Tahamida Banu

Hydrolysis of ammonia borane and metal amidoboranes: A comparative study

A gas phase mechanistic investigation has been carried out theoretically to explore the hydrolysis pathway of ammonia borane (NH3BH3) and metal amidoboranes (MNH2BH3, M = Li,Na). The Solvation Model based on Density (SMD) has been employed to show the effect of bulk water on the reaction mechanism. Gibbs free energy of solvation has also been computed to evaluate the stabilization of the participating systems in water medium which directly affects the barrier heights in the potential energy surface of hydrolysis reaction. To validate the experimentally observed kinetics studies, we have carried out transition state theory calculations on these hydrolysis reactions. Our result shows that the hydrolysis of both the metal amidoboranes exhibits greatly improved kinetics over the neat NH3BH3 hydrolysis which corroborates well with the experimental observation. Between the two amidoboranes, hydrolysis of LiNH2BH3 is found to be kinetically favored over that of NaNH2BH3, making it a better candidate for releasing molecular hydrogen.

Molecular hydrogen binding affinities of metal cation decorated substituted benzene systems: insight from computational exploration

The binding affinity of hydrogen molecules towards Li+ and Mg2+ decorated C6H5X (X = −CH3, −NH2, −CN, −COOH) systems has been investigated theoretically with special emphasis on the nature of the interaction between metal cations and H2 molecules. Our calculations show that binding of H2 over C6H5X−M (where M = Li+, Mg2+) is improved on moving from Li+ to Mg2+. For both C6H5X−M complexes the electron donating substituents weaken the H2 binding energy considerably whereas electron withdrawing substituents slightly strengthen the interaction relative to the C6H6−M complex. The interaction of H2 molecules with the metal centers in Li+ and Mg2+ decorated C6H5X systems has been explored in the light of AIM formalism, NBO analysis and LMOEDA analysis. The polarization and the charge transfer together stabilize the system whereas the pairwise steric exchange interaction renders destabilization of the system. In the case of Mg2+ decorated systems, the amount of charge transfer from the bonding orbital of the hydrogen molecule to the antibonding lone pair orbital of the metal cation and thereby the polarization factor is much higher than that found in corresponding Li+ decorated systems.

Exploration of Binding Interactions of Cu2+ with d-Penicillamine and its O- and Se- Analogues in Both Gas and Aqueous Phases: A Theoretical Approach

We have theoretically explored the entire binding phenomena of d-penicillamine and its O- and Se-analogues with Cu(2+) in both gas and aqueous phases. At first, a brief conformational analysis has been performed via -XH and -COOH rotations to investigate such conformers that are suitable for binding in both bidentate as well as tridentate fashions. The stability of each bidentate and tridentate complex is determined on the basis of relative energy (ΔE) and gas phase metal ion affinity (MIA) along with the bonding analysis by using atoms in molecule theory. The effect of conformational change on the stability of the complexes is also examined thoroughly. By analyzing the MIA values, we have shown that the side chain substitution makes an impact on the binding process. To delve into the binding phenomena in aqueous phase, we have introduced both the first and second hydration sphere models. In first hydration sphere model, to realize the precise effect of water molecules we have considered stable octahedral hexa-aqua copper complex, [Cu(H2O)6](+2) and accordingly substituted water molecules depending on the bidentate or tridentate nature of the chelating agents. The influence of bulk water molecules on the energetics and geometries of the first hydrated sphere complexes have also been investigated by employing second hydration sphere model assuming physiological pH through the implementation of implicit COSMO and polarizable continuum models, respectively. In the second hydration sphere model, the zwitterionic structures of the amino acids and their side chain deprotonated forms are also included to study the binding phenomena with Cu(2+). The complete work furnishes both the binding properties and the energetics of the copper-artificial amino acid complexes in both gas and aqueous phases that will reflect a realistic overview of the entire binding phenomena.

Exploration of Unimolecular Gas-Phase Detoxication Pathways of Sarin and Soman: A Computational Study from the Perspective of Reaction Energetics and Kinetics

A mechanistic investigation has been carried out to explore all possible gas phase unimolecular isomerization as well as decomposition pathways of toxic organophosphorus compounds (OPCs), namely, sarin (GB) and soman (GD), which are better known as nerve agents. We have identified a total of 13 detoxication pathways for sarin, where the α-H, β-H, and γ-H take part in the H-transfer process. However, for soman, due to the presence of ω-H, three additional detoxication pathways are obtained, where the ω-H is involved in the H-transfer process. Among all the pathways, the D3 decomposition pathway, where the phosphorus oxoacid derivative and alkene are generated via the formation of a six-membered ring in the transition state, is identified as the most feasible pathway from the perspective of both activation barrier and reaction enthalpy values. Moreover, we have studied the feasibility of the isomerization and decomposition pathways by performing the reaction kinetics in the temperature range of 300 K-1000 K using the one-dimensional Rice-Ramsperger-Kassel-Marcus (RRKM) master equation. From the RRKM calculation also, D3 pathway is confirmed as the most feasible pathway for both OPCs. The rate constant values associated with the D3 pathway within the temperature range of 600 K-700 K imply that the degradation of the OPCs is possible within this temperature range via the D3 pathway, which is in good agreement with the earlier reported experimental result. It is also observed that at higher temperature range (∼900 K), the increased rate constant values of other detoxication pathways indicate that along with D3, all other pathways become more or less equally feasible. Therefore, the entire work provides a widespread idea about the kinetic as well as thermodynamic feasibility of the explored detoxication pathways of the titled OPCs.

Cyclic amine-borane adducts [CnH2n+1N·BH3 (n = 2–6)] as chemical hydrogen storage systems: a computational analysis

A detailed theoretical analysis of the cyclic amine-borane adducts has been performed to explore their efficiency towards hydrogen storage. The proton affinities, gas phase basicities and heats of formation of cyclic amines, e.g., aziridine, azetidine, pyrrolidine, piperidine and azepane are calculated at the G4MP2 level. The thermodynamic properties of the borane adducts of these five cyclic aliphatic amines and their associated dehydrogenated products are also investigated. The potential energy surface (PES) associated with the dehydrogenation reaction of all these cyclic amine-borane systems has also been explored. The dehydrogenation reaction enthalpies being close to thermoneutral for all these five ring compounds indicate their potentiality as efficient hydrogen storage materials.

Towards a comprehensive understanding of the chemical vapor deposition of titanium nitride using Ti(NMe2)4: a density functional theory approach

A gas phase mechanistic investigation of the chemical vapor deposition (CVD) of titanium nitride (TiN) from the decomposition of Ti(NMe2)4, tetrakis(dimethylamido)titanium (TDMAT) as a single source precursor as well as from the reaction of Ti(NMe2)4 with NH3, i.e., the ammonia assisted mechanism is carried out and reported herein within the framework of density functional theory. Contrary to the theoretical result reported previously for a model TDMAT, metallacycle formation and β-H elimination pathways are found to be the major decomposition pathways responsible for the decomposition of TDMAT, and this finding is in accord with the experimental observation. Interestingly, agostic interaction is found to play a key role in promoting β-H elimination in the decomposition of TDMAT. A new additional pathway of decomposition of TDMAT has been identified theoretically in this present study. Exploration of the complex gas phase mechanism and thereby a detailed identification of the reaction intermediates enable us in realizing the origin of incorporation of carbon contamination in TiN films produced from TDMAT alone and then how the contamination is removed in the presence of ammonia. The ammonia assisted mechanism is found to proceed through the formation of a pre-equilibrium complex. The computed barrier height of 7.3 kcal mol(-1) for the initial transamination process associated with the Ti(NMe2)4 + NH3 reaction is found to be in very good agreement with the experimental activation energy. The total rate constant ktot for the ammonia assisted mechanism is calculated to be 1.28 × 10(-51) cm(3) molecule(-1) s(-1) at 298.15 K.

Structure, stability, and dissociation of small ionic silicon oxide clusters (SiOn +(n = 3, 4)): Insight from density functional and topological exploration

The structures, energies, isomerization, and decomposition pathways of small ionic silicon oxide clusters, SiO(n)(+) (n = 3, 4), on doublet and quartet energy surfaces are investigated by density functional theory. New structural isomers of these ionic clusters have been obtained with this systematic study. The energy ordering of the isomeric cluster ions on doublet spin surface is found to follow the same general trend as that of the neutral ones, while it differs on the quartet surface. Our computational results reveal the energetically most preferred decomposition pathways of the ionic clusters on both spin surfaces. To comprehend the reaction mechanism, bonding evolution theory has also been employed using atoms in molecules formalism. The possible reasons behind the structural deformation of some isomers on quartet surface have also been addressed. Our results are expected to provide important insight into the decomposition mechanism and relative stability of the SiO(n)(+) clusters on both the energy surfaces.

Investigation of agostic interaction through NBO analysis and its impact on β-hydride elimination and dehydrogenation: a DFT approach

Agostic interactions have been investigated through NBO analysis tuning each part of H2LMMZCHR2 by varying metal centre and intramolecular groups in different positions of the molecule. We have calculated the energy transfer phenomena between donor and acceptor orbitals associated with the agostic bond by second-order perturbation analysis and characterized the acceptor orbitals for whether it is a vacant metal lone pair or molecular orbital associated with both metal and ligand. The investigation of agostic phenomenon by altering the orientation of the metal-associated ligand is another aspect of this work to reveal the orientation effect as one of the controlling factors of the agostic interaction. We have further studied the impact of agostic interaction on the two well-recognized reaction processes, namely β-hydride elimination and dehydrogenation for all the d0 systems under investigation. Both the reactions are found to be facilitated by agostic interaction, where the nature of agostic hydrogen varies from reaction to reaction. The complete work furnishes both characterization and reaction pathways to portray agostic phenomena by interconnecting the molecular geometry with the reaction processes.

Catalytic role of borane and alane in hydrogen release from cyclic amine adducts CnH2n+1N·XH3 [X = B, Al; n = 2–5]: a theoretical interpretation

The reaction pathways for H2 release from cyclic amine–alane adducts in the presence and absence of alane (AlH3) are investigated and the associated potential energy surfaces are constructed by employing the composite G4MP2 method. The computed Gibbs free energy change values are potential proof of the thermodynamic feasibility of the unimolecular dehydrogenation reactions. Unlike cyclic amine–borane adducts, the unimolecular dehydrogenation barrier in cyclic amine–alane adducts is found to be lower than the Al–N bond dissociation energy in all cases but one. The calculated results clearly demonstrate that alane plays a significant catalytic role in H2 release from cyclic amine–alane adducts. The catalytic effect of molecular borane (BH3) on the dehydrogenation of cyclic amine–borane adducts is also investigated and it is observed that AlH3 exhibits a larger catalytic effect than BH3. A systematic comparison of the unimolecular as well as catalyzed H2 loss from cyclic amine–alane and amine–borane systems reveals that the former is a better candidate for releasing molecular hydrogen than the latter.

Comprehensive study of methylation on the silicon (100)-2 × 1 surface: a density functional approach.

A detailed mechanistic investigation of Si-Me formation over the silicon (100)-2 × 1 surface using the Si9H12 cluster model has been performed using various reagents, based on two basic mechanisms: dissociation and substitution. The reagents CH4, CH3Cl for dissociation and CH3Li, CH3MgBr for substitution mechanism are used to explore the methylation process on the silicon surface at the M062X/6-311+G(2d, p) level of theory. The associated potential energy surfaces explored here are aimed to unveil the most favored pathway of methylation with appropriate reagents. Dissociation of methane forms a monomethylated product (D1) through an energetically unfavorable pathway. All the adsorption modes of CH3Cl over the silicon surface are also detected and analyzed. Methyl chloride dissociates to form another monomethylated product D2 and its derivative D3 in the entrance channel, while, in the next step, bridged compounds I1 (Cl-bridged) and I2 (H-bridged) are produced from them, respectively. The C-Cl dissociation leads to the formation of D2 having a lower activation barrier. With a comparably high activation barrier in the C-H dissociation, producing D3, very interestingly carbene intermediate has been detected in the reaction pathway. Detection of energetically unfavored conversions from D2 to I1 and D3 to I2 ensured that the methylation process will not be hampered through these interconversions. For substitution, HCl- and Cl2-passivated Si surfaces are taken, where chlorine is to be substituted by the methyl group of both of the methylating agents. With both substituents, HCl-passivated Si9H12 gives D1. The substitution process on Cl2-passivated Si9H12 leads to the formation of D2 in the first step and dimethylated product (S1) in the final step. In all the above substitution processes, methyl lithium proved to be the better substituent for the formations of D1, D2, and S1 on HCl- or Cl2-passivated surfaces. The present work not only demonstrated methyl lithium as one of the best methylating agents but also revealed the interrelation among the dissociative adsorption modes of CH3Cl, reported earlier, in a single potential energy surface with a remarkable detection of carbene intermediate formed in the pathway of C-H dissociation.