Potential Energy Diagram Research Articles

Investigating the Atmospheric Fate and Kinetics of OH Radical-Initiated Oxidation Reactions for Epoxybutane Isomers: Theoretical Insight.

Epoxides, which belong to the category of oxygenated volatile organic compounds (OVOCs), are emitted into the atmosphere by an array of sources and can impact both human and environmental well-being significantly. This study involves comprehensive computational analyses aimed at investigating the mechanism, thermodynamic aspects, and reaction kinetics associated with hydrogen abstraction reactions of cis-2,3-epoxybutane, trans-2,3-epoxybutane, and 1,2-epoxybutane by OH radicals. The potential energy diagrams involving all of the species for each specific pathway were constructed at the CCSD(T)/aug-cc-pVTZ//M06-2X/cc-pVTZ level of theory. The rate coefficients for all possible pathways were calculated using the Rice-Ramsperger-Kassel-Marcus master equation (RRKM-ME) corrected by Eckart tunneling within the 200-350 K temperature range and 1 atm pressure. The overall rate coefficients of the reaction of cis-2,3-epoxybutane, trans-2,3-epoxybutane, and 1,2-epoxybutane with OH radicals at 298.15 K were found to be 0.32 × 10-12, 0.33 × 10-12, and 0.66 × 10-12 cm3 molecule-1 s-1, respectively. We also studied the atmospheric lifetime and photochemical ozone creation potential (POCP) of all three compounds. In addition, we have provided extensive degradation pathways for the product radicals formed from the initial reaction with OH radicals in the presence of O2 and NO. The study showed that the product radicals can result in various harmful end products, including grade 1 and grade 2 carcinogens, as listed by the World Health Organization (WHO).

A Bichromophoric Approach to Induce Luminescence - A Blend of Experimental and Theoretical Studies.

Two homoleptic terpyridyl complexes of Ru(II), 1 and Fe(II), 2 were synthesized using a ligand L1 that contained a phenyl spacer between an anthracenyl (An) and a terpyridyl (tpy) moiety. An equilibrated-bichromophoric strategy was adopted to induce photoluminescence in 1 and 2. A glimpse into the excited state photophysical properties of 1 and 2 revealed that 1 exhibited NIR emission at ~700 nm with an excited state lifetime components of 1.33 and 6.52 ns. On the other hand, 2 was found to be non-luminescent. The origin of emission in case of 1 was attributed to the effect of phenyl spacer which rendered the 3An state to be nearly isoenergetic to the emissive 3MLCT state of 1 facilitating 3MLCT-3An equilibrium. This fact was supported by experimental (photocurrent generation) and theoretical (potential energy diagram) evidences.

UV photolysis of oxalyl chloride: ClCO radical decomposition and direct Cl2${\rm Cl}_2 {\rm }$ formation pathways

AbstractOxalyl chloride, , is widely used as a photolytic source of Cl atoms in reaction kinetics studies. photolysis is typically assumed to produce Cl atoms with an overall yield of 2 via three‐body dissociation, , followed by fast subsequent ClCO unimolecular decomposition of either the energetically excited fragment, , or the thermalized ClCO radical, . However, a study by Huang et al. (J. Phys. Chem. A 121 (2017) 2888–2895) found that UV photolysis of at directly yields with a photolysis quantum yield of . This new product pathway may complicate the use of as a clean source of Cl atoms and challenges the previously accepted photodissociation scheme. The purpose of the present work was 2‐fold. Firstly, the unimolecular decomposition of and ClCO radicals has been investigated in / gas mixtures after UV photolysis at and . Cl atoms were captured by added in excess such that concentration‐time profiles of HCl measured by means of mid‐infrared frequency modulation spectroscopy reflect the temporally separated Cl formation pathways. The low‐pressure thermal ClCO decomposition rate constant was determined to be at , which is in very good agreement with previously reported literature values. Secondly, the photolysis quantum yield of direct formation from photofragmentation was studied with time‐of‐flight mass spectrometry using a photoelectron photoion coincidence setup. Calibrated concentration‐time profiles were recorded and analyzed using kinetic simulations accounting for both direct and secondary formation of from photolysis and reactions involving Cl, ClCO, and , respectively. Direct formation could be confirmed, where wavelength‐dependent quantum yields of , , and at , , and were determined. Complementary quantum‐chemical calculations of the potential energy diagram for ground‐state photodissociation reveal a low‐lying energy barrier for the formation of phosgene, . We suggest that subsequent and Cl formation from energetically excited may actually play a role for the overall photodissociation of .

Orbital Engineering Mediated by Cation Conjugation in Luminescent Uranyl-Organic Hybrid Materials.

A series of compounds of the form [HAr]2 [UO2 X4 ] is reported here, wherein Ar is systematically varied between pyridine (1-X), quinoline (2-X), acridine (3-X), 2,5-dimethylpyrazine (4-X), quinoxaline (5-X), and phenazine (6-X), and X=Cl or Br. With greater conjugation in the organic cation, a larger quenching in uranyl luminescence is observed in the solid state. Supporting our luminescence experiments with computation, we map out the potential energy diagrams for the singlet and triplet states of both the [HAr]+ cations and [UO2 Cl4 ]2- anion in the crystalline state, and of the assembly. The distinct energy transfer pathways in each compound are discussed.

Fast Water Transport in UTSA‐280 via a Knock‐Off Mechanism

AbstractWater and other small molecules frequently coordinate within metal‐organic frameworks (MOFs). These coordinated molecules may actively engage in mass transfer, moving together with the transport molecules, but this phenomenon has yet to be examined. In this study, we explore a unique water transfer mechanism in UTSA‐280, where an incoming water molecule can displace a coordinated molecule for mass transfer. We refer to this process as the “knock‐off” mechanism. Despite UTSA‐280 possessing one‐dimensional channels, the knock‐off transport enables water movement along the other two axes, effectively simulating a pseudo‐three‐dimensional mass transfer. Even with a relatively narrow pore width, the knock‐off mechanism enables a high water flux in the UTSA‐280 membrane. The knock‐off mechanism also renders UTSA‐280 superior water/ethanol diffusion selectivity for pervaporation. To validate this unique mechanism, we conducted 1H and 2H solid‐state NMR on UTSA‐280 after the adsorption of deuterated water. We also derived potential energy diagrams from the density functional theory to gain atomic‐level insight into the knock‐off and the direct‐hopping mechanisms. The simulation findings reveal that the energy barrier of the knock‐off mechanism is marginally lower than the direct‐hopping pathway, implying its potential role in enhancing water diffusion in UTSA‐280.

Fast Water Transport in UTSA-280 via a Knock-Off Mechanism.

Water and other small molecules frequently coordinate within metal-organic frameworks (MOFs). These coordinated molecules may actively engage in mass transfer, moving together with the transport molecules, but this phenomenon has yet to be examined. In this study, we explore a unique water transfer mechanism in UTSA-280, where an incoming water molecule can displace a coordinated molecule for mass transfer. We refer to this process as the "knock-off" mechanism. Despite UTSA-280 possessing one-dimensional channels, the knock-off transport enables water movement along the other two axes, effectively simulating a pseudo-three-dimensional mass transfer. Even with a relatively narrow pore width, the knock-off mechanism enables a high water flux in the UTSA-280 membrane. The knock-off mechanism also renders UTSA-280 superior water/ethanol diffusion selectivity for pervaporation. To validate this unique mechanism, we conducted 1 H and 2 H solid-state NMR on UTSA-280 after the adsorption of deuterated water. We also derived potential energy diagrams from the density functional theory to gain atomic-level insight into the knock-off and the direct-hopping mechanisms. The simulation findings reveal that the energy barrier of the knock-off mechanism is marginally lower than the direct-hopping pathway, implying its potential role in enhancing water diffusion in UTSA-280.

Probing neck growth mechanisms and tensile properties of sintered multi-nanoparticle Al-Cu systems via MD simulation

In the present work, a series of molecular dynamics simulations are conducted to figure out how sintering parameters would influence the atomic structure, sintering mechanisms, and elastoplastic properties of sintered Al-Cu nanoparticulate systems. For this purpose, first, utilizing the atomic potential energy diagrams, the suitable sintering temperatures for the introduced nanoparticles (NPs) have been calculated. NPs are then sintered at the temperatures of 410, 510, 600 and 680 K, and microstructural changes have been probed during the process. Finally, employing a uniaxial tensile test, the sintering temperature and holding time effects on the tensile behavior of the sintered products are studied in detail. Our simulation results indicate a strong correlation between the crystalline structure of the sintered NPs and the process temperature. It is also concluded that the main sintering mechanism at low-temperature conditions is dislocation slip, while at elevated temperatures, the sintering mechanism switches to diffusion-based phenomena. Moreover, it is revealed that increasing the sintering temperature from 410 to 680 K enhances the Young modulus of the samples monotonically, while the greatest yield strength is achieved at 600 K. Similar correlation is also found between the holding time and mechanical properties of the final product.

Ab initio chemical kinetics of Isopropyl acetate oxidation with OH radicals

ABSTRACT Global reactivity descriptors of isopropyl acetate (IPA) and thermo-kinetic aspects of its oxidation via OH radicals have been studied. Transition state theory (TST) was utilized to estimate the bimolecular rate constants. Ten oxidation pathways have been investigated, and all of them are exothermic. The potential energy diagram has been sketched using different pre- and post-reactive complexes for all reaction pathways. Rate coefficient calculations were obtained directly by connecting the separated reactants with different transition states. The results indicate that the reaction of IPA with OH radicals occurs in the ground state rather than the excited state, and the rate constants obtained directly and from the effective approach are the same, which confirmed the accuracy of the estimated pre-reactive complexes and the reaction mechanism. Rate constants and branching ratios show that hydrogen atom abstraction from the iso C − H (C2 atom) bond is the most kinetically preferable route up to 1000 K, while at higher temperatures, H-atom abstraction from the out-of-plane CH3 group (C3 atom) became the most dominant route with high competition with that of the in-plane CH3 group (C4 atom).

Novel dispersant‐free disperse dyes containing polyethylene oxide moieties: Synthesis and eco‐friendly dyeing on polyethylene terephthalate fabrics

AbstractTo reduce pollutant emission during dyeing process with disperse dyes, novel disperse dyes containing polyethylene oxide moieties were synthesized. The amphiphilic PEG 400 was engaged in nucleophile substitution with prepared N‐ethyl‐N‐phenylglycinoyl chloride to give coupling component containing polyethylene oxide. Diazonium salts were reacted with synthesized coupling component to obtain aimed eco‐friendly disperse dyes. Fourier transform infrared and 1H‐nuclear magnetic resonance method were used to manifest structures of synthesized dyes and their intermediates. These dyes were applied on polyethylene terephthalate (PET) fabric using traditional high temperature dyeing method under pressure. Dyeing process with and without dispersants were compared, the synthesized dyes showed no dependency on dispersants. Dyed PET fabrics using dispersant‐free methods exhibited excellent levelness, excellent washing and rubbing fastness. Quantum simulations were applied to evaluate structure–property relationship of designed dyes. The optimized geometries of designed dyes and electrostatic potential energy diagrams of molecules were simulated by density functional theory method to explain dye‐fiber interaction. The designed dispersant‐free disperse dyes exhibited no dependency on dispersants and excellent rubbing and washing fastness. Reduction of dispersants could help to eliminate environmental damage related to discharge of effluent containing dispersants.

Atmospheric degradation, mechanism and kinetics of ethyl vinyl ketone (CH2=CHCOCH2CH3) initiated by Cl atom: an insight from DFT study

The mechanism and kinetics of the H-abstractions of ethylvinylketone (CH2=CHCOCH2CH3) with Cl atom have been carried out using density functional theory (DFT). The electronic structures and frequencies of reaction species are carried out at M06-2X/6-31+G(d,p) level. The energy calculation is performed for optimised species at the same functionality but using a 6-311++G(d,p) basis set. We characterised transition states (TSs) in each H-abstraction channel and explored reaction species along with TS involved in CH2=CHCOCH2CH3+Cl reaction on the potential energy diagram. Among the various H-abstraction channels, H-abstraction from the methylene group (–CH2–) of CH2=CHCOCH2CH3 is found to be a more dominant reaction channel which is further confirmed by thermochemical analysis. The rate constants of all H-abstraction reaction channels and overall rate constant are calculated using canonical transition state theory (TST) within the temperature range of 200–400 K. The value of the overall rate constant at 298.15 K and 1atm pressure is found to be 0.94 × 10−10 cm3 mol−1 s−1, which is in close with the experimental reported rate constant value, i.e. (2.91 ± 1.10) × 10−10 cm3 mol−1 s−1. The percentage branching ratios of each H-abstraction reaction channel, as well as the lifetime of the titled compound, are also reported herein.

Mechanical properties are affected by coalescence mechanisms during sintering of metal powders: Case study of Al-Cu nanoparticles by molecular dynamics simulation

In the present study, molecular dynamics simulation is utilized to explore atomic-scale evolution during the solid-state sintering of Al-Cu nanoparticles (NPs). Our main goal is to determine the ideal process parameters and desired mechanical properties of these metal NPs sintered at temperatures of 430, 510, 600, and 680 K. For this purpose, after examining the proper sintering temperature range using the atomic potential energy diagrams, we have probed the microstructural changes within the NPs during the process. Finally, employing uniaxial tensile tests, the temperature-dependent mechanical properties of the final products are studied. The results reveal a direct correlation between the temperature and the amount of stacking faults. It is also found that the process is mainly controlled by the dislocation slip at lower temperatures. In contrast, diffusion-based mechanisms play a vital role at higher temperatures. Moreover, it is concluded that rising the sintering temperature results in higher mechanical features.

A new approach of kinetic modeling: Kinetically consistent energy profile and rate expression analysis

We report a scaling relation-based L-H kinetic model to assess reaction rate, where general principles for making a kinetically consistent free energy profile are suggested. Water-gas shift, Fischer-Tropsch synthesis, and steam methane reforming are employed as three model reactions to illustrate this approach. Herein, potential energies and free energies are calculated by improved scaling relations. The different methods to construct energy profiles are compared, and free energy diagrams rather than potential energy diagrams are more favorable to be employed in the field of reaction mechanism discriminations and kinetics investigations due to consisting of thermodynamic information. The sequence of various elementary steps is important to generate appropriate rate-determining step and rate expressions. It is suggested that the sequence of the elementary step follows a principle that the reaction step is set just before it is required, and desorption occurs just after the formation of the intermediates. Based on the procedure elucidated here, it is easy to get turnover frequency straightforwardly from the free energy diagram of reaction and the surface site coverages from the free energy diagram of intermediates. The scaling relation-based L-H kinetic model exhibits a similar accuracy for predicting the surface coverages and reaction rates with full microkinetic modeling toward all the three model reactions. It demonstrates that the approach proposed herein greatly simplifies the calculation but maintains high accuracy as compared to full microkinetic modeling. This approach can be widely applied for kinetic modeling to elucidate the reaction mechanism and kinetics for chemical processes of interest.

Nonlinear stiffness mechanism for high-efficiency and broadband raft-type wave energy converters

In this study, a simple and effective nonlinear stiffness mechanism (NSM) is proposed to enhance the capture efficiency and broaden the bandwidth of a raft-type wave energy converter (WEC). A prototype of a simple and compact NSM is first proposed in which a simple elastic element is arranged between hinged floating rafts. The governing equations of motion for the nonlinear WEC are then established based on linear wave theory and the Cummins equation. The harmonic balance method is employed to solve the set of nonlinear governing equations. A numerical simulation for a two-raft WEC is performed to demonstrate the effectiveness of the robust performance improvement. The numerical results show that the NSM can enhance the energy capture efficiency and broaden the low-frequency bandwidth effectively when the structural parameters are adjusted appropriately. The passive phase control mechanism of the NSM is revealed using the Lissajous figure and the potential energy diagram.

Prior knowledge of potential energy and the understanding of quantum mechanics

Quantum mechanics (QM) has become part of many secondary school curricula. These curricula often do not include the mathematical tools for a formal, mathematical introduction of QM. QM therefore needs to be taught at a more conceptual level, but making secondary school students understand counterintuitive QM concepts without introducing mathematical formalism is a challenge. In order to accept QM, students not only have to see the need of it, but also have to see that QM is understandable and logical. Dutch secondary school students are familiar with potential energy (PE) in the context of gravitational and elastic energy. Therefore, the introduction of QM by using the potential wells and tunneling with emphasis on students’ prior knowledge of PE could be a way to make QM more understandable and logical. To explore this, we investigated the relation between the understanding of energy diagrams and the understanding of the potential well and tunneling. A module was created to promote students’ understanding of PE in classical context. Then, a quasi-experimental intervention was used, in which the experimental group received additional lessons using the module on classical energy diagrams before being taught QM. Two tests were developed in order to determine students’ understanding of PE and QM. The results of the tests showed that the experimental group not only had better understanding of PE diagrams, but also of QM even before they were being taught QM. Analysis of the tests also showed that there was a significant correlation between the understanding of PE diagrams and the understanding of QM. Therefore, the results of this study indicate that emphasis on PE can be used to reduce the gap between students’ prior knowledge and QM.

Theoretical study of mechanisms and kinetics of reactions of the O(3P) atom with alkyl hydroperoxides (ROOH) where (R = CH3 & C2H5)

An analysis of O(3P) reaction mechanisms and kinetics with CH3OOH and CH3CH2OOH has been examined. Potential energy diagrams for the named reactions have been evaluated at the W1 and CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ levels of theory. For the two reactions, a total of seven abstraction channels have been described. The reaction enthalpies and standard reaction Gibbs free energy are calculated at the W1 level. The kinetic study is conducted within the temperature range of 210–350 K using transition state theory (TST), TST with Wigner correction (TST/W), and TST with zero curvature tunneling (TST/ZCT). The rate coefficients were also calculated using canonical variational transition state theory (CVT), CVT with zero curvature tunneling (CVT/ZCT), and CVT with small curvature tunneling (CVT/SCT). Branching ratios have been computed using the CVT/SCT rate coefficients. Hydrogen abstraction from the hydroxyl group is identified as a significant pathway in both reactions.

Theoretical study of the reactions of propargyl radical with methanol and ethanol

The C3H3• + CH3OH and C3H3• + C2H5OH reactions were characterised using MP2 and M06-2X theories. For the two reactions, a total of seven channels have been described, including the H-abstraction channels and OH-transfer channels. Potential energy diagrams for the named reactions have been evaluated at the CCSD(T)/CBS//MP2/cc-pVTZ level of theory. The kinetic study was conducted over the temperature range of 300–1500 K using CVT/SCT, CVT and TST methods. Note that the tunnelling effect was significant over low temperatures and the fitting parameters of Arrhensive formula were given. H-abstraction from the –CH3 group (CH3OH) and –CH2 group (C2H5OH) were identified as dominant channels less than 1000 K. While, the OH-transfer channel producing C3H4OH was an important channel higher than 1300 K.

Design and Prototyping of Rotational Bi-Stable Mechanism Using Permanent Magnets

Abstract Diverse applications including switches, deployable structures, and reconfigurable robots can benefit from bi-stability characteristics. However, the complexity of the implementation and the limitation of the structural configuration makes it difficult to apply conventional bi-stable mechanisms to the structures that require rotational bi-stability. In this paper, an implementation method using cylindrical magnets for the rotational bi-stable mechanism is proposed. The proposed bi-stable mechanism consists of a revolute joint with two links. It has rotational bi-stability through the magnetic force relationship between the array of magnets on each link. To identify the characteristics of the proposed bi-stable mechanism, a cylindrical permanent magnet is considered as an electromagnet model that consists of one ring with a virtual electric current. The magnetic field of the cylindrical permanent magnet can be calculated using the Biot–Savart law. Similarly, the magnetic force between two cylindrical permanent magnets is calculated using the Lorentz force law. The criteria of the magnet array for symmetric bi-stability are described and the potential energy diagram of the rotation link is considered as the performance criterion to identify the stable state. The proposed bi-stable mechanism was applied to the prototype of a deployable structure consisting of two links. The load testing of the structure against external torque was performed and it was obtained that the rotation link can stay within 5 deg angle to the maximum load applied and was experimentally verified with good agreement.

Density Functional Study of Methanol Synthesis from CO Hydrogenation on Cu-Based Catalysts

Abstract The reaction mechanism of carbon monoxide (CO) hydrogenation to methanol has been carried out theoretically in this paper. The atomic configuration was analyzed by an unlimited B3LYP calculation method in density functional theory. The reaction model for CO hydrogenation to methanol was established by using Gaussian 09. The adsorption sites, bond angles, bond lengths, reaction intermediates, transition-state structures, adsorption energy, reaction-energy barriers, and reaction heat of CO hydrogenation to methanol with different amounts of copper-based catalysts were calculated. The calculations provided the elementary reaction of methanol in the synthesis, and the reaction potential-energy diagram for methanol synthesis was plotted. The optimum reaction path for CO hydrogenation to methanol was as follows: CO→HCO*→H2CO*→H3CO*→CH3OH. The rate-limiting step was the hydrogenation of the methoxy (H3CO) species with an activation barrier of 1.28 eV.

Molecular Dynamics Simulations, Reaction Pathway and Mechanism Dissection, and Kinetics Modeling of the Nitric Acid Oxidation of Dicyanamide and Dicyanoborohydride Anions.

Direct dynamics simulations of HNO3 with dicyanamide anion DCA- (i.e., N(CN)2-) and dicyanoborohydride anion DCBH- (i.e., BH2(CN)2-) were performed at the B3LYP/6-31+G(d) level of theory in an attempt to elucidate the primary and secondary reactions in the two reaction systems. Guided by trajectory results, reaction coordinates and potential energy diagrams were mapped out for the oxidation of DCA- and DCBH- by one and two HNO3 molecules, respectively, in the gas-phase and in the condensed-phase ionic liquids using the B3LYP/6-311++G(d,p) method. The oxidation of DCA- by HNO3 is initiated by proton transfer. The most important pathway leads to the formation of O2N-NHC(O)NCN-, and the latter reacts with a second HNO3 to produce O2N-NHC(O)NC(O)NH-NO2-(DNB-). The oxidation of DCBH- by HNO3 may follow a similar mechanism as that of DCA-, producing two analogue products: O2N-NHC(O)BH2CN- and O2N-NHC(O)BH2C(O)NH-NO2-. Moreover, two new, unique reaction pathways were discovered for DCBH- because of its boron-hydride group: (1) isomerization of DCBH- to CNBH2CN- and CNBH2NC- and (2) H2 elimination in which the proton in HNO3 combines with a hydride-H in DCBH-. The Rice-Ramsperger-Kassel-Marcus (RRKM) theory was utilized to calculate reaction kinetics and product branching ratios. The RRKM results indicate that the formation of DNB- is exclusively important in the oxidation of DCA-, whereas the same type of reaction is a minor channel in the oxidation of DCBH-. In the latter case, H2 elimination becomes dominating. The RRKM modeling also indicates that the oxidation rate constant of DCBH- is higher than that of DCA- by an order of magnitude. This rationalizes the enhanced preignition performance of DCBH- over DCA- with HNO3.

Ring-opening pathway of 2, 4, 6-trichlorophenol initiated by OH radical: an insight from first principle study

Ring-opening studies of Chlorophenols (CPs) are an important issue to recognise the various end degradation products which may affect the environment and human health. Therefore, here we have explored the ring-opening pathway of 2, 4, 6-trichlorophenol (2, 4, 6-TCP) initiated by OH radical using quantum chemical investigations. Electronic structures and frequencies of all species of the titled reaction are obtained from the density functional theory method using M06-2X functional along with a 6-311++G(d,p) basis set. We have further performed energetic calculations of all optimised species using the CCSD(T) coupled-cluster method combined with the same basis set to achieve more accurate energy values. The full degradation pathway of 2, 4, 6-TCP + •OH reaction is shown by potential energy diagram at 298 K and 1 atm. The values of standard enthalpy change (ΔrH0) and Gibbs free energy change (ΔrG0) of all the reaction steps involved in the ring-opening pathway are also reported herein. From the energy calculations, we found that chloroethyne (HC≡CCl), carbon dioxide (CO2) and C(Cl)O• radical are the end degradation products in the ring-opening pathway.