Construction of a series of pH stable Ca-based MOFs, their CO2 adsorption and catalytic activity.

In this study, three different solvent systems have been employed to investigate the effect of reaction parameters on the synthesis of four alkaline earth metal-based MOFs namely [Ca(0.5 1,4-phenyl diacetic acid)2(H2O)DMF]∞ (Ca-MOF-1), [Ca(1,4-naphthalene dicarboxylate)DMF]∞ (Ca-MOF-2), [Ca2(0.5 1,2,4,5-benzene tetracarboxylate)2(H2O)3DMF]∞ (Ca-MOF-3) and [Ca2(2,6-naphthalene dicarboxylate)2(H2O)6]∞ (Ca-MOF-4). The crystal structures of these four MOFs have been resolved through single crystal X-ray analysis and the bulk phase purity of these MOFs was assessed using PXRD and FT-IR analysis. To check the stability of these MOFs, thermogravimetric analysis (TGA) was carried out. To analyze the robustness of these MOFs, the PXRD of the samples was also collected at different pH levels. These MOFs were further explored as Lewis acid catalysts for the alcoholysis of epoxides and the activity of these catalysts depend on the open metal sites present in the MOFs. The catalytic activity follows the order: Ca-MOF-2 > Ca-MOF-4 > Ca-MOF-1 > Ca-MOF-3. The activity was also checked with various epoxide substrates using Ca-MOF-2. Density functional theory (DFT) calculations also support this trend with the help of the thermodynamic feasibility of epoxide binding, considering model MOF structures. The weak interaction between the epoxide oxygen and the metal centre of the most stable MOF structure has also been clarified by computational studies.

UV-assisted photochemical transformation of a tetranuclear copper(II) complex: a DFT supported study on ß-lactamase inhibitory activity towards antibiotic resistance.

Herein, we present a dark-green crystalline tetranuclear Cu(II) Schiff base complex {C1 = [Cu4L4](ClO4)4(DMF)4(H2O)} using a N,N,O donor ligand (HL), namely 2-(((2-hydroxypropyl)imino)methyl)-6-methoxyphenol. Spectro-photometrical investigation on the ß-lactamase-like activity of this coordinately saturated system revealed its catalytic inefficiency towards hydrolysis of nitrocefin as a model substrate. This complex has attracted significant interest as a promising photo-catalyst owing to its narrow band gap (2.40 eV) as predicted from DFT calculations and its higher responsivity towards UV light. Therefore, C1 is effectively involved in the photocatalytic reduction of perchlorate to Cl- in the presence of a hole scavenger (H2O-MeOH) under prolonged UV irradiation and itself becomes photo-cleaved to yield a new dark-brown colored chlorobridged dinuclear crystalline complex C2 {[CuL(H2O)2Cl3]H2O}. Furthermore, C2 was deployed as a functional ß-lactamase model and was found to show a remarkable catalytic proficiency towards the hydrolysis of nitrocefin in 70 : 30 (V/V) MeOH-H2O medium. This pro-catalyst C2 has been speculated to generate an aqua bridged active catalyst that plays a crucial factor in hydrolysis. This phenomenon was again experimentally established by potentiometric pH titration where C2 displays only one pKa value (7.11) in the basic pH range, indicating the deprotonation of the bridged water molecule. Based on several other kinetic studies, it may be postulated that the hydrolysis of nitrocefin is initiated by the nucleophilic attack of a bridging hydroxide, followed by very fast protonation of the intermediate to furnish the hydrolyzed product. It is noteworthy that the rate of nitrocefin hydrolysis is greatly inhibited in the presence of external chloride concentration. To the best of our knowledge, this is the first report on the photochemical behavior of such a tetranuclear copper(II) Schiff base complex. Our current interest is focused on inventing a potent ß-lactamase inhibitory therapeutic as well as elucidating its mechanism through comprehensive chemical analysis.

Superalkalis with Hydrogen as Central Electronegative Atom and their Possible Applications: Ab Initio and DFT Study.

Superalkalis are unusual species having ionization energies lower than that of the alkali metals. These species with various applications are of great importance in chemistry due to their low ionization energies and strong reducing property. A typical superalkali contains a central electronegative core decorated with excess metal ligands. In the quest for novel superalkalis, we have designed the superalkalis HLi2, HLiNa and HNa2 using hydrogen as central electronegative atom for the first time employing high level ab initio (CCSD(T), MP2) and density functional theory (ωB97X-D) methods. The superalkalis exhibit very low ionization energies, even lower than that of cesium. Stability of these species is verified from binding energy and dissociation energy values. The superalkalis are capable of reducing SO2, NO, CO2, CO and N2 molecules by forming stable ionic complexes and therefore can be used as catalysts for the reduction or activation of systems possessing very low electron affinities. The superalkalis form stable supersalts with tailored properties when interact with a superhalogen. They also show remarkably high non-linear optical responses, hence could have industrial applications. It is hoped that this work will enrich the superalkali family and spur further theoretical and experimental research in this direction.

Enhancement of nutritional quality in maize kernel through marker-assisted breeding for vte4, crtRB1, and opaque2 genes.

Traditional maize is poor in vitamin-E [α-tocopherol (α-T): 6-8 ppm], vitamin-A [provitamin-A (proA): 1-2ppm], lysine (0.150-0.2-50%), and tryptophan (0.030-0.040%). Here, we combined favourable alleles of vte4, crtRB1, and opaque2 (o2) genes in the parents of maize hybrids, viz., APQH-10 (PMI-PV-9 × PMI-PV-14) and APQH-11 (PMI-PV-9 × PMI-PV-15) using molecular breeding. Gene-specific markers were successfully used to select vte4, crtRB1, and o2 in BC1F1, BC2F1, and BC2F2 generations. Simple sequence repeats (104-109) were used for background selection, leading to an average recovery of 94% recurrent parent genome. The introgressed inbreds possessed significantly higher α-T: 18.38 ppm, α-/γ-tocopherol (α-/γ-T: 52%), and α-/total tocopherol (α-/TT: 32%) compared to original inbreds (α-T: 8.17 ppm, α-/γ-T: 25%, α-/TT: 18%). These newly derived inbreds also possessed higher ß-carotene (BC: 8.91 ppm), ß-cryptoxanthin (BCX: 1.27 ppm), proA (9.54 ppm), lysine (0.348%), and tryptophan (0.082%) compared to traditional maize inbreds. The reconstituted hybrids recorded higher α-T (2.1-fold), α-/γ-T (1.9-fold), and α-/TT (1.6-fold) over the original hybrids. These reconstituted hybrids were also rich in BC (5.7-fold), BCX (3.3-fold), proA (5.3-fold), lysine (1.9-fold), and tryptophan (2.0-fold) over the traditional hybrids. The reconstituted hybrids had similar grain yield and phenotypic characteristics to original versions. These multinutrient-rich maize hybrids hold great potential to alleviate malnutrition in sustainable and cost-effective manner.

New kind of electride sandwich complexes based on the cyclooctatetraene ligand M12(η8-C8H8)2M22 (M1 = Na, K and M2 = Ca, Mg): a theoretical study.

In the present study, the electronic structures of a series of binuclear sandwich complexes based on the cyclooctatetraene ligand M12(η8-C8H8)2M22 (M1 = Na, K and M2 = Ca, Mg) are studied theoretically. Each cyclooctatetraene ligand binds with the metal in the η8 binding mode. The M2-M2 bond length agrees well with the reported bimetallic covalent Ca2 and Mg2 bond lengths. The Wiberg bond index (WBI) also indicates the presence of covalent M2-M2 bonds, which gives additional stability to the complex. A non-nuclear attractor (NNA) is found in-between the M2-M2 bond and the negative Laplacian of the electron density is found at the NNA. Noncovalent interaction (NCI) plot shows that electron density is localized at the M2-M2 bond. Based on the performed analysis, we have concluded that the designed sandwich complexes are electrides. We herein report, for the first time, the electride sandwich complexes of the cyclooctatetraene ligand. Due to the presence of a diffuse electron system, the electride complexes exhibit higher values of the static second hyperpolarizability within the range of 2.6 × 105 to 1.4 × 106 a.u. Among the studied complexes, M12(η8-C8H8)2Ca2 exhibit a higher value of static second hyperpolarizability.

Dual modification to stabilize Non-IPR C72 fullerene: A new theoretical strategy.

The stabilization of non-IPR fullerenes for their isolation and characterization is an area of recent interest. In the present study, we have explored the stabilization techniques of C72 isomers via endo and exo-modifications and finally approached dual modification. A total of four isomers of C72 have been considered in this study; among them, one is IPR derivative (1), and the rest are non-IPR derivatives with one (2) and two (3 and 4) fused pentagon rings. First, we have studied the endohedral modification by encapsulating one and two La atoms in the C72 cavity. Secondly, we have exohedrally modified the C72 isomers via chlorination by adding four and eight chlorides, respectively. Our final approach is to study the dual modification, where we have implemented both endo exo-modifications together. This dual modification can be achieved in two ways: exo followed by endo and endo followed by exo. For each modification, the relative stability of every modified C72 derivative has been checked by calculating the relative energy with respect to the most stable modified analogue. To find out whether these modifications are energetically feasible or not, we have calculated the binding energy of each modified C72 isomer. The binding energy calculation reveals that the encapsulation and exo-modification techniques are good enough to stabilize the non-IPR C72 derivatives. Moreover, the effectiveness of dual modification has also been established from the enhanced binding energy compared to either endo- or exo-modification. We have also studied the NPA charges on the encapsulated La atoms for each endo- and dual-modified C72 derivative. Furthermore, the AIM study has also been perceived to find out the interaction between the La atom and the fullerene cages for both mono- and di-encapsulated fullerene derivatives and also between La-La centres for di-encapsulated derivatives. Overall, the present theoretical study will provide an idea about the stability of the modified C72 derivatives, which will help the experimentalists to design new strategies for synthesizing modified non-IPR fullerene derivatives that have vast applications in the medicinal and industrial fields.

ZnAl2O4 Nanomaterial as a Naked-Eye Arsenate Sensor: A Combined Experimental and Computational Mechanistic Approach.

Raising public awareness over the emerging health risk due to intake of arsenic-contaminated potable water is a matter of great concern. Exploration of cost-effective, self-testing kits is a substantial way to reach out to the masses and detect the presence of arsenate in water. With this agenda, a photoluminescent Mannich base Zn(II) complex (ZnMC = [Zn2(ML)2]·(ClO4)2·(H2O); HML = Mannich base ligand) has been synthesized, and its dinuclearity was verified with single-crystal X-ray diffraction structural analysis. Among a range of anions, ZnMC was found to detect arsenate selectively by showing a turn-off emission with a color change from bright green to dark under UV light. The real-life applicability of the ZnMC probe is somewhat restricted to only sensing of arsenate, but not its removal owing to the fact of its homogeneity. Considering the efficacy of ZnMC as well as a need for its easy removal from water, slight modification has been done with chloride ions in the form of ZnMC″ (=[Zn2(ML)2(Cl)2]), and finally, an interface between homogeneous and heterogeneous solid support has been explored with a strategic fabrication of ZnMC″ grafted ZnAl2O4, named as ZAZ nanomaterial. This not only imparts successful segregation of arsenate from drinking water but also provides naked-eye detection under ambient light as well as UV light. Thermodynamic parameters associated with the binding of arsenate to ZnMC and ZAZ have been evaluated through isothermal calorimetric (ITC) measurements. Steady-state and time-resolved fluorescence titration study, absorption titration study, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and computational calculations have been performed to get deep insights into the sensing properties. Proper justification of the sensing mechanism is the highlight of this work. ZAZ nanomaterial has been exploited to produce a self-test paper kit for arsenate detection with a limit of 9.86 ppb, which potentially enables applications in environmental monitoring.

Effect of O-substitution in imidazole based Zn(II) dual fluorescent probes in the light of arsenate detection in potable water: a combined experimental and theoretical approach.

Efficient detection of arsenate (AsO43-) from contaminated drinking water extracted from underground has become a matter of utmost necessity and an exquisite challenge owing to the growing public health issue due to arsenicosis. In order to combat this we planned to detect arsenate with the naked eye under UV light using a novel chemosensor material whose structure and functioning as a sensor could be certified mechanistically. Hence we were encouraged to synthesize two differently O-substituted imidazole based homologous ligands: C1 (HL1 = 2-((E)-(3-(1H-imidazole-1-yl)propylimino)methyl)-6-ethoxyphenol) and C2 (HL2 = 2-((E)-(3-(1H-imidazole-1-yl)propylimino)methyl)-6-methoxyphenol). To accomplish the purposeful exploration of the luminescent sensor, we considered Chelation Enhanced Fluorescence (CHEF) and kept on searching for a metal cation that would be able to turn on the fluorescence of the ligands. Considering Zn(II) as the most suitable candidate, luminescent complexes D1 and D2 ({[Zn2(L1)2(I)2](DMF)} and [Zn2(L2)2(I)2](DMF), respectively) were synthesized and characterized by SXRD, UV-Vis, FT-IR, and photoluminescence spectroscopy. In spite of the resemblance in the solid state structures of D1 and D2, the selective response of D1 towards arsenate with high quenching constants (2.13 × 106), unlike D2, has been demonstrated mechanistically with steady state and time resolved fluorescence titration, solution phase ESI-MS spectral analysis and DFT studies. The selectivity and sensitivity of the sensor D1 explicitly make this material a potent candidate for arsenate detection due to its very low detection limit (8.2 ppb), low cost and user friendly characteristics. Real life implementation of this work in a test strip is expected to prove beneficial for public health to identify arsenate polluted water.

Dehydrogenation of ammonia-borane to functionalize neutral and Li+-encapsulated C60, C70 and C36 fullerene cages: a DFT approach.

Mechanistic investigations into the functionalization of three fullerene cages, viz. C60, C70, and C36 through dehydrogenation of ammonia-borane (AB) have been conducted using Density Functional Theory (DFT). In this process of functionalization, different ring fusions, namely (6-6), (6-5) positions for C60 and C70, and an additional (5-5) for C36 fullerene have been investigated. The optimized geometries of all the complexes and transition states have been characterized using the M06-2X functional in conjunction with the 6-31G(d) basis set. The effect of Li+-encapsulation on the energetics and activation barriers of H2 attachment has also been examined. Although the process of functionalization of neutral fullerenes proceeds extensively through concerted pathways, a step-wise route has been observed for the encapsulated systems. NPA charge analysis and Wiberg bond index (WBI) have been used in order to detect the change in the nature of participating hydrogen atoms and validate the variation in the bond order of the C-C connectivity respectively upon hydrogenation. GCRD parameters have also been calculated to explicate the electronic properties of the hydrogenated products. The (6-6) hydrogenation is observed to be favoured thermodynamically and kinetically for both neutral and Li+-encapsulated C60 and C70, while (5-5) is found to be the most preferred site for C36 systems. Our theoretical exploration suggests that the covalent functionalization of the fullerene cages can be done successfully viaAB resulting in the stabilization of these systems. In short, the present work will provide a general idea about the detailed mechanism related to the functionalization of fullerene cages, which will further motivate researchers in fullerene chemistry.

Theoretical study of gas-phase detoxication of DMMP and DMPT using ammonia-borane and its analogous compound.

The detoxication of DMMP (Dimethyl methylphosphonate) and DMPT (O, S-dimethyl methylphosphonothiolate) via hydrogenation have been investigated computationally employing density functional theory (DFT). In this present study, we aim to explore the direct molecular H2 assisted as well as ammonia-borane (NH3BH3) and 3-methyl-1,2-BN-cyclopentane (denoted as cy-AB) assisted hydrogenation pathways of DMMP and DMPT in order to detoxify them. The detoxication of DMMP has been carried out by successive elimination of two -OMe groups. However, in the case of DMPT, two possibilities have been identified because of two different substituents, -OMe and -SMe. In possibility-I, the elimination of the -OMe group occurs at the beginning, followed by the -SMe group, whereas in possibility-II, the reverse order of elimination occurs for -OMe and -SMe groups. During the detoxication of DMMP using both NH3BH3 and cy-AB as the assisting reagents, the first step has been identified as the rate-determining step (RDS) in which the hydrogens attached to the N- and B-centers of NH3BH3 are transferred to the O-center of PO and P-center, respectively. In harmony with DMMP detoxication, for DMPT also, analyzing the activation barriers, it can be articulated that for both NH3BH3 and cy-AB assisted pathways, both the possibilities are equally feasible as in both the possibilities the common first step is the RDS. Therefore, our computational study is designed to explore the assisting efficiency of NH3BH3 and its cyclic analogue for detoxifying the OPCs.

Development of Maize Hybrids With Enhanced Vitamin-E, Vitamin-A, Lysine, and Tryptophan Through Molecular Breeding.

Malnutrition is a widespread problem that affects human health, society, and the economy. Traditional maize that serves as an important source of human nutrition is deficient in vitamin-E, vitamin-A, lysine, and tryptophan. Here, favorable alleles of vte4 (α-tocopherol methyl transferase), crtRB1 (ß-carotene hydroxylase), lcyE (lycopene ε-cyclase), and o2 (opaque2) genes were combined in parental lines of four popular hybrids using marker-assisted selection (MAS). BC1F1, BC2F1, and BC2F2 populations were genotyped using gene-based markers of vte4, crtRB1, lcyE, and o2. Background selection using 81-103 simple sequence repeats (SSRs) markers led to the recovery of recurrent parent genome (RPG) up to 95.45%. Alpha (α)-tocopherol was significantly enhanced among introgressed progenies (16.13 µg/g) as compared to original inbreds (7.90 µg/g). Provitamin-A (proA) (10.42 µg/g), lysine (0.352%), and tryptophan (0.086%) were also high in the introgressed progenies. The reconstituted hybrids showed a 2-fold enhancement in α-tocopherol (16.83 µg/g) over original hybrids (8.06 µg/g). Improved hybrids also possessed high proA (11.48 µg/g), lysine (0.367%), and tryptophan (0.084%) when compared with traditional hybrids. The reconstituted hybrids recorded the mean grain yield of 8,066 kg/ha, which was at par with original hybrids (mean: 7,846 kg/ha). The MAS-derived genotypes resembled their corresponding original hybrids for the majority of agronomic and yield-related traits, besides characteristics related to distinctness, uniformity, and stability (DUS). This is the first report for the development of maize with enhanced vitamin-E, vitamin-A, lysine, and tryptophan.

Atmospheric Fate of Criegee Intermediate Formed During Ozonolysis of Styrene in the Presence of H2O and NH3: The Crucial Role of Stereochemistry.

A gas-phase mechanistic investigation of the unimolecular, water/ammonia-assisted decomposition reactions of the α-hydroxy hydroperoxides (HPs) and hydroperoxide arylamines (a-HPs) produced during the styrene ozonolysis has been carried out theoretically in the present article. The instrumental role of stereochemistry in controlling the outcome of individual reactions has been discussed. Thermodynamic parameters (Δ G298K, Δ H298K, Δ E0K) associated with individual reactions have also been computed. The rate constants estimated for individual reactions using conventional transition state theory (TST) combined with statistical mechanics provide a comprehensive understanding of the reaction mechanism and also elucidate the atmospheric fate of Criegee intermediates. Considering the feasibility of reactions from thermodynamic and kinetic points of view, while aldehyde (PhCHO) formation pathway originating from bimolecular decomposition of HP is found to be kinetically favored, benzoic acid formation pathway remains favored thermodynamically. A similar consideration for the bimolecular reactions of a-HP reveals the phenylmethanimine formation pathway to be kinetically favored, while the benzamide formation pathway is favored thermodynamically. Our findings appear to be in excellent agreement with the experimental observations.

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 ß-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.

Exploration of Binding Interactions of Cu(2+) 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.

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.

Decomposition of O,S-dimethyl methylphosphonothiolate by ammonia on magnesium oxide: a theoretical study of catalytic detoxification of a chemical warfare agent.

The adsorption of a model nerve agent, O,S-dimethyl methylphosphonothiolate (DMPT), on the hydroxylated and unhydroxylated nano-crystalline magnesium oxide surface followed by the nucleophilic attack of ammonia (NH3) is investigated at the M06-2X/6-311++G(d,p) level of theory using the representative cluster models. The geometries of DMPT and NH3 are fully optimized, while the geometry of the oxide fragment is kept frozen. The main insight of this study is the incorporation of the Eley-Rideal mechanism for the first time in the detoxification process, where one of the reactant molecules (DMPT) is adsorbed and the other one (NH3) reacts with it directly impinging from the gas phase. There are two possible pathways of nucleophilic detoxification, either concerted or stepwise. The nature of the first transition state of nucleophilic attack in both pathways is the vital step for degradation. Our calculated results predict that the reaction of DMPT with NH3 gives rise to both P-S and P-O bond cleavage completely. Also, the P-S cleavage is found to be the favorable one over P-O bond breaking. The exploration of the overall reaction mechanism has established the catalytic activity of nano-crystalline MgO in nucleophilic DMPT degradation, as in all cases the activation barriers have reduced compared to the previously reported aminolysis of DMPT in the gas phase. Interestingly, the hydroxylated model has better catalytic performance than the unhydroxylated one.

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.

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 ß-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 ß-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 [SiO(n)+ (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.

Performance of dispersion-corrected double hybrid density functional theory: a computational study of OCS-hydrocarbon van der Waals complexes.

The performance of double hybrid density functionals (DHDFs) has been assessed by studying the spectroscopic properties and potential energy curves of OCS-C2H4 (carbonyl sulfide-ethylene) and OCS-C4H6 (carbonyl sulfide-dimethylacetylene) van der Waals complexes. Both dispersion corrected and uncorrected DHDF theories have been applied to study the intermolecular interaction energies, stability, spectroscopic parameters, rigidity, and binding energies or depths of the potential well of the weakly bound complexes and also to explore the possibility of formation of three isomers of each complex. The correlation consistent valence triple zeta quality basis set is used to investigate the complexes. The calculated results provide insight into the computational methods applied to the weakly bound complexes. The double hybrid density functional B2PLYP and mPW2PLYP methods with dispersion corrections (B2PLYP-D2, B2PLYP-D3 and mPW2PLYP-D2, mPW2PLYP-D3) performed better over the B2PLYP and mPW2PLYP density functional methods without dispersion correction to deal with the weak dispersion interaction that prevails in these complexes. The results obtained by the dispersion-corrected density functional mPW2PLYP-D2 and mPW2PLYP-D3 methods agree very well with the earlier experimental values wherever available. The contributing components of the interaction energy have been analyzed by the symmetry-adapted perturbation theory (SAPT, here, SAPT0) to get insight into the interaction energy.