG–H2O and Na–DMPG–4H2O complexes.

A DFT based assay for tailor-made terpyridine ligand–metal complexation properties

Electron-rich terpyridine ligand and its metal complexes have a potential to grow as responsive surfaces by adapting their physicochemical properties as a function of environment. The responsiveness is brought about by judicious molecular level designing that is currently hindered due to lack of information and control on terpyridine (TPy)–metal (M) interactions at single molecule level. So far there is no organised understanding on the binding of different metals with TPy ligand and ways to modulate it. Being a large conjugated system, TPy has a large scope to be functionalised with electron exchanging groups to alter its electronic structure and consequently its binding with metal atoms. In first report of such a kind, using density functional theory (DFT), we demonstrate that convenient modulation of TPy-M binding is possible through functionalisation of TPy for , Ru, Fe, Mo and Au. Electron donating groups viz., CH, OCH, CH, NH and electron withdrawing groups viz., CF, COOH, CN and NO are considered for functionalisation of TPy ligand. Significantly, the present work focuses on the functionalisation at 4 and 4 positions of TPy molecule. The role of such a functionalisation in influencing the ligands structure–property correlation is missing in the literature to the best of our knowledge. The present investigation quantifies that by pertinent functionalisation of TPy, TPy-M binding energies can be modified up to 60 kcal/mol. Our results reveal that functionalisation leads to a considerable charge redistribution within the TPy-M complex with carbon atoms in pyridine rings functioning as major electron sink/source with a corresponding red/blue shift of stretching frequency. This modifies the red-ox, optical and other chemical properties of TPy-M complexes. In brief, the present report illustrates a way to design ligands such as TPy for diverse applications through tailor-made functionalisation using electronic structure methodology.