Deepanwita Ghosh

Arsine and its fluoro, chloro derivatives: a computational thermochemical study

The structures, vibrational frequencies, enthalpies of formation and dissociation energies of arsine, arsenic hydrides and their fluoro, chloro derivatives have been studied using density functional B3LYP/cc-pVDZ, ab-initio MP2/cc-pVDZ and composite CBS-QB3 and CBS-Q methods. Computed standard enthalpies of formation at 298 K by atomisation scheme are compared with reported values. Bond dissociation energies at 0 K are calculated for all possible thermal dissociation of the molecular species in gas phase, from which the energetically most favourable dissociation pathways are predicted. The calculated enthalpies of formation and bond dissociation energies are correlated with the nature of bonding in arsine and its fluoro, chloro derivatives. Energy barriers at 0 K are calculated and transition states are located for the molecular fragment elimination of the thermal dissociation reactions.

Interaction Between Group IIb Divalent Transition-Metal Cations and 3-Mercaptopropionic Acid: A Computational and Topological Perspective

Density functional theory was applied to study the interaction of group IIb transition-metal cations (Zn(2+), Cd(2+), and Hg(2+)) with one and two fully or partially deprotonated 3-mercaptopropionic acid ligands. In this investigation, we determined the geometries of all possible complexes resulting from the coordination of the metal ions with the ligands at different binding sites selected on each ligand. The relative energies of the complexes, metal-ion affinities, free energies, and entropies were also determined. The natures of the bonds were critically analyzed by natural bond orbital (NBO) analysis and clarified further using the atoms-in-molecules (AIM) approach. The substantial influence of the solvent (water) polarization on the energetics, geometries, and bonding of the molecular complexes was also investigated by the conductor-like screening solvation model (COSMO). In an attempt to simulate the complexes in an aqueous environment, water molecules were added explicitly to complete the coordination sphere of the metal cations, and the corresponding metal-ion affinities were calculated to study the effect of microhydration.

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.

Computational study on the growth of gallium nitride and a possible source of oxygen impurity.

The reaction pathways for the gallium nitride GaN growth by gas phase reaction of trimethylgallium (TMG) with ammonia is studied theoretically. Water is the most important impurity in ammonia, therefore its reaction with TMG is investigated as a possible source of oxygen impurity in GaN. Gallium oxide (GaO) formed by the reaction between TMG and H(2)O is predicted to be one of the possible source of oxygen impurity in GaN. The mechanisms and energetics of these reactions in the gas phase have been investigated by density functional B3LYP/[LANL2DZ-ECP + 6-31G(d,p)] method and ab initio MP2/[LANL2DZ-ECP + 6-31G(d,p)], CCSD(T)/[LANL2DZ-ECP + 6-31G(d,p)]//B3LYP/[LANL2DZ-ECP + 6-31G(d,p)], CCSD(T)/[LANL2DZ-ECP + 6-31G(d,p)]//MP2/[LANL2DZ-ECP + 6-31G(d,p)], and CCSD(T)/[LANL2DZ-ECP + Ahlrichs-VTZP]//MP2/[LANL2DZ-ECP + Ahlrichs-VTZP] methods. Both the reactions of TMG with NH(3) and H(2)O are modeled using pre-equilibrium charge-transfer complexes (CH(3))(3)Ga:NH(3) (C1) and (CH(3))(3)Ga:OH(2) (C2) having binding energies of 18.8 and 12.4 kcal/mol, respectively. The first step of the methane elimination reaction from the complexes proceeds through the saddle points TS1 and TS1a having activation barriers 37.0 and 22.6 kcal/mol for C1 and C2, respectively. The first CH(4) elimination step is exothermic for both the cases, but the exothermicity is 15.0 kcal/mol greater for CH(4) elimination from C2. The next step of methane elimination from the stable reaction intermediates (CH(3))(2)GaNH(2) and (CH(3))(3)GaOH has a very high activation barrier of 76.0 and 67.8 kcal/mol via saddle points TS2 and TS2a, respectively. The calculated reaction rates at 298.15 K for both the reactions are low but are comparable to each other. The total rate constant k(tot) for GaN formation is 2.07 x 10(-60) cm(3) molecule(-1) s(-1), and that for GaO formation is 6.85 x 10(-62) cm(3) molecule(-1) s(-1).

Efficient White-Light Generation from Ionically Self-Assembled Triply-Fluorescent Organic Nanoparticles.

Low cost, simple, and environmentally friendly strategies for white-light generation which do not require rare-earth phosphors or other toxic or elementally scare species remain an essentially unmet challenge. Progress in the area of all-organic approaches is highly sought, single molecular systems remaining a particular challenge. Taking inspiration from the designer nature of ionic-liquid chemistry, we now introduce a new strategy toward white-light emission based on the facile generation of nanoparticles comprising three different fluorophores assembled in a well-defined stoichiometry purely through electrostatic interactions. The building blocks consist of the fluorophores aminopyrene, fluorescein, and rhodamine 6G which represent blue, green, and red-emitting species, respectively. Spherical nanoparticles 16(±5) nm in size were prepared which display bright white-light emission with high fluorescence quantum efficiency (26 %) and color coordinate at (0.29, 0.38) which lie in close proximity to pure white light (0.33, 0.33). It is noteworthy that this same fluorophore mixture in free solution yields only blue emission. Density functional theory calculations reveal H-bond and ground-state proton transfer mediated absolute non-parallel orientation of the constituent units which result in frustrated energy transfer, giving rise to emission from the individual centers and concomitant white-light emission.

Theoretical study of electronic structure and complexation of PbII(S2COR)2 [R = Me, Et, Ph] complexes

The complexes of PbII(S2COR)2 [R = Me, Et, Ph] having three possible coordination patterns S,S/S,S or S,O/S,S or S,O/S,O are studied. The formation energy and relative energy of these isomers reveal that the order of stability is S,S/S,S > S,O/S,S > S,O/S,O. Complexation energy of the isomers decreases substantially from gas phase to water. The natural bond orbital (NBO) analysis has been performed to explore the metal–ligand coordination. Similar types of metal–ligand coordinations are observed for methyl and ethyl substituent. In S/S,S/S coordination, lead mainly uses its 6p sub-shells along with 6d and 7s orbitals to coordinate with 3p sub-shells of the surrounding sulfur atoms. Significant mixing of 6p and 6d orbitals of lead is also observed. In contrast, for phenyl substituent, lead uses only its 6p sub-shells to coordinate with sulfur atoms. In S,O/S,O or S,O/S,S coordination, oxygen uses its 2s and 2p sub-shells while sulfur uses its 3p sub-shells to coordinate with 7s and 6p sub-shells of lead. The binding energy and NBO analysis of these chelates indicate that lead–sulfur (soft–soft) coordination is preferred over lead–oxygen (soft–hard). In all these chelates, valence 6s orbital of lead shows inert pair effect with no participation in chelation.

Theoretical study of catalytic oxidation of CO on free PdxO2+ (x = 4–6) clusters: size dependent comparison of combustion

The mechanistic details of the adsorption and complete dissociation of oxygen on cationic palladium clusters [Pdx+ (x = 4–6)] to form pre-oxidized PdxO2+ (x = 4–6) clusters and their catalytic role in the oxidation of carbon monoxide (CO) have been investigated in the gas phase by performing density functional theory calculations. The nature of the dissociation of O2 on palladium clusters is the governing step for CO oxidation prior to CO adsorption. The presence of individual oxygen atoms, either at the bridge or at the hollow site of the cluster, controls the CO oxidation barrier effectively. These barriers for PdxO2+ (x = 4–5) are lower than for the Pd6O2+ cluster. The barrier heights for CO oxidation on PdxO2+ (x = 4–5) clusters are in good agreement with the range of barrier heights of 16.14 to 20.75 kcal mol−1 reported previously for various supported systems and lower than the experimental barrier height of 23.06 kcal mol−1 on the bulk Pd(111) surface. The present study predicted the complete reaction mechanisms of the catalytic cycle and compared the size dependency of the CO oxidation barrier for free PdxO2+ (x = 4–6) clusters. It is observed that Pd4O2+ and Pd5O2+ are more suitable catalysts compared to Pd6O2+.

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.