Simple Accurate Verification of Enthalpy-Entropy Compensation and Isoequilibrium Relationship

In many experimental investigations of thermodynamic equilibrium or kinetic properties of series of similar reactions it is found that the enthalpies and entropies derived from Van ′t Hoff or Arrhenius plots exhibit a strong linear correlation. The origin of this Enthalpy-Entropy compensation, which is strongly related to the coalescence tendency of Van ′t Hoff or Arrhenius plots, is not necessarily due to a physical/chemical/biological process. It can also be a merely statistical artefact. A new method, called Combined K-CQF makes it possible both to quantify the degree of coalescence of experimental Van ‘t Hoff lines and to verify whether or not the Enthalpy-Entropy Compensation is of a statistical origin at a desired confidence level. The method is universal and can handle data sets with any degree of coalescence of Van ‘t Hoff (or Arrhenius) plots. The new method requires only a standard least square fit of the enthalpyΔH versus entropy ΔS plot to determine the two essential dimensionless parameters K and CQF. The parameter K indicates the position (in inverse temperature) of the coalescence region of Van ‘t Hoff plots and CQF is a quantitative measure of the smallest spread of the Van ‘t Hoff plots. The position of the (K, CQF) couple with respect to universal confidence contours determined from a large number of simulations of random Van ‘t Hoff plots indicates straightforwardly whether or not the ΔH-ΔS compensation is a statistical artefact.     

Tailoring Thermodynamics and Kinetics for Hydrogen Storage in Complex Hydrides towards Applications

Solid‐state hydrogen storage using various materials is expected to provide the ultimate solution for safe and efficient on‐board storage. Complex hydrides have attracted increasing attention over the past two decades due to their high gravimetric and volumetric hydrogen densities. In this account, we review studies from our lab on tailoring the thermodynamics and kinetics for hydrogen storage in complex hydrides, including metal alanates, borohydrides and amides. By changing the material composition and structure, developing feasible preparation methods, doping high‐performance catalysts, optimizing multifunctional additives, creating nanostructures and understanding the interaction mechanisms with hydrogen, the operating temperatures for hydrogen storage in metal amides, alanates and borohydrides are remarkably reduced. This temperature reduction is associated with enhanced reaction kinetics and improved reversibility. The examples discussed in this review are expected to provide new inspiration for the development of complex hydrides with high hydrogen capacity and appropriate thermodynamics and kinetics for hydrogen storage.

     

CS2水解催化反应动力学补偿效应

考察了常压下ＣＳ２水解催化剂活性组分含量和温度对ＣＳ２水解的综合影响，并进行了ＣＳ２水解动力学研究，发现在所研究的４个系列催化剂中都存在着补偿效应，在反应温度范围内，Ａ、Ｅ两类催化剂符合“顺Ａｒｒｈｅｎｉｕｓｆｗｔｖ”；而Ｏ、Ｃ两类催化剂符合“反Ａｒｒｔｈｅｎｉｕｓ规律”，另外，针对多相表面催伦反应过程，提出了修正的阿累尼乌斯公式，即：ｋ＝Ａ‘ｅｘｐ（△Ｅ／ＲＴ）ｅｘｐ（－Ｅａ／ＲＴ），且从此角度     

新型储氢材料及其热力学与动力学问题

氢能作为一种理想的二次能源受到了国内外科研工作者的广泛关注,研制可以在室温和较低压力下方便、安全、高效地储存氢能的材料是氢能发展的瓶颈.到目前为止,固态储氢材料以能量密度高及安全性好等优势被认为极具应用前景,其中以轻质元素构成的氢化物(包括硼氢化物/铝氢化物(可用通式A(MH4)n表示,其中A是碱金属(Li,Na,K)或碱土金属(Be,Mg,Ca);M是硼或铝;n=1～4)、氨基氢化物(如LiNH2等))、氨硼烷(NH3BH3)、金属有机骨架材料(MOFs)是新型储氢材料研究领域的热点,本文将着重就目前这几类储氢材料的研究当中所涉及到的一些热力学及动力学问题进行总结探讨.     

Li-Mg-N-H体系储氢材料的热力学和动力学调控

Mg（NH₂）₂-2LiH体系储氢材料具有较高的储氢容量和较适宜的热力学性能,并且其吸放氢过程完全可逆,是目前最有望实现大规模应用的固态储氢材料之一。然而,由于该体系在吸放氢过程中具有较高的动力学壁垒,导致其在200℃以上才能实现快速地吸放氢。因此,国际上对该体系储氢材料的研究主要集中在热力学和动力学的调控方面。本文从成分调变、纳米化和掺杂改性等方面,详细综述了Mg（NH₂）₂-2LiH体系储氢材料热力学和动力学调控的研究现状,并提出了其中存在的问题和相应对策,同时指出了将来的研究方向。     

ThermoSlope: A Software for Determining Thermodynamic Parameters from Single Steady-State Experiments

The determination of the temperature dependence of enzyme catalysis has traditionally been a labourious undertaking. We have developed a new approach to the classical Arrhenius parameter estimation by fitting the change in velocity under a gradual change in temperature. The evaluation with a simulated dataset shows that the approach is valid. The approach is demonstrated as a useful tool by characterizing the Bacillus pumilus LipA enzyme. Our results for the lipase show that the enzyme is psychrotolerant, with an activation energy of 15.3 kcal/mol for the chromogenic substrate para-nitrophenyl butyrate. Our results demonstrate that this can produce equivalent curves to the traditional approach while requiring significantly less sample, labour and time. Our method is further validated by characterizing three α-amylases from different species and habitats. The experiments with the α-amylases show that the approach works over a wide range of temperatures and clearly differentiates between psychrophilic, mesophilic and thermophilic enzymes. The methodology is released as an open-source implementation in Python, available online or used locally. This method of determining the activation parameters can make studies of the temperature dependence of enzyme catalysis more widely adapted to understand how enzymes have evolved to function in extreme environments. Moreover, the thermodynamic parameters that are estimated serve as functional validations of the empirical valence bond calculations of enzyme catalysis.     

Microstructure and First Hydrogenation Properties of TiHfZrNb1−xV1+x Alloy for x = 0, 0.1, 0.2, 0.4, 0.6 and 1

The effect of the substitution of Nb by V on the microstructure and hydrogen storage properties of TiHfZrNb_1−xV_1+x alloys (x = 0.1, 0.2, 0.4, 0.6 and 1) was investigated. For x = 0, the alloy was pure BCC and upon the substitution of niobium by vanadium, the BCC was progressively replaced by HCP and FCC phases. For x = 0.6, a C15 phase was also present and becomes the main phase for x = 1. The substitution greatly enhanced the first hydrogenation and makes it possible at room temperature under 20 bars of hydrogen. The capacity of all substituted alloys was around 2 wt.%.     

Generation, characterization, and electrochemical behavior of the palladium-hydride cluster [Pd3(dppm)3(mu3-CO)(mu3-H)]+ (dppm=Bis(diphenylphosphinomethane)

Addition of formate on the dicationic cluster [Pd(3)(dppm)(3)(mu(3)-CO)](2+) (dppm=bis(diphenylphosphinomethane) affords quantitatively the hydride cluster [Pd(3)(dppm)(3)(mu(3)-CO)(mu(3)-H)](+). This new palladium-hydride cluster has been characterised by (1)H NMR, (31)P NMR and UV/Vis spectroscopy and MALDI-TOF mass spectrometry. The unambiguous identification of the capping hydride was made from (2)H NMR spectroscopy by using DCO(2) (-) as starting material. The mechanism of the hydride complex formation was investigated by UV/Vis stopped-flow methods. The kinetic data are consistent with a two-step process involving: 1) host-guest interactions between HCO(2) (-) and [Pd(3)(dppm)(3)(mu(3)-CO)](2+) and 2) a reductive elimination of CO(2). Two alternatives routes to the hydride complex were also examined : 1) hydride transfer from NaBH(4) to [Pd(3)(dppm)(3)(mu(3)-CO)](2+) and 2) electrochemical reduction of [Pd(3)(dppm)(3)(mu(3)-CO)](2+) to [Pd(3)(dppm)(3)(mu(3)-CO)](0) followed by an addition of one equivalent of H(+). Based on cyclic voltammetry, evidence for a dual mechanism (ECE and EEC; E=electrochemical (one-electron transfer), C=chemical (hydride dissociation)) for the two-electron reduction of [Pd(3)(dppm)(3)(mu(3)-CO)(mu(3)-H)](+) to [Pd(3)(dppm)(3)(mu(3)-CO)](0) is provided, corroborated by digital simulation of the experimental results. Geometry optimisations of the [Pd(3)(H(2)PCH(2)PH(2))(3)(mu(3)-CO)(mu(3)-H)](n) model clusters were performed by using DFT at the B3 LYP level. Upon one-electron reductions, the Pd--Pd distance increases from a formal single bond (n=+1), to partially bonding (n=0), to weak metal-metal interactions (n=-1), while the Pd--H bond length remains relatively the same.     

A novel predictive model for formation enthalpies of Si and Ge hydrides with propane- and butane-like structures

Butane- and propane-like silicon-germanium hydrides and chlorinated derivatives represent a new class of precursors for the fabrication of novel metastable materials at low-temperature regimes compatible with selective growth and commensurate with the emerging demand for the reduced thermal budgets of complementary metal oxide semiconductor integration. However, predictive simulation studies of the growth process and reaction mechanisms of these new compounds, needed to accelerate their deployment and fine-tune the unprecedented low-temperature and low-pressure synthesis protocols, require experimental thermodynamic data, which are currently unavailable. Furthermore, traditional quantum chemistry approaches lack the accuracy needed to treat large molecules containing third-row elements such as Ge. Accordingly, here we develop a method to accurately predict the formation enthalpy of these compounds using atom-wise corrections for Si, Ge, Cl, and H. For a test set of 15 well-known hydrides of Si and Ge and their chlorides, such as Si(3)H(8), Ge(2)H(6), SiGeH(6), SiHCl(3), and GeCl(4), our approach reduces the deviations between the experimental and predicted formation enthalpies obtained from complete basis set (CBS-QB3), G2, and B3LPY thermochemistry to levels of 1-3 kcal/mol, or a factor of ～5 over the corresponding uncorrected values. We show that our approach yields results comparable or better than those obtained using homodesmic reactions while circumventing the need for thermochemical data of the associated reaction species. Optimized atom-wise corrections are then used to generate accurate enthalpies of formation for 39 pure Si-Ge hydrides and a selected group of 20 chlorinated analogs, of which some have recently been synthesized for the first time. Our corrected enthalpies perfectly reproduce the experimental stability trends of heavy butane-like compounds containing Ge. This is in contrast to the direct application of the CBS-QB3 method, which yields erratic predictions. Our approach also provides quantitative bond-additivity rules for the chlorination of these heavier species. Finally, we discuss structure and bonding trends across the entire sequence of butane-, propane-, and ethane-like molecules with a special focus on the isomeric variations.     

Terminology of kinetics,thermodynamics, and mechanisms of polymerization

This article, after a short introduction, written by a coordinator of the International Union of Pure and Applied Chemistry (IUPAC) Project devoted to the Terminology of the Kinetics, Thermodynamics, and Mechanisms of Polymerization presents the Provisional Document prepared by the IUPAC Macromolecular Division (Commission Nomenclature). It contains 95 entries, from “activated monomer polymerization” to “zwitterionic polymerization.” © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1665–1676, 2002     

The Thermodynamic Driving Force for Kinetics in General and Enzyme Kinetics in Particular

The thermodynamic driving force of a reaction is usually taken as the chemical potential difference between products and reactants. The forward and backward reaction rates are then related to this force. This procedure is of very limited validity, as the resulting expression contains no kinetic factor and gives little information on reaction kinetics. The transformation of the reaction rate as a function of concentration (and temperature) into a function of chemical potential should be more properly performed, as illustrated by a simple example of an enzymatic reaction. The proper thermodynamic driving force is the difference between the exponentials of the totaled chemical potentials of reactants and products.     

Thermodynamics and kinetics of the hydride-transfer cycles for 1-aryl-1,4-dihydronicotinamide and its 1,2-dihydroisomer

Five 1-(p-substituted phenyl)-1,4-dihydronicotinamides (GPNAH-1,4-H(2)) and five 1-(p-substituted phenyl)-1,2-dihydronicotinamides (GPNAH-1,2-H(2)) were synthesized, which were used to mimic NAD(P)H coenzyme and its 1,2-dihydroisomer reductions, respectively. When the 1,4-dihydropyridine (GPNAH-1,4-H(2)) and the 1,2-dihydroisomer (GPNAH-1,2-H(2)) were treated with p-trifluoromethylbenzylidenemalononitrile (S) as a hydride acceptor, both reactions gave the same products: pyridinium derivative (GPNA(+)) and carbanion SH(-) by a hydride one-step transfer. Thermodynamic analysis on the two reactions shows that the hydride transfer from the 1,2-dihydropyridine is much more favorable than the hydride transfer from the corresponding 1,4-dihydroisomer, but the kinetic examination displays that the former reaction is remarkably slower than the latter reaction, which is mainly due to much more negative activation entropy for the former reaction. When the formed pyridinium derivative (GPNA(+)) was treated with SH(-), the major reduced product was the corresponding 1,4-dihydropyridine along with a trace of the 1,2-dihydroisomer. Thermodynamic and kinetic analyses on the hydride transfer from SH(-) to GPNA(+) all suggest that the 4-position on the pyridinium ring in GPNA(+) is much easier to accept the hydride than the 2-position, which indicates that when the 1,4-dihydropyridine is used the hydride donor to react with S, the formed pyridinium derivative GPNA(+) may return to the 1,4-dihydropyridine by a hydride transfer cycle; but when the 1,2-dihydropyridine is used as the hydride donor, the formed pyridinium derivative can not return to the 1,2-dihydropyridine by the hydride reverse transfer from SH(-) to GPNA(+). These results clearly show that the hydride-transfer cycle is favorable for the 1,4-dihydronicotinamides, but unfavorable for the corresponding 1,2-dihydroisomers.     

Microstructure,Kinetics, and Transport in Oil‐in‐Water Microemulsion Polymerizations

The kinetics of microemulsion polymerization depend on the structure of the initial microemulsion and the transport of species between the aqueous domain, the micelles, and the polymer particles. The water solubility of the monomer and the proximity of the initial microemulsion composition to a phase boundary are key considerations for studying microemulsion polymerization kinetics and producing the desired products. Complications frequently arise in the synthesis of copolymers or the incorporation of controlled polymerization mechanisms because of the compartmentalized nature of microemulsion polymerizations.

     

Production,Biochemical Characterization,and Kinetic/Thermodynamic Study of Inulinase from Aspergillus terreus URM4658

Inulinases are enzymes involved in the hydrolysis of inulin, which can be used in the food industry to produce high-fructose syrups and fructo-oligosaccharides. For this purpose, different Aspergillus strains and substrates were tested for inulinase production by solid-state fermentation, among which Aspergillus terreus URM4658 grown on wheat bran showed the highest activity (15.08 U mL⁻¹). The inulinase produced by this strain exhibited optimum activity at 60 °C and pH 4.0. A detailed kinetic/thermodynamic study was performed on the inulin hydrolysis reaction and enzyme thermal inactivation. Inulinase was shown to have a high affinity for substrate evidenced by very-low Michaelis constant values (0.78–2.02 mM), which together with a low activation energy (19.59 kJ mol⁻¹), indicates good enzyme catalytic potential. Moreover, its long half-life (t_1/2 = 519.86 min) and very high D-value (1726.94 min) at 60 °C suggested great thermostability, which was confirmed by the thermodynamic parameters of its thermal denaturation, namely the activation energy of thermal denaturation (E*_d = 182.18 kJ mol⁻¹) and Gibbs free energy (106.18 ≤ ΔG*_d ≤ 111.56 kJ mol⁻¹). These results indicate that A. terreus URM4658 inulinase is a promising and efficient biocatalyst, which could be fruitfully exploited in long-term industrial applications.     

Ground‐State Equilibrium Thermodynamics and Switching Kinetics of Bistable [2]Rotaxanes Switched in Solution,Polymer Gels,and Molecular Electronic Devices

We report on the kinetics and ground‐state thermodynamics associated with electrochemically driven molecular mechanical switching of three bistable [2]rotaxanes in acetonitrile solution, polymer electrolyte gels, and molecular‐switch tunnel junctions (MSTJs). For all rotaxanes a π‐electron‐deficient cyclobis(paraquat‐p‐phenylene) (CBPQT⁴⁺) ring component encircles one of two recognition sites within a dumbbell component. Two rotaxanes (RATTF⁴⁺ and RTTF⁴⁺) contain tetrathiafulvalene (TTF) and 1,5‐dioxynaphthalene (DNP) recognition units, but different hydrophilic stoppers. For these rotaxanes, the CBPQT⁴⁺ ring encircles predominantly (>90 %) the TTF unit at equilibrium, and this equilibrium is relatively temperature independent. In the third rotaxane (RBPTTF⁴⁺), the TTF unit is replaced by a π‐extended analogue (a bispyrrolotetrathiafulvalene (BPTTF) unit), and the CBPQT⁴⁺ ring encircles almost equally both recognition sites at equilibrium. This equilibrium exhibits strong temperature dependence. These thermodynamic differences were rationalized by reference to binding constants obtained by isothermal titration calorimetry for the complexation of model guests by the CBPQT⁴⁺ host in acetonitrile. For all bistable rotaxanes, oxidation of the TTF (BPTTF) unit is accompanied by movement of the CBPQT⁴⁺ ring to the DNP site. Reduction back to TTF⁰ (BPTTF⁰) is followed by relaxation to the equilibrium distribution of translational isomers. The relaxation kinetics are strongly environmentally dependent, yet consistent with a single electromechanical‐switching mechanism in acetonitrile, polymer electrolyte gels, and MSTJs. The ground‐state equilibrium properties of all three bistable [2]rotaxanes were reflective of molecular structure in all environments. These results provide direct evidence for the control by molecular structure of the electronic properties exhibited by the MSTJs.     

Density Functional Investigations of Electronic Structure and Dehydrogenation Reactions of Al‐ and Si‐Substituted Magnesium Hydride

The effect on the hydrogen storage attributes of magnesium hydride (MgH₂) of the substitution of Mg by varying fractions of Al and Si is investigated by an ab initio plane‐wave pseuodopotential method based on density functional theory. Three supercells, namely, 2×2×2, 3×1×1 and 5×1×1 are used for generating configurations with varying amounts (fractions x=0.0625, 0.1, and 0.167) of impurities. The analyses of band structure and density of states (DOS) show that, when a Mg atom is replaced by Al, the band gap vanishes as the extra electron occupies the conduction band minimum. In the case of Si‐substitution, additional states are generated within the band gap of pure MgH₂—significantly reducing the gap in the process. The reduced band gaps cause the Mg? H bond to become more susceptible to dissociation. For all the fractions, the calculated reaction energies for the stepwise removal of H₂ molecules from Al‐ and Si‐substituted MgH₂ are much lower than for H₂ removal from pure MgH₂. The reduced stability is also reflected in the comparatively smaller heats of formation (ΔH_f) of the substituted MgH₂ systems. Si causes greater destabilization of MgH₂ than Al for each x. For fractions x=0.167 of Al, x=0.1, 0.167 of Si (FCC) and x=0.0625, 0.1 of Si (diamond), ΔH_f is much less than that of MgH₂ substituted by a fraction x=0.2 of Ti (Y. Song, Z. X. Guo, R. Yang, Mat. Sc. & Eng. A 2004 , 365, 73). Hence, we suggest the use of Al or Si instead of Ti as an agent for decreasing the dehydrogenation reaction and energy, consequently, the dehydrogenation temperature of MgH₂, thereby improving its potential as a hydrogen storage material.     

Polyaniline Modified CNTs and Graphene Nanocomposite for Removal of Lead and Zinc Metal Ions: Kinetics,Thermodynamics and Desorption Studies

A novel polyaniline-modified CNT and graphene-based nanocomposite (2.32–7.34 nm) was prepared and characterized by spectroscopic methods. The specific surface area was 176 m²/g with 0.232 cm³/g as the specific pore volume. The nanocomposite was used to remove zinc and lead metal ions from water; showing a high removal capacity of 346 and 581 mg/g at pH 6.5. The data followed pseudo-second-order, intraparticle diffusion and Elovich models. Besides this, the experimental values obeyed Langmuir and Temkin isotherms. The results confirmed that the removal of lead and zinc ions occurred in a mixed mode, that is, diffusion absorption and ion exchange between the heterogeneous surface of the sorbent containing active adsorption centers and the solution containing metal ions. The enthalpy values were 149.9 and 158.6 J.mol⁻¹K⁻¹ for zinc and lead metal ions. The negative values of free energies were in the range of −4.97 to −26.3 kJ/mol. These values indicated an endothermic spontaneous removal of metal ions from water. The reported method is useful to remove the zinc and lead metal ions in any water body due to the high removal capacity of nanocomposite at natural pH of 6.5. Moreover, a low dose of 0.005 g per 30 mL made this method economical. Furthermore, a low contact time of 15 min made this method applicable to the removal of the reported metal ions from water in a short time. Briefly, the reported method is highly economical, nature-friendly and fast and can be used to remove the reported metal ions from any water resource.     

Effect of Methyl,Hydroxyl, and Chloro Substituents in Position 3 of 3′,4′,7‐Trihydroxyflavylium: Stability,Kinetics, and Thermodynamics

The effect of methyl, hydroxyl, and chloride substituents in position 3 of the 3′,4′,7‐trihydroxyflavylium core structure was studied. The stability, relative energy of each of chemical species (thermodynamics), and their rates of interconversion (kinetics) are very dependent on these substituents. By comparing the mole fraction distribution at equilibrium of the three multistate systems with the parent 3′,4′,7‐trihydroxyflavylium, introduction of a methyl substituent in position 3 increases the mole fraction of hemiketal at the expense of the trans‐chalcone and increases the hydration rate very significantly; a hydroxyl substituent in position 3 gives rise to a degradation process, as observed in anthocyanidins. In the case of 3‐chloro‐3′,4′,7‐trihydroxyflavylium, a dramatic increase of the flavylium cation acidity was observed and a photochromic system can be operated upon irradiation of the respective trans‐chalcone in 1 m HCl. According to the photochromic response of 3,3′,4′,7‐tetrahydroxyflavylium and 3′,4′,7‐trihydroxyflavylium, some requirements for a good photochromic performance are discussed.     

阴离子型有机化合物在LDHs上的动力学和热力学研究

本文考察水滑石焙烧产物MgAl-LDO吸附3种阴离子染料AcidRed88（AR88）、AcidOrange 3（AO3）、AcidViolet90（AV90）过程中的热力学和动力学机理,并在不同温度下探究该吸附过程的热力学参数。实验结果表明：MgAl-LDO对染料阴离子的吸附过程复合Langmuir吸附等温模型,且为自发、放热的过程。3种染料在MgAl-LDO上的吸附过程均符合准二级反应动力学模型,且该吸附过程是由MgAl-LDO与阴离子染料之间的反应速率控制而不是两者之间的扩散作用。计算所得的吉布斯自由能绝对值在7~15kJ·mol^-1,这主要是由染料阴离子与MgAl-LDHs层板的氢键作用产生,结合MaterialsStudio 5.5软件模拟染料分子在MgAl-LDHs上的排列分布,推测MgAl-LDO对阴离子染料的吸附机理是表面吸附（占优势）与层间插层的协同作用。