RESUMO
Degradation of the mechanical properties of α-titanium, which is used to manufacture parts of jet engines, due to high-temperature oxidation is detrimental for the engine components. Therefore, to overcome this problem there are ongoing endeavors to develop novel oxidation-resistant titanium alloys and improve the properties of the existing ones. In an effort to understand the effect of alloying on oxidation of the α-Ti(0001) surface and to identify descriptors for rational design of oxidation-resistant alloys, in this work, using density functional theory-based calculations, we studied oxygen sorption and surface to subsurface diffusion on pure and alloyed α-Ti(0001) surfaces. Zr, Hf, Nb, and Mo from the d block and Al, Ga, Si, and Ge from the p block were used as alloying elements. We find that the alloying elements prefer to segregate on the surface compared to the subsurface layers. Our calculations show that the diffusion barrier correlates with the difference in the electronegativity between the alloying element and Ti. Elements which are more electropositive than Ti are found to hinder the oxygen dissolution in Ti and vice versa. We propose that the electronegativity difference can act as a good descriptor for choosing alloying elements. Our results are in reasonably good agreement with experimental reports on the growth of oxide layers on these alloyed Ti surfaces.
RESUMO
Abatement of CO, due to its poisonous nature, is an extensively researched topic. Oxidation to CO2 is one of the strategies deployed and finds application in automobiles and fuel cells. Gold nanoparticles on an oxide support is a pioneering catalyst in this field, but need improvement in cost, stability, and O2 activation. Doping with Cu can open up avenues for improvement in these attributes. In the present investigation, we have explored the possibility of using bimetallic AunCum (n + m = 4) clusters supported on Ti2CO2 MXene. We find that AuCu3 is the most stable cluster on the support. The complete CO oxidation cycle on this supported cluster proceeds through a mix of Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms. Our calculations predict that the first cycle is expected to proceed only via the LH mechanism due to kinetic and thermodynamic limitations ascribed to ER and Mars van Krevelen (MvK) mechanisms, respectively. The second cycle, however, prefers ER over the LH mechanism. Overall, with the highest barrier of 0.56 eV, this low cost novel catalyst performs better in terms of stability and/or activity in comparison with many of the catalysts reported in the literature.
RESUMO
Thermoelectric materials are used for the conversion of waste heat to electrical energy. The transport coefficients that determine their thermoelectric properties depend on the band structure and the relaxation time of the charge carriers. Both of these are significantly affected by electron-phonon coupling. In this work, using a combination of density functional theory and semiclassical Boltzmann transport theory, we have studied the effect of electron-phonon coupling in monolayers of ZrS2, BiI3 and PbI2. Our results show that in these ionic materials charge carriers are primarily scattered by the optical modes that strongly couple with them. From our study it is conclusively shown that neglecting the contributions of optical modes to electron-phonon coupling in these low-dimensional ionic solids, as is usually done in the computation of relaxation time from deformation theory, results in severe overestimation of the relaxation time. In particular, neglecting the scattering of the optical phonons results in about two orders of magnitude overestimation of relaxation times in these materials. Moreover, we also find that the renormalization of the band structure not only results in the reduction of band gaps but also changes the band dispersion, which strongly affect different transport properties like the electrical conductivity and Seebeck coefficient. Amongst these three materials, we observe that carrier relaxation time due to electron-phonon scattering is reduced as the ionicity of the material decreases.
RESUMO
In the current context of green mobility and sustainability, the use of new generation natural fillers, namely, α-cellulose, has gained significant recognition. The presence of hydroxyl groups on α-cellulose has generated immense eagerness to map its potency as filler in an elastomeric composite. In the present work, α-cellulose-emulsion-grade styrene butadiene rubber (E-SBR) composite is prepared by conventional rubber processing method by using variable proportions of α-cellulose (1 to 40 phr) to assess its reinforce ability. Rheological, physical, visco-elastic and dynamic-mechanical behavior have clearly established that 10 phr loading of α-cellulose can be considered as an optimized dosage in terms of performance parameters. Morphological characterization with the aid of scanning electron microscope (SEM) and transmission electron microscopy (TEM) also substantiated that composite with 10 phr loading of α-cellulose has achieved the morphological threshold. With this background, synthetic filler (silica) is substituted by green filler (α-cellulose) in an E-SBR-based composite. Characterization of the compound has clearly established the reinforcement ability of α-cellulose.
Assuntos
Butadienos/química , Celulose/química , Elastômeros/química , Estirenos/química , Resinas Compostas/química , Emulsões/química , Dióxido de Silício/químicaRESUMO
Atmospheric CO2 is one of the main components of the greenhouse effect. To overcome this problem there are ongoing efforts to convert CO2 to some other useful and harmless products. The capture, activation and dissociation of CO2 are the preliminary steps in this process. In an effort to understand the role of surface composition and structure in CO2 adsorption and dissociation, in this work, with the help of first principles density functional theory based calculations, we have studied the same on the {100} surface of cubic Ti2C and MXene (also the {0001} surface of trigonal Ti2C). Our results show that CO2 undergoes barrierless chemisorption on both of these surfaces with a preference towards {100} cubic Ti2C. We attribute the reason for this to a lower value of the work function of the {100} surface. Furthermore, on MXene, the barrier for CO2 dissociation is lower compared to that on the {100} surface. Coverage dependent CO2 chemisorption studies on these two surfaces show that on the Ti2C surface the CO2 molecules form clusters around the C-vacancies while on MXene they are uniformly spread on the surface.
RESUMO
Pt-water interfaces have been of immense interest in the field of energy storage and conversion. Studying this interface using both experimental and theoretical tools is challenging. On the theoretical front, typically one uses classical molecular dynamics (MD) simulations to handle large system sizes or time scales while for a more accurate quantum mechanical description Born Oppenheimer MD (BOMD) is typically used. The latter is limited to smaller system sizes and time-scales. In this study using quantum-mechanics-molecular-mechanics (QMMM), we have performed atomistic MD simulations to have a microscopic understanding of the structure of the Pt-water interface using a system size that is much larger than that accessible when using BOMD simulations. In contrast to recent reports using BOMD simulations, our study reveals that the water molecules typically form two distinct layers above the Pt-surface before they form bulk like structures. Further, we also find that a significant fraction of the water molecules at the interface are pointed towards the surface thereby disrupting the H-bond network. Consistent with this observation, the layer resolved oxygen-oxygen radial distribution function for the water molecules belonging to the solvating water layer shows a high density liquid like behaviour even though the overall water behaves like a low density liquid. A charge transfer analysis reveals that this solvating water layer donates electrons to the Pt atoms in contact with it thereby resulting in the formation of an interface dipole that is pointing towards the surface. Our results suggest that, using QMMM-MD, on one hand it is possible to study more realistic models of solid-liquid interfaces that are inaccessible with BOMD, while on the other hand one also has access to information about such systems that are not obtained from conventional classical MD simulations.
RESUMO
Traction forces exerted by adherent cells are quantified using displacements of embedded markers on polyacrylamide substrates due to cell contractility. Fourier Transform Traction Cytometry (FTTC) is widely used to calculate tractions but has inherent limitations due to errors in the displacement fields; these are mitigated through a regularization parameter (γ) in the Reg-FTTC method. An alternate finite element (FE) approach computes tractions on a domain using known boundary conditions. Robust verification and recovery studies are lacking but essential in assessing the accuracy and noise sensitivity of the traction solutions from the different methods. We implemented the L2 regularization method and defined a maximum curvature point in the traction with γ plot as the optimal regularization parameter (γ*) in the Reg-FTTC approach. Traction reconstructions using γ* yield accurate values of low and maximum tractions (Tmax) in the presence of up to 5% noise. Reg-FTTC is hence a clear improvement over the FTTC method but is inadequate to reconstruct low stresses such as those at nascent focal adhesions. FE, implemented using a node-by-node comparison, showed an intermediate reconstruction compared to Reg-FTTC. We performed experiments using mouse embryonic fibroblast (MEF) and compared results between these approaches. Tractions from FTTC and FE showed differences of â¼92% and 22% as compared to Reg-FTTC. Selection of an optimum value of γ for each cell reduced variability in the computed tractions as compared to using a single value of γ for all the MEF cells in this study.
RESUMO
A density functional theory (DFT) investigation performed at the B3LYP/TZVP//B3LYP/6-31G(d)-LANL2DZ level of theory on the hydroamination of dimethylamine (Me2NH) on an activated olefin (namely, acrylonitrile (CH2âCHCN)), as catalyzed by a 1,2,4-triazol based nickel(II) N-heterocyclic carbene complex (namely, [1,4-dimethyl-1,2,4-triazole-5-ylidene]2 nickel dichloride) revealed that the olefin coordination pathway is favorable over the amine coordination pathway, although the initial olefin coordination step is higher in energy than the initial amine coordination step. Significantly enough, the reaction involved a crucial 1,3-proton transfer step between the resonance intermediates, i.e., the C-bound [(NHC)2Ni(CH(CN)CH2NHMe2)]+ (D) species or N-bound [(NHC)2Ni(NCCHCH2NHMe2)]+ (E) species and the intermediate [(NHC)2Ni(NCCH2CH2NMe2)]+ (F), depicting the cleavage of a N-H bond and the formation of a C-H bond facilitated by a water-assisted/amine-assisted proton shuttle. Overall, among the various pathways explored, the lowest energy pathway involved alkene coordination, followed by an amine-assisted 1,3-proton transfer step.
RESUMO
Using a glycerine-water solution with various concentrations, we investigate the dispersion characteristics of photonic crystal fibers by selective filling of holes. Our analysis is based on a simple but accurate semi-vectorial solution of Helmholtz's equation by the finite difference method devised with a mode-field convergence technique and crosschecked by results with those from a deeply involved multipole method. Significantly, a better ultra-flatness but near-zero group velocity dispersion is revealed with a 20% glycerine-water solution that is superior to pure water of a very recent case when the holes of the first ring of the fiber are filled. This versatile effect in management of holes of identical diameter with liquid is expected to play a guiding role in studies of supercontinuum generation.
RESUMO
Colloidal CsPbBr3 perovskite nanocrystals (NCs) have emerged as an excellent light emitting material in last one year. Using time domain and time-resolved THz spectroscopy and density functional theory based calculations, we establish 3-fold free carrier recombination mechanism, namely, nonradiative Auger, bimolecular electron-hole recombination, and inefficient trap-assisted recombination in 11 nm sized colloidal CsPbBr3 NCs. Our results confirm a negligible influence of surface defects in trapping charge carriers, which in turn results into desirable intrinsic transport properties, from the perspective of device applications, such as remarkably high carrier mobility (â¼4500 cm(2) V(-1) s(-1)), large diffusion length (>9.2 µm), and high luminescence quantum yield (80%). Despite being solution processed and possessing a large surface to volume ratio, this combination of high carrier mobility and diffusion length, along with nearly ideal photoluminescence quantum yield, is unique compared to any other colloidal quantum dot system.
RESUMO
Well-defined palladium N-heterocyclic carbene (NHC) complexes were employed in the one-pot tandem Heck alkynylation/cyclization sequence for preparing biologically relevant benzofuran compounds under copper-free conditions in a time-efficient step-reduced fashion. In particular, a series of binuclear palladium complexes, 1b-1e and 2b-2e, of the alkyl-bridged NHC ligands, namely, {1,1'-di-R1-4,4'-R2-di-1,2,4-triazoline-5,5'-diylid-2-ene] (R1 = i-Pr; R2 = -(CH2)2-, -(CH2)3-), and their mononuclear analogues, trans-(NHC)PdBr2(pyridine) (3b) and cis-(NHC)PdBr2(PPh3) (3c), successfully catalyzed the one-pot tandem Heck alkynylation/cyclization reaction of 2-iodophenol with a variety of terminal alkyne substrates, yielding 2-substituted benzofuran derivatives. The mononuclear complexes 3b and 3c were nearly half as active as the representative dinuclear analogue 1c under analogous reaction conditions, thereby implying that, at the same mole percent of the palladium loading, the monometallic 3b and 3c and the bimetallic 1c complexes were equally effective as catalysts. The two sites of the bimetallic complex 1c performed as two separate independent catalytic sites, displaying no cooperativity effect in the catalysis. Finally, the practical utility of the aforementioned catalysts was demonstrated for a representative catalyst 1c through the convenient synthesis of a key intermediate, 3-[2-(benzo[d][1,3]dioxol-5-yl)-7-methoxybenzofuran-5-yl]propan-1-ol, in a total-synthesis protocol of the natural product Egonol.
Assuntos
Alcinos/química , Benzofuranos/química , Compostos Heterocíclicos/química , Metano/análogos & derivados , Paládio/química , Ciclização , Espectroscopia de Ressonância Magnética , Metano/química , Modelos Moleculares , Espectrometria de Massas por Ionização por ElectrosprayRESUMO
Ellipticine is a natural product that is currently being actively investigated for its inhibitory cancer and HIV properties. Here we use path-integral molecular dynamics coupled with excited state calculations to characterize the role of nuclear quantum effects on the structural and electronic properties of ellipticine in water, a common biological solvent. Quantum effects collectively enhance the fluctuations of both light and heavy nuclei of the covalent and hydrogen bonds in ellipticine. In particular, for the ellipticine-water system, where the proton donor and acceptor have different proton affinities, we find that nuclear quantum effects (NQEs) strengthen both the strong and the weak H bonds. This is in contrast to what is observed for the cases where the proton affinity of the donors and acceptors is same. These structural fluctuations cause a significant red-shift in the absorption spectra and an increase in the broadening, bringing it into closer agreement with the experiments. Our work shows that nuclear quantum effects alter both qualitatively and quantitatively the optical properties of this biologically relevant system and highlights the importance of the inclusion of these effects in the microscopic understanding of their optical properties. We propose that isotopic substitution will produce a blue shift and a reduction in the broadening of the absorption peak.
Assuntos
Fármacos Anti-HIV/química , Antineoplásicos/química , Elipticinas/química , Teoria Quântica , Absorção Fisico-Química , Fármacos Anti-HIV/farmacologia , Antineoplásicos/farmacologia , Elipticinas/farmacologia , Conformação Molecular , Simulação de Dinâmica Molecular , Solventes/química , Água/químicaRESUMO
Based on a simple but accurate semivectorial solution of Helmholtz's equation by the finite difference method devised with a mode-field convergence technique, we have shown an interesting and significant effect showing an almost ultraflat zero group velocity dispersion in photonic crystal fiber when the holes of the first ring of the fiber are filled with water. Crosschecking our results with earlier results involving a deeply involved multipole method for the central core of photonic crystal fiber filled with water and fused silica, our observation in the case of filling the first ring holes with water reveals potential information in studies of supercontinuum generation.
RESUMO
High temperature ethanol fermentation from sugarcane molasses B using thermophilic Crabtree-positive yeast Kluyveromyces sp. IIPE453 was carried out in batch bioreactor system. Strain was found to have a maximum specific ethanol productivity of 0.688 g/g/h with 92 % theoretical ethanol yield. Aeration and initial sugar concentration were tuning parameters to regulate metabolic pathways of the strain for either cell mass or higher ethanol production during growth with an optimum sugar to cell ratio 33:1 requisite for fermentation. An assessment of ethanol recovery from fermentation broth via simulation study illustrated that distillation-based conventional recovery was significantly better in terms of energy efficiency and overall mass recovery in comparison to coupled solvent extraction-azeotropic distillation technique for the same.
Assuntos
Adaptação Fisiológica , Etanol/metabolismo , Fermentação , Temperatura Alta , Kluyveromyces/metabolismo , Melaço , Biomassa , Kluyveromyces/crescimento & desenvolvimento , Kluyveromyces/fisiologia , SolventesRESUMO
The coupling of aryl and aliphatic azides with isocyanides yielding carbodiimides (8-17) were efficiently catalyzed by well-defined structurally characterized trans-(MIC)PdI2(L) [MIC = 1-CH2Ph-3-Me-4-(CH2N(C6H4)2S)-1,2,3-triazol-5-ylidene, L = NC5H5 (4), MesNC (5)], trans-(MIC)2PdI2 (6), and cis-(MIC)Pd(PPh3)I2 (7) type palladium complexes, which incidentally mark the first instances of the use of mesoionic singlet palladium carbene complexes for the said application. As observed from the product yields, the catalytic activity varied in the order 4 > 5 â¼ 6 > 7 for these complexes. A detailed mechanistic studies indicated that the catalysis proceeded via a palladium(0) (4a-7 a) species. Using a representative palladium precatalyst (4), the azide-isocyanide coupling was successfully extended to synthesizing two different bioactive heteroannular benzoxazole (18-22) and benzimidazole (23-27) derivatives, thereby broadening the scope of the catalytic application.
RESUMO
Heterocyclic polynuclear aromatic hydrocarbons (PAH) are characterized by higher aqueous solubility and enhanced bioavailability due to presence of nitrogen, sulfur or oxygen heteroatoms in their chemical structure and are referred to as nitrogen (PANH), sulfur (PASH) and oxygen (PAOH) heterocyclic PAHs, respectively. Inspite of their significant ecotoxicity and human health impacts, these compounds have not yet been included in the U.S. EPA's list of "priority PAH". The current paper presents a comprehensive review of the environmental fate, various detection techniques and toxicity of heterocyclic PAH compounds, highlighting their significant environmental impacts. Heterocyclic PAHs have been detected at 0.03 to 11,000 ng/L in various aquatic bodies and at 0.1 to 3210 ng/g in contaminated land. PANHs are the most polar heterocyclic PAHs, having aqueous solubility at least 10 to 10,000 times higher than PAH, PASH, and PAOH compounds, which make them more bioavailable. Aquatic fate of heterocyclic PAHs is dominated by volatilization and biodegradation processes for low molecular weight (MW) compounds and photochemical oxidation for high MW compounds. Sorption of heterocyclic PAHs on soil is governed by partitioning to soil organic carbon, cation exchange, and surface complexation mechanisms for PANHs and non-specific interactions, such as van der Waals forces with soil organic carbon for PASHs and PAOHs. Various chromatographic and spectroscopic techniques, such as HPLC and GC, NMR, and TLC have been employed to elucidate their distribution and fate in the environment. PANHs are also the most acutely toxic heterocyclic PAHs with EC50 values ranging from 0.001 to 1100 mg/L in various species of bacteria, algae, yeast, invertebrate, and fish. Heterocyclic PAHs also induce mutagenicity, genotoxicity, carcinogenicity, teratogenicity, and phototoxicity in various aquatic and benthic organisms and terrestrial animals. Compounds, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) and some acridine derivatives are proven human carcinogens and several other heterocyclic PAHs are suspected human carcinogens.
Assuntos
Hidrocarbonetos Policíclicos Aromáticos , Poluentes do Solo , Animais , Humanos , Solo/química , Carbono , Hidrocarbonetos Policíclicos Aromáticos/análise , Invertebrados , Água/análise , Poluentes do Solo/análise , Carcinógenos/análiseRESUMO
Double proton transfers (DPTs) are important for several physical processes, both in molecules and in the condensed phase. While these have been widely studied in biological systems, their study in crystalline environments is rare. In this work, using path integral molecular dynamics simulations, we have studied temperature dependent DPT in molecular crystals of terephthalic acid (TPA). In accordance with experimental reports, we find evidence for a double proton transfer induced order-to-disorder transition that is sensitive to the inclusion of nuclear quantum effects. Our simulations show that in addition to the presence of L and R tautomers of terepthalic acid, there are a small but non-negligible concentration of positive and negatively charged pairs of TPA molecules. At the onset of the transition at low temperatures, DPT likely occurs through a tunneling mechanism while at room temperature, likely involving the dominance of activated hopping. Through an analysis of the electronic structure of the system using Wannier functions, we show that the H atom shuttling between the donor and acceptor O atoms involves a proton.
RESUMO
Hydrohydrazination of terminal alkynes with hydrazides yielding hydrazones 5-14 were successfully catalyzed by a series of gold(I) acyclic aminooxy carbene complexes of the type [{(4-R2-2,6-t-Bu2-C6H2O)(N(R1)2)}methylidene]AuCl, where R2 = H, R1 = Me (1b); R2 = H, R1 = Cy (2b); R2 = t-Bu, R1 = Me (3b); R2 = t-Bu, R1 = Cy (4b). The mass spectrometric evidence corroborated the existence of the catalytically active solvent-coordinated [(AAOC)Au(CH3CN)]SbF6 (1-4)A species and the acetylene-bound [(AAOC)Au(HC≡CPhMe)]SbF6 (3B) species of the proposed catalysis cycle. The hydrohydrazination reaction was successfully employed in synthesizing several bioactive hydrazone compounds (15-18) with anticonvulsant properties using a representative precatalyst (2b). The DFT studies favored the 4-ethynyltoluene (HC≡CPhMe) coordination pathway over the p-toluenesulfonyl hydrazide (NH2NHSO2C6H4CH3) coordination pathway, and that proceeded by a crucial intermolecular hydrazide-assisted proton transfer step. The gold(I) complexes (1-4)b were synthesized from the {[(4-R2-2,6-t-Bu2-C6H2O)(N(R1)2)]CH}+OTf- (1-4)a by treatment with (Me2S)AuCl in the presence of NaH as a base. The reactivity studies of (1-4)b yielded the gold(III) [{(4-R2-2,6-t-Bu2-C6H2O)(N(R1)2)}methylidene]AuBr3 (1-4)c complexes upon reaction with molecular bromine and the gold(I) perfluorophenylthiolato derivatives, [{(4-R2-2,6-t-Bu2-C6H2O)(N(R1)2)}methylidene]AuSC6F5 (1-4)d, upon treatment with C6F5SH.
RESUMO
Metallophilic interactions were observed in four pairs of 12-membered metallamacrocyclic silver and gold complexes of imidazole-derived N-heterocyclic carbenes (NHCs), [1-(R1)-3-N-(2,6-di-(R2)-phenylacetamido)-imidazol-2-ylidene]2M2 [R1 = p-MeC6H4, R2 = Me, M = Ag (1b) and Au (1c); R1 = Me, R2 = i-Pr, M = Ag (2b) and Au (2c); R1 = Et, R2 = i-Pr, M = Ag (3b) and Au (3c)], and a 1,2,4-triazole-derived N-heterocyclic carbene (NHC), [1-(i-Pr)-4-N-(2,6-di-(i-Pr)-phenylacetamido)-1,2,4-triazol-2-ylidene]2M2 [M = Ag (4b) and Au (4c)]. The X-ray diffraction, photoluminescence, and computational studies indicate the presence of metallophilic interactions in these complexes, which are significantly influenced by the sterics and the electronics of the N-amido substituents of the NHC ligands. The argentophilic interaction in the silver 1b-4b complexes was stronger than the aurophilic interaction in the gold 1c-4c complexes, with the metallophilic interaction decreasing in the order 4b > 1b > 1c > 4c > 3b > 3c > 2b > 2c. The 1b-4b complexes were synthesized from the corresponding amido-functionalized imidazolium chloride 1a-3a and the 1,2,4-triazolium chloride 4a salts upon treatment with Ag2O. The reaction of 1b-4b complexes with (Me2S)AuCl gave the gold 1c-4c complexes.
RESUMO
Lead-free halide perovskites have gained immense popularity in photovoltaic and energy harvesting applications because of their excellent optical and electrical attributes with minimal toxicity. We synthesized composite films of lead-free Cs3Bi2Br9 perovskite embedded in the polyvinylidene fluoride (PVDF) matrix and have investigated their piezoelectric energy harvesting. Five PVDF@Cs3Bi2Br9 composite films were fabricated with varying wt% of the perovskite in the PVDF. The composite with a 4 wt% of the perovskite shows 85% activation of the electroactive ß-phase of PVDF. Additionally, this composite exhibits a maximum polarisation of â¼0.1 µC cm-2 and the best energy storage density of â¼0.8 mJ cm-3 at an applied field of â¼16 kV cm-1 among all the synthesized composites. A nanogenerator fabricated using 4 wt% loading in the composite film produced an instantaneous output voltage of â¼40 V, an instantaneous current of â¼4.1 µA, and a power density of â¼17.8 µW cm-2 across 10 MΩ resistance when repeatedly hammered by the human hand. The nanogenerator is further employed to light up several LEDs and to charge capacitors with a small active area demonstrating significant promise for prospective wearables and portable devices and paving the way for high-performance nanogenerators using lead-free halide perovskites. Density functional theory calculations were performed to understand the interaction of the electroactive phase of the PVDF with different perovskite surface terminations to unravel the various interaction mechanisms and their ensuing charge transfer properties.