Density functional theory study on mechanisms of epoxy‐phenol curing reaction

A comprehensive picture on the mechanism of the epoxy‐phenol curing reactions is presented using the density functional theory B3LYP/ 6‐31G(d,p) and simplified physical molecular models to examine all possible reaction pathways. Phenol can act as its own promoter by using an addition phenol molecule to stabilize the transition states, and thus lower the rate‐limiting barriers by 27.0–48.9 kJ/mol. In the uncatalyzed reaction, an epoxy ring is opened by a phenol with an apparent barrier of about 129.6 kJ/mol. In catalyzed reaction, catalysts facilitate the epoxy ring opening prior to curing that lowers the apparent barriers by 48.9–50.6 kJ/mol. However, this can be competed in highly basic catalysts such as amine‐based catalysts, where catalysts are trapped in forms of hydrogen‐bonded complex with phenol. Our theoretical results predict the activation energy in the range of 79.0–80.7 kJ/mol in phosphine‐based catalyzed reactions, which agrees well with the reported experimental range of 54–86 kJ/mol. © 2014 Wiley Periodicals, Inc.     

Density functional theory study of mechanism of epoxy‐carboxylic acid curing reaction

A comprehensive picture on the mechanism of the epoxy‐carboxylic acid curing reactions is presented using the density functional theory B3LYP/6‐31G(d,p) and simplified physical molecular models to examine all possible reaction pathways. Carboxylic acid can act as its own promoter by using the OH group of an additional acid molecule to stabilize the transition states, and thus lower the rate‐limiting barriers by 45 kJ/mol. For comparison, in the uncatalyzed reaction, an epoxy ring is opened by a phenol with an apparent barrier of about 107 kJ/mol. In catalyzed reaction, catalysts facilitate the epoxy ring opening prior to curing that lowers the apparent barriers by 35 kJ/mol. However, this can be competed in highly basic catalysts such as amine‐based catalysts, where catalysts can enhance the nucleophilicity of the acid by forming hydrogen‐bonded complex with it. Our theoretical results predict the activation energy in the range of 71 to 94 kJ/mol, which agrees well with the reported experimental range for catalyzed reactions. © 2017 Wiley Periodicals, Inc.     

Kinetics and Mechanism of Oxidation of 1‐Methoxy‐2‐propanol and 1‐Ethoxy‐2‐propanol by Ditelluratocuprate(III) in Alkaline Medium

The kinetics of oxidation of 1‐methoxy‐2‐propanol and 1‐ethoxy‐2‐propanol by ditelluratocuprate(III) (DTC) in alkaline liquids has been studied spectrophotometrically in the temperature range of 293.2–313.2 K. The reaction rate showed first order dependence in DTC and fractional order with respect to 1‐methoxy‐2‐propanol or 1‐ethoxy‐2‐propanol. It was found that the pseudo‐first order rate constant k_obs increased with an increase in concentration of OH^? and a decrease in concentration of TeO₄^2?. There is a negative salt effect. A plausible mechanism involving a pre‐equilibrium of a adduct formation between the complex and 1‐methoxy‐2‐propanol or 1‐ethoxy‐2‐propanol was proposed. The rate equations derived from mechanism can explain all experimental observations. The activation parameters along with the rate constants of the rate‐determining step were calculated.     

Density functional theory study of tautomerization of 2‐aminothiazole in the gas phase and in solution

The amino/imino tautomeric equilibrium in the isolated, mono‐, di‐, and trihydrate forms and dimer of 2‐aminothiazole, and the effects of hydration or self‐assistance on the transition state structures corresponding to proton transfer from the amino to imino form, have been investigated by the B3LYP method in conjunction with 6‐31+G(d,p) and 6‐311+G(3df,2p) basis sets in the gas phase and in solution. The amino form has been found to be the predominant tautomer. The tautomeric barrier heights for water‐ and self‐assisted tautomerization reactions are significantly lower than that from the amino to imino form by the intramolecular proton transfer, showing the catalytic effect of water molecule(s) and the important role of 2‐aminothiazole itself for intermolecular proton transfer. Comparison between the tautomeric barriers demonstrates that the self‐association tautomerization through the dimerization is the most favorable pathway. Bulk solvent effects have been taken into account using the polarizable continuum model (PCM) of water and CCl₄. The polar medium is favorable for the population of the imino form. The amino/imino equilibrium is also analyzed using the aromaticity index nucleus‐independent chemical shift (NICS); the NICS values for the amino form (about ?10 ppm) are more negative than the imino species (about ?8 ppm), showing that the amino form is more stable. The time‐dependent density functional theory (TDDFT) calculations of electronic absorption spectra suggest that the λ_max of dimer is 255 nm. The oscillator strength of the imino forms is less than the amino form, and increases with the polarity of the solvents. All calculations for the tautomerization of 2‐aminothiazole are in reasonable line with the available experiments. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Reactivation pathway of the hydrogenase H‐cluster: Density functional theory study

This work puts forth a reaction pathway for the reactivation of exogenous ligand inhibited H‐cluster, the active site of Fe‐only hydrogenases. The H‐cluster is a dimetal complex, Fe–Fe, with the metal centers bridged by di(thiomethyl)amine. Exogenous ligands, H₂O, and OH^?, are bound to the distal iron (Fe_d). Density functional theory (DFT) calculations on the native and ruthenium‐modified H‐cluster have been performed using the B3LYP functional with 6‐31+G** and 6‐311+G** basis sets. We have ascertained that there is a thermodynamically favorable pathway for the reactivation of the OH^? inhibited H‐cluster, which proceeds by an initial protonation of the Fe_d–OH^? complex. The proposed reaction pathway has all its intermediate reactions ensue exothermically. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Effects of introduction of α‐carboxylate,N‐methyl,and N‐formyl groups on intramolecular cyclization of o‐quinone amines: Density functional theory‐based study

o‐Quinone amines, which are relevant to various biological processes, can undergo spontaneous intramolecular cyclization (ring closure reaction by amino‐terminated hydrocarbon side chain) that deactivates them toward another possible reactions, that is, thiol binding. Density functional theory‐based calculation is employed for obtaining the potential energy curves along the C? N bond formation in the intramolecular cyclization of various o‐quinone amines, viz., dopaminequinone, dopaquinone, N‐methyl‐dopaminequinone, N‐formyl‐dopaminequinone, and the corresponding methylene‐inserted analogues. The activation barrier is decreased by introduction of α‐carboxylate and N‐methyl group whereas increased by introduction of N‐formyl group. A negative correlation between the activation barriers and the level of highest occupied molecular orbital is pointed out. Furthermore, the methylene‐inserted analogues show decreased activation barriers. This is explained by reduction of steric repulsion in the transition state.     

A density functional theory study on diazotization and nitration of 3,5‐diamino‐1,2,4‐triazole

The reaction mechanism, thermodynamic and kinetic properties for diazotization and nitration of 3,5‐diamino‐1,2,4‐triazole were studied by a density functional theory. The geometries of the reactants, transition states, and intermediates were optimized at the B3LYP/6‐31G (d, p) level. Vibrational analysis was carried out to confirm the transition state structures, and the intrinsic reaction coordinate (IRC) method was used to explore the minimum energy path. The single‐point energies of all stagnation points were further calculated at the B3LYP (MP2)/6‐311+G (2d, p) level. The statistical thermodynamic method and Eyring transition state theory with Wigner correction were used to study the thermodynamic and kinetic characters of all reactions within 0–25°C. Two reaction channels are computed, including the diazotization and nitration of 3‐NH₂ or 5‐NH₂, and there are six steps in each channel. The reaction rate in each step is increased with temperature. The last step in each channel is the slowest step. The first, second, and fifth steps are exothermic reactions, and are favored at lower temperature in the thermodynamics. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

白藜芦醇清除自由基活性的密度泛函理论研究

采用密度泛函理论(DFT)方法,从静态与动态两大方面分析了白藜芦醇分子酚羟基在不同溶剂中清除自由基活性的能力大小.通过白藜芦醇的结构参数、前线轨道理论、3种抽氢反应机制等方面分析了分子活性位与其性质的关系.探讨了白藜芦醇分子不同位置酚羟基清除·OH和·OOH的抗氧化机理,得到了该分子与·OOH发生抽氢反应时的过渡态结构.计算结果表明,在任何溶剂中白藜芦醇分子C(4')位上酚羟基活性最高,发生抽氢反应时反应热最小,是高活性位点.     

Density functional theory on phase space

Forty‐five years after the point de départ [Hohenberg and Kohn, Phys Rev, 1964, 136, B864] of density functional theory, its applications in chemistry and the study of electronic structures keep steadily growing. However, the precise form of the energy functional in terms of the electron density still eludes us—and possibly will do so forever [Schuch and Verstraete, Nat Phys, 2009, 5, 732]. In what follows we examine a formulation in the same spirit with phase space variables. The validity of Hohenberg–Kohn–Levy‐type theorems on phase space is recalled. We study the representability problem for reduced Wigner functions, and proceed to analyze properties of the new functional. Along the way, new results on states in the phase space formalism of quantum mechanics are established. Natural Wigner orbital theory is developed in depth, with the final aim of constructing accurate correlation‐exchange functionals on phase space. A new proof of the overbinding property of the Müller functional is given. This exact theory supplies its home at long last to that illustrious ancestor, the Thomas–Fermi model. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Density functional theory study of the interaction between 3‐nitro‐1,2,4‐triazole‐5‐one and ammonia

Two fully optimized geometries of 3‐nitro‐1,2,4‐triazol‐5‐one (NTO)–NH₃ complexes have been obtained with the density function theory (DFT) method at the B3LYP/6‐311++G** level. The intermolecular interaction energy is calculated with zero point energy (ZPE) correction and basis set superposition error (BSSE) correction. The greatest corrected intermolecular interaction of the NTO–NH₃ complexes is ?37.58 kJ/mol. Electrons in complex systems transfer from NH₃ to NTO. The strong hydrogen bonds contribute to the interaction energies dominantly. Natural bond orbital (NBO) analysis is performed to reveal the origin of the interaction. Based on vibrational analysis, the changes of thermodynamic properties from the monomer to complexes with the temperature ranging from 200 K to 800 K have been obtained using the statistical thermodynamic method. It is found that two NTO–NH₃ complexes can be produced spontaneously from NTO and NH₃ at normal temperature. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

A DFT study on the mechanisms for the cycloaddition reactions between 1‐Aza‐2‐azoniaallene cations and carbodiimides

The mechanisms of the title reactions between 1‐aza‐2‐azoniaallene cations and carbodiimides in the gas phase have been examined using the Becke‐3‐parameter‐Lee‐Yang‐Parr (B3LYP) at 6‐31++G** level. The theoretical results revealed that the reaction is a domino reaction that comprises two consecutive reactions: an ionic [3+2] cycloaddition reaction between 1‐aza‐2‐azoniallene cation and carbodiimide to yield the cycloadduct 3 and then a [1,2]‐shift to yield the thermodynamically more stable adduct 4 . Both stepwise and concerted pathways are accessible in the first cycloaddition in the model reaction. The activation barriers of them are almost equivalent. For the [1,2]‐shift reactions, both of the electron‐withdrawing chlorine substituent and the electron‐releasing methyl substituent on the 1‐aza‐2‐azoniaallene cation can facilitate the reaction but have little effects when substituted in the carbodiimide moiety. The model reaction has also been investigated at the QCISD (quadratic configuration interaction using single and double substitutions)/6‐31++G** and CCSD(T) (coupled cluster calculations with single and double excitations and a perturbative estimate of triple contributions calculations)/6‐31++G** levels as well as by the density functional theory. In addition, solvent effects with the isodensity‐surface polarized continuum model are also reported for all the reactions. In solvent dichloromethane, the cycloadducts of all the reactions, except model reaction and reaction d, were obtained from reactants directly as the result of the solvent effect. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Density functional study of protonation sites of α‐Keggin isopolyanions

Density functional theory (DFT) calculations were carried out to characterize the optimal site of the protons and the precise protonation state in the polyoxometalate (POM) anions [V₁₃O₄₀]^15? and [H₁₂V₁₃O₄₀]^3?. Six kinds of possible protonated stable isomers with the whole Keggin anion units are discussed. The calculations reveal that the preferred protonation site corresponds to bridging oxygens that belong to the same trimetallic group (isomers B and C). Both isomers B and C are comparatively stable in the gas phase, but only isomer B could exist stably in aqueous solution because of being stabilized by the electrostatic interaction. The solvent effects and protonation are also discussed. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

The photoisomerization of 11‐cis‐retinal protonated schiff base in gas phase: Insight from spin‐flip density functional theory

This extensive theoretical study employed the spin‐flip density functional theory (SFDFT) method to investigate the photoisomerization of 11‐cis‐retinal protonated Schiff base (PSB11) and its minimal model tZt‐penta‐3,5‐dieniminium cation (PSB3). Our calculated results indicate that SFDFT can perform very well in describing the ground‐ and excited‐state geometries of PSB3 and PSB11. We located the conical intersection (CI) point and constructed the photoisomerization reaction path of PSB3 and PSB11 by using the SFDFT method. To further verify the SFDFT results, we computed the energy profiles along the constructed linearly interpolated internal coordinate (LIIC) pathways by using high‐level theoretical methods, such as the EOM‐CCSD, CR‐EOM‐CCSD(T), CASPT2, NEVPT2, and XMCQDPT2 methods. The SFDFT method predicts that the photoisomerization of PSB3 is barrierless, in accordance with previous complete‐active‐space self‐consistent‐field (CASSCF) results. However, an energy barrier is predicted along the LIIC pathways of PSB11. This finding is different from previous CASSCF results and may indicate that the photoisomerization of PSB11 in gas phase is similar to that in solution. However, the higher spin contamination of the SFDFT method in the vicinity of the CI point caused the located CI geometry to deviate from that of the real CI. In addition, the LIIC pathways are only approximations to the minimum energy path (MEP). Thus, further experimental and theoretical studies are needed to verify the existence of an energy barrier along the photoisomerization reaction path of PSB11 in gas phase. © 2013 Wiley Periodicals, Inc.     

左旋葡聚糖热解机理的密度泛函理论研究

采用密度泛函理论B3LYP/6-31++G(d,p)方法,对纤维素热解的主要产物左旋葡聚糖的热解反应机理进行了理论计算分析,设计了四种可能的热解反应途径, 对各种反应的反应物、产物和过渡态的结构进行了能量梯度全优化。计算结果表明,左旋葡聚糖开环成链状中间体时,首先,左旋葡聚糖中的两个半缩醛键C(1)-O(7)和C(6)-O(8)断裂,经过渡态TS₁形成中间体IM₁,同时,C(6)-O(7)结合成键使C(5)-C(6)-O(7)形成环状结构,该反应的能垒较高,为296.53 kJ/mol,然后IM₁经过渡态TS₂转变为中间体IM₂,该反应的能垒为234.09 kJ/mol;对IM₂设计了四条可能的反应路径,反应路径2和3能垒较低,是IM₂最可能的热解反应途径;在反应路径1和4中都包含了脱羰基反应,其反应能垒较高,不易发生。     

Density functional theory study of high‐pressure effect on crystalline 4,4′,6,6′‐tetra(azido)hydrazo‐1,3,5‐triazine

Periodic density functional theory calculations are performed to study the hydrostatic compression effects on the structure, electronic, and thermodynamic properties of the energetic polyazide 4,4′,6,6′‐tetra(azido)hydrazo‐1,3,5‐triazine (TAHT) in the range of 0?100 GPa. At the ambient pressure, the local density approximation/Ceperley‐Alder exchange‐correlation potential parameterized by Perdew and Zunger relaxed crystal structure compares well with the experimental results. The predicted heat of sublimation is 38.68 kcal/mol, and the evaluated condensed phase of formation (414.04 kcal/mol) approximates to the experimental value. The detonation velocity and detonation pressure for the solid TAHT are calculated to be 7.44 km/s and 23.71 GPa, respectively. When the pressure is exerted less than 35 GPa, the crystal structure and geometric parameters change slightly. However, at 36 GPa, the molecular structure, band structure, and density of states change abnormally because of the azide‐tetrazole transformation that has not been observed in gas phase or polar solvents. The azido group cyclizes to form a five‐membered tetrazole ring that is coplanar with the riazine ring and contributes to a larger conjunction system. As the pressure augments further to 80 GPa, the hydrogen transfer is found and a new covalent bond H2? N9 is formed. In the studied pressure range, the band gap decreases generally except for some breaks due to the molecular transformation and drops to nearly zero at 100 GPa, which means the electronic character of the crystal changes toward a metallic system. An analysis of the electronic structure shows that an applied pressure increases the impact sensitivity of TAHT. © 2012 Wiley Periodicals, Inc.     

Density functional theory of polymer-polymer phase separation behavior

Polyatomic density functional theory is applied to a binary polymer blend. The polymer reference interaction site model (PRISM) liquid state theory provides the homogeneous state correlation functions necessary for the application of density functional theory. An effective chi parameter can be recognized from the density functional expression; however, the phase separation criteria does not depend solely upon the chi parameter, rather it depends upon various combinations of the species-dependent direct correlation functions of the blend. The Flory-Huggins chi parameter along with the associated phase diagram is obtained when the monomer volumes of the blend species are equal and for a range of monomer-monomer attractive interactions. Calculations are performed both with and without the assumption of incompressibility. The density functional theory along with the PRISM determined “input” predict that an isotopic polymer blend shows an upper critical solution temperature (UCST) phenomena. © 1995 John Wiley & Sons, Inc.     

Mechanistic study on gold(I)‐catalyzed crosscoupling of diazo compounds: A DFT study

The reaction mechanisms of the gold(I)‐catalyzed cross‐coupling reaction of aryldiazoacetate R1 with vinyldiazoacetate R2 leading to N‐substituted pyrazoles have been theoretically investigated using density functional theory calculations. Two possible reaction mechanisms were examined and discussed. The preferred reaction mechanism (mechanism A) can be characterized by five steps: the formation of the gold carbenoid A2 via the attack of catalyst to R1 (step I), nucleophilic addition of another reactant R2 to generate intermediate A3 (step II), intramolecular cyclization of A3 to form intermediate A4 (step III), hydrogen migration to give intermediate A5 (step IV), and catalyst elimination affording the final product P1 (step V). Step IV is found to be the rate‐determining step with an overall free energy barrier of 28.3 kcal/mol. Our calculated results are in good agreement with the experimental observations. The present study may provide a useful guide for understanding these kinds of gold(I)‐catalyzed cross‐coupling reactions of diazo compounds.     

Electrochemical properties of niosomes‐modified Au electrode and its interaction with 1‐anilino‐8‐naphthalene‐sulfonate

The gold electrode was modified by noisomes, prepared by sonicating the mixed solution of PEG 6000/Tween 80/Span 80/H₂O. Electrochemical cyclic voltammetry (CV) and impedance spectroscopy (EIS) were performed for characterizing the modification. The construction of multilayer films of octadecanethiol (ODT)‐niosomes had almost no pinholes, which elicited that niosomes‐coated electrode through ODT assembly was more effective immobilization compared to direct modification method. A large semicircle formation in the entire range of frequency indicated the complete electron‐transfer control for the redox reaction, implying a perfect blocking behavior. The Nyquist plot was consisting of two depressed semicircles after storage for 36 h in air, which implied a progressive and uniform construction manner, as expected for porous coatings. The modified electrode was used to investigate the interaction between 1‐anilino‐8‐naphthalene‐sulfonate (ANS) and noisomes. It was found that the low‐frequency semicircular arcs increase in diameter, indicating that the increase in the resistance R_p attributed mainly to the niosomes/solution interface. It was due to the binding of ANS to the niosomes‐modified electrode surface, exhibited against diffusing species through the pores. Copyright © 2009 John Wiley & Sons, Ltd.     

Theoretical Study on Gas—phase Pyrolytic Reactions of N—Ethyl,Isopropyl and N—t—Butyl Substituted 2—Aminopyrazine

Density functional theory (DFT) and ab initio methods were used to study gas‐phase pyrolytic reaction mechanisms of iV‐ethyl, N‐isopropyl and N‐t‐butyl substituted 2‐aminopyrazine at B3LYP/6–31G* and MP2/6–31G*, respectively. Single‐point energies of all optimized molecular geometries were calculated at B3LYP/6–311 + G(2d,p) level. Results show that the pyrolytic reactions were carried out through a unimolecular first‐order mechanism which were caused by the migration of atom H(17) via a six‐member ring transition state. The activation energies which were verified by vibrational analysis and correlated with zero‐point energies along the reaction channel at B3LYP/6–311 + G(2d,p) level were 252.02 kJ. mo^?1 (N‐ethyl substituted), 235.92 kJ‐mol^?1 (N‐t‐isopropyl substituted) and 234.27 kJ‐mol^?1 (N‐t‐butyl substituted), respectively. The results were in good agreement with available experimental data.     

Density functional theory study of a molecular allosteric switch for 2,2′‐bipyridyl‐3,3′‐15‐crown‐5

A classical model of “molecular machine,” which acts as an ON–OFF switch for 2,2′‐bipyridyl‐3,3′‐15‐crown‐5 ( L ), has been theoretically studied. It is highly important to understand the mechanism of this switch. The alkali‐metal cations (Na⁺ and K⁺) and W(CO)₄ fragment are introduced to coordinate with the different active sites of L , respectively. The density functional theory (DFT) method is used for understanding the stereochemical structural natures and thermodynamic properties of all the target molecules at B3LYP/6‐31G(d) and SDD (Stuttgart–Dresden) level, together with the corresponding effective core potential (ECP) for tungsten (W). The fully optimized geometries have been performed with real frequencies, which indicate the minima states. The nucleophilicity of L has been investigated by the Fukui functions. The natural bond orbital analysis is used to study the intermolecular charge‐transfer interactions and explore the origin of the internal forces of the molecular switch. In addition, the binding energies, enthalpies, Gibbs free energies, and the cation exchange energies have been studied for L , W(CO)₄ L , and their corresponding complexes. The properties of the complexes displayed by in presence or absence of the W(CO)₄ fragment are also analyzed. The calculated results of allosterism displayed by L are in a good agreement with the experimental results. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2011