Tamalika Ash

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.

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.

Mechanistic Insight into the Molecular TiO2-mediated Gas Phase Detoxication of DMMP: A Theoretical Approach

The detoxication of DMMP (dimethyl methylphosphonate) mediated by molecular TiO2 has been investigated computationally using density functional theory (DFT). From our previous studies, it is evident that the unimolecular detoxication of OPCs (organophosphorus compounds) is kinetically unfeasible at room temperature due to the significantly high activation barrier. Thus, the aim of our work is to find out whether molecular TiO2 can make any significant impact on the kinetic feasibility of the detoxication processes or not. Here, we have identified a total of three detoxication pathways, where in the first step the detoxication occurs through H-abstraction with the assistance of TiO2, and in the second step, the titanium complex is separated from the respective phospho-titanium complexes. The outcomes reveal that the TiO2-mediated detoxication pathways are at least 20.0 kcal/mol more favorable than their respective unimolecular pathways and that among them, the α-H-mediated isomerization is found to be the most feasible pathway. When the separation of a titanium complex is under consideration, the double H2O-assisted mechanism is found to be the favored pathway. Overall, the entire work provides a widespread idea about the efficiency of molecular TiO2-assisted detoxication of DMMP, which can be well applicable to other OPCs also.

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.

Multiple Li+- and Mg2+-decorated PAHs: potential systems for reversible hydrogen storage

The hydrogen binding efficiency of multiple metal-ion-(Li+, Mg2+)-decorated small Polycyclic Aromatic Hydrocarbons (PAHs) has been investigated using Density Functional Theory (DFT). The CAM-B3LYP method and 6-311+G(d,p) basis set have been used for the systematic calculation of the interaction energy of the metal ion with the aromatic system (ΔE) and the average binding energy (ΔBE) of the hydrogen molecules to these metal-ion-decorated PAHs. Our results show that the aromatic ring current associated with a PAH can bind at most one pair of metal ions, with the metal ions ensconcing themselves with appreciable values of ΔE both on the same and opposite faces of a PAH with certain preferences. The ΔBE values associated with H2 binding on Li+-decorated systems are found to be almost similar for anthracene and phenanthrene, which are higher in comparison to naphthacene. The Mg2+-decorated counterparts, however, exhibit ΔBE values of around four to five times higher than the Li+-decorated ones. The nature of interaction between hydrogen molecules and metal ions is predicted by the topological analysis of the atoms in molecules formalism (AIM), employing an AIMAll program. The Natural Population Analysis (NPA) method is used for the evaluation of charge distribution between donor hydrogen molecules and acceptor metal ions. The value of charge transfer from H2 to metal ions (ΔECT) is evaluated employing Natural Bond Orbital (NBO) analysis. The charge transfer from the bonding orbital of the hydrogen molecule to the antibonding lone pair orbital of the metal ion is taken into account for stability of the complexes. All the complexes possess high gravimetric storage capacity, and are found to be maximum for Mg2+-decorated anthracene (9.6 wt% H2).

Theoretical analysis of tautomerization of succinimide and analogous compounds: insights from DFT approach

Tautomerizations of biologically and medicinally important heterocyclic compound, 2,5-pyrrolidinedione, commonly known as succinimide and two of its analogous compounds, 2,4-pyrrolidinedione and 3,4-pyrrolidinedione have been investigated at the density functional theory (DFT)/M06-2X level in aqueous medium, implementing polarizable continuum model (PCM). We have extended our investigation of tautomerism to the sulfur analogues of the aforementioned compounds also, i.e., 2,5-pyrrolidinedithione, 2,4-pyrrolidinedithione, and 3,4-pyrrolidinedithione. Tautomerism observed in these compounds are mainly keto-enol, thio-thiol and amine-imine, but we have detected two new kind of tautomerization shown by some of the abovementioned compounds, named as keto-epoxy (for the oxygen analogue) and thio-thioepoxy (for the sulfur analogue) which have not yet been reported in the literature. Relative energies (Er) and activation energies (Ea) have been calculated for all the tautomers and tautomerization processes. The potential energy surfaces (PESs) have been constructed using the M06-2X energy values. It has been observed that the energy difference found in the tautomers of sulfur analogues is relatively lower than that of the corresponding oxygen analogues, so does the activation energy barrier. As one-electron redox properties play an important role in biological systems, we have also explored the effect of conformational changes on the overall redox properties of the said compounds by calculating the adiabatic and vertical ionization potentials (AIPs and VIPs, respectively) and adiabatic electron affinities (AEAs).

Towards a comprehensive understanding of the Si(100)-2×1 surface termination through hydrogen passivation using methylamine and methanol: a theoretical approach

Abstract Using density functional theory, we explored the termination process of Si (100)-2 × 1 reconstructed surface mechanistically through the dehydrogenation of small molecules, considering methyl amine and methanol as terminating reagents. At first, both the terminating reagents form two types of adduct through adsorption on the Si (100)-2 × 1 surface, one in chemisorption mode and the other via physisorption, from which the dehydrogenation process is initiated. By analyzing the activation barriers, it was observed that termination of the Si-surface through the dehydrogenation is kinetically almost equally feasible using either reagent. We further examined in detail the mechanism for each termination process by analyzing geometrical parameters and natural population analysis charges. From bonding evaluation, it is evident that hydrogen abstraction from adsorbates on the Si-surface is asymmetric in nature, where one hydrogen is abstracted as hydride by the electrophilic surface Si and the other hydrogen is abstracted as proton by the neucleophilic surface Si. Moreover, it was also observed that hydride transfer from adsorbate to the Si-surface occurs first followed by proton transfer. Overall, our theoretical interpretation provides a mechanistic understanding of the Si (100)-2 × 1 reconstructed surface termination by amine and alcohol that will further motivate researchers to design different types of decorated semiconductor devices. Graphical AbstractSurface termination process of Si(100)-2×1 through formation of non-polar Si–H bonds via dehydrogenation of methylamine and methanol as terminating reagents

Identification and characterization of intramolecular γ-halo interaction in d0 complexes: a theoretical approach

A mechanistic investigation to detect intramolecular M⋯X–C type interactions in d0 neutral and cationic complexes was carried out through a benchmark study employing different density functional methods. As γ-halogen is involved in M⋯X–C type interactions, it is denoted as a γ-halo interaction and the respective conformers are designated as halo-conformers. By analyzing the geometrical parameters of halo-conformers, it was observed that, irrespective of the nature of the metal and the halogen, the Cγ–X bond distance increases compared to the usual C–X bond, which brings the M and X centers close enough to generate a weak interaction. Generation of the M⋯X–C interaction was confirmed by performing NBO, AIM and Wiberg bond index analyses, from which the persistence of γ-halo interaction was seen to be prominent. Moreover, for each neutral and cationic complex, the values of Wiberg bond order are in good agreement with the AIM results. The effect of the metal center, as well as γ-halogen substitution, on γ-halo interaction was also studied in the present work. To justify the practical subsistence of the halo-conformers, we checked the stability of the conformers with respect to their β-conformers by comparing the zero-point-corrected electronic energies. Therefore, the entire study was designed in such a way that it can provide evidence in support of intramolecular M⋯X–C interactions, where, instead of the C–H bond, the Cγ–X bond will interact with the central transition metal.

Structural and thermodynamic aspects of Li n @C x endohedral metallofullerenes: a DFT approach

We have determined the electronic, thermodynamic and geometrical properties of Lin@Cx fullerenes by employing DFT study, which are better known as endohedral metallofullerenes (EMFs). In this work, we have considered C20, C32, C42 and C60 fullerenes and predicted the maximum number of Li atoms that can be encapsulated in their respective cavities. The identification of the geometrical structures of Lin clusters formed inside the fullerene cavity has been also included in our study. The binding energies as well as the thermodynamic properties (enthalpy, entropy and Gibbs free energy) of each EMF have been determined to acquire an idea about the stability of the EMFs. Further to ensure the stability of the EMFs, we have also calculated the deformation energy of the EMFs with respect to normal C60. By analyzing the results obtained from our calculation, we have shown that our results are in reasonably good agreement with the earlier available theoretical results. We have also performed the NPA analysis to calculate the charges on both the fullerene surface and the encapsulated Lin clusters for each Lin@Cx and further investigated its variation with respect to n and x. Energy transfer phenomena from