Predicting the Standard Enthalpy (ΔHo f) and Entropy (So) of Alkanes by Artificial Neural Networks

Abstract

Artificial Neural Networks (ANNs) with Extended Delta-Bar-Delta (EDBD) back propagation learning algorithm have been developed to predict the standard enthalpy and entropy of 87 acyclic alkanes. Molecular weight, boiling point and density of the compounds were used as input parameters. The network's architecture and parameters were optimized to give maximum performances. The best network was a 3-6-2 ANN, and the optimum learning epoch was about 1320. The results show that the maximum relative errors of enthalpy and entropy are less than 3%. They reveal that the performances of ANNs for predicting the enthalpy and entropy of alkanes are satisfying.     

n- and isoalkane adsorption mechanisms on zeolite MCM-22

Low-coverage adsorption properties (Henry constants, adsorption enthalpy, and entropy) of linear and branched alkanes (C3-C8) on zeolite MCM-22 were determined using the chromatographic technique at temperatures between 420 and 540 K. It was found that adsorption enthalpy and entropy of linear alkanes vary in a nonmonotonic way with carbon number. The adsorption behavior of alkanes was rationalized on the basis of the pore geometry. Short molecules prefer to reside in the pockets of the MCM-22 supercage, where they maximize energetic interaction with the zeolite. Longer molecules reside in the larger central part of the supercage. For carbon numbers up to six, singly branched alkanes are selectively adsorbed over their linear counterparts. This preference originates from the entropic advantage of singly branched molecules inside MCM-22 supercages, where these species have high rotational freedom because of their small length.     

Copper(II) 12-metallacrown-4 complexes of alpha-, beta- and gamma-aminohydroxamic acids: a comparative thermodynamic study in aqueous solution

A complete thermodynamic study of the protonation and Cu(II) complex formation equilibria of a series of alpha- and beta-aminohydroxamic acids in aqueous solution was performed. The thermodynamic parameters obtained for the protonation of glycine-, (S)-alpha-alanine-, (R,S)-valine-, (S)-leucine-, beta-alanine- and (R)-aspartic-beta-hydroxamic acids were compared with those previously reported for gamma-amino- and (S)-glutamic-gamma-hydroxamic acids. The enthalpy/entropy parameters calculated for the protonation microequilibria of these three types of ligands are in very good agreement with the literature values for simple amines and hydroxamic acids. The pentanuclear complexes [Cu5L4H(-4)]2+ contain the ligands acting as (NH2,N-)-(O,O-) bridging bis-chelating and correspond to 12-metallacrown-4 (12-MC-4) which are formed by self-assembly between pH 4 and 6 with alpha-aminohydroxamates (HL), while those with beta- and gamma-derivatives exist in a wider pH range (4-11). The stability order of these metallomacrocycles is beta- > alpha- > gamma-aminohydroxamates. The formation of 12-MC-4 with alpha-aminohydroxamates is entropy-driven, and that with beta-derivatives is enthalpy-driven, while with gamma-GABAhydroxamate both effects occur. These results are interpreted on the basis of specific enthalpies or entropy contributions related to chelate ring dimensions, charge neutralization and solvation-desolvation effects. The enthalpy/entropy parameters of 12-MC-4 with alpha-aminohydroxamic acids considered are also dependent on the optical purity of the ligands. Actually, that with (R,S)-valinehydroxamic acid presents an higher entropy and a lower enthalpy value than those of enantiopure ligands, although the corresponding stabilities are almost equivalent. Moreover, DFT calculations are in agreement with a more exothermic enthalpy found for metallacrowns with enantiomerically pure ligands.     

A Thermochemical Study on Complex Nickel(Ⅱ) L-α-Histidine

IntroductionNickel is an essential trace biological element.L-α- Amino acids are the structural units of pro-teins.L- α- Histidine is one of the eight species ofamino acids which have to be absorbed from foodbecause they are not synthesized by organism.Thus,the investigation on the complexation ofnickel and L -α- histidine is of considerable practicaland fundamental importance.For the nickel com-plexes of amino acids,more extensive work hasbeen carried out[1— 3 ] . However,the thermochem…     

Complexation of Np(V) with oxalate at 283-343 K: spectroscopic and microcalorimetric studies

Thermodynamic parameters including the equilibrium constants and enthalpy of complexation of Np(V) with oxalate at variable temperatures (T = 283-343 K, ionic strength = 1.05 mol kg(-1) NaClO(4)) were determined by spectrophotometric and microcalorimetric titrations. The results show that the complexation of Np(V) with oxalate is moderately strong and becomes weaker at higher temperatures. The complexation is exothermic and driven by both enthalpy (negative) and entropy (positive) in the temperature range from 283 K to 343 K. As the temperature is increased, both the enthalpy and entropy of complexation increase (ΔH becomes less negative and ΔS becomes more positive), having opposing effects on the complexation. Because the increase in the enthalpy (ΔH) exceeds that of the entropy term (TΔS), the complexation of Np(V) with oxalate becomes weaker at higher temperatures. The effect of temperature on the complexation is discussed in terms of the energetics of ion solvation and hydrogen bonding involved in the complexation.     

配合物稳定性与配位体酸碱强度间的直线自由能关系 III:N-(间取代苯基)氨基乙酸的铜(II)、镍(II)配合物稳定性及热力学性质的研究

The stepwise formation constants of complex compounds formed from N-(m-substituted-phenyl) glycines (m-RC6H4NHCH2COOH(m-RPhG), R = H, CH3, CH3O, Cl, NO2) with Cu(II) and Ni(II) were determined by pH method at 15, 25 35`C in 30% (V/V) ethanol solution in the presence of 0.1m NaClO4. Their thermodynamical parameters were calculated. It was found that the N-(m-substituted-phenyl) glycine complexes also showed linear free energy relationships between the stability of complex compounds and the base strength of the ligands just as the para-substituted isomers reported previously. Linear relationships not only existed between their thermodynamical parameters and the base strength of the ligands but also existed between the formation enthalpy of complex compounds and the neutralization enthalpy of the ligands and between the formation entropy of complex compounds and neutralization entropy of the ligands as well.     

Transition state characterization for the reversible binding of dihydrogen to bis(2,2'-bipyridine)rhodium(I) from temperature- and pressure-dependent experimental and theoretical studies

Thermodynamic and kinetic parameters for the oxidative addition of H2 to [Rh(I)(bpy)2]+ (bpy = 2,2'-bipyridine) to form [Rh(III)(H)2(bpy)2]+ were determined from either the UV-vis spectrum of equilibrium mixtures of [Rh(I)(bpy)2]+ and [Rh(III)(H)2(bpy)2]+ or from the observed rates of dihydride formation following visible-light irradiation of solutions containing [Rh(III)(H)2(bpy)2]+ as a function of H2 concentration, temperature, and pressure in acetone and methanol. The activation enthalpy and entropy in methanol are 10.0 kcal mol(-1) and -18 cal mol(-1) K(-1), respectively. The reaction enthalpy and entropy are -10.3 kcal mol(-1) and -19 cal mol(-1) K(-1), respectively. Similar values were obtained in acetone. Surprisingly, the volumes of activation for dihydride formation (-15 and -16 cm(3) mol(-1) in methanol and acetone, respectively) are very close to the overall reaction volumes (-15 cm(3) mol(-1) in both solvents). Thus, the volumes of activation for the reverse reaction, elimination of dihydrogen from the dihydrido complex, are approximately zero. B3LYP hybrid DFT calculations of the transition-state complex in methanol and similar MP2 calculations in the gas phase suggest that the dihydrogen has a short H-H bond (0.823 and 0.810 Angstroms, respectively) and forms only a weak Rh-H bond (1.866 and 1.915 Angstroms, respectively). Equal partial molar volumes of the dihydrogenrhodium(I) transition state and dihydridorhodium(III) can account for the experimental volume profile found for the overall process.     

Environmental swap energy and role of configurational entropy in transfer of small molecules from water into alkanes

We studied the effect of segmented solvent molecules on the free energy of transfer of small molecules from water into alkanes (hexane, heptane, octane, decane, dodecane, tetradecane, and hexadecane). For these alkanes we measured partition coefficients of benzene, 3-methylindole (3MI), 2,3,4,6-tetrachlorophenol (TeCP), and 2,4,6-tribromophenol (TriBP) at 3, 11, 20, 33 [corrected], and 47 degrees C. For 3MI, TeCP, and TriBP the dependence of free energy of transfer on length of alkane chains was found to be very different from that for benzene. In contrast to benzene, the energy of transfer for 3MI, TeCP, and TriBP was independent of the number of carbons in alkanes. To interpret data, we used the classic Flory-Huggins (FH) theory of concentrated polymer solutions for the alkane phase. For benzene, the measured dependence of energy of transfer on the number of carbons in alkanes agreed well with predictions based on FH model in which the size of alkane segments was obtained from the ratio of molar volumes of alkanes and the solute. We show that for benzene, the energy of transfer can be divided into two components, one called environmental swap energy (ESE), and one representing the contribution of configurational entropy of alkane chains. For 3MI, TeCP, and TriBP the contribution of configurational entropy was not measurable even though the magnitude of the effect predicted from the FH model for short chain alkanes was as much as 20 times greater than experimental uncertainties. From the temperature dependence of ESE we obtained enthalpy and entropy of transfer for benzene, 3MI, TeCP, and TriBP. Experimental results are discussed in terms of a thermodynamic cycle considering creation of cavity, insertion of solute, and activation of solute-medium attractive interactions. Our results suggest that correcting experimental free energy of transfer by Flory-Huggins configurational entropy term is not generally appropriate and cannot be applied indiscriminately.     

铁(III)卟啉催化β-胡萝卜素分解动力学研究

使用BeckmannDU-8B紫外可见分光光度计研究了以氯合四－间三甲苯基卟啉铁（Ⅲ）（FeTMPCl）为催化剂，间氯过氧化苯甲酸（mCPBA）为氧化剂，咪唑（I_m）、2－甲基咪唑（MeI_m）、2－乙基－4－甲基咪唑（EMI_m）为轴向配体，催化β-胡萝卜素（β－cte）氧化分解为维生素A的动力学规律，提出了反应机理，研究了温度、催化剂浓度、氧化剂浓度及轴向配体对反应速率的影响，应用Gauss－Newton－Marquardt方法求得各基元反应的有关动力学参数．     

Unusual chain length dependent adsorption of linear and branched alkanes on UiO-66

In this work, the zero coverage adsorption properties of C₅–C₁₀ n- and iso-alkanes on the UiO-66, UiO-66-Me and UiO-66-NO₂ metal–organic frameworks are studied by gas phase pulse chromatography. Analysis of enthalpy values, entropy values, Gibbs free energies and Henry constants reveals unusual chain length dependent adsorption behaviour of linear and branched alkanes, caused by the complex structure of the zirconium metal–organic framework UiO-66. The UiO-66 structure consists of a small, tetrahedral and large, octahedral cage. It is shown that at specific carbon chain lengths (e.g. C₆–C₇ for n-alkanes), distinctive jumps in adsorption enthalpy, entropy values and Henry constants occur. This chain length dependent effect is even more pronounced for 2- and 3-methyl alkanes and double branched alkanes. This distinctive shift in adsorption behaviour occurs at a molecular size that coincides with the cavity dimensions of the smallest, tetrahedral cage. The resulting selective adsorption arises from confinement effects and is function of both the molecular shape and size.     

Removal of Cr(VI) from wastewater using rice bran

The novel biosorbent rice bran has been successfully utilized for the removal of Cr(VI) from wastewater. The maximum removal of Cr(VI) was found to be 99.4% at pH 2.0, initial Cr(VI) concentration of 200 mg l(-1), and temperature 20 degrees C. The effect of different parameters such as contact time, adsorbate concentration, pH of the medium, and temperature was investigated. The adsorption kinetics was tested for first-order reversible, pseudo-first-order, and pseudo-second-order; reaction and the rate constants of kinetic models were calculated. Mass transfer of Cr(VI) from the bulk to the solid phase (rice bran) was studied at different temperatures. Different thermodynamic parameters, viz., changes in standard free energy, enthalpy, and entropy, have also been evaluated and it has been found that the reaction was spontaneous and endothermic in nature. The Langmuir and Freundlich equations for describing adsorption equilibrium were applied to data. The constants and correlation coefficients of these isotherm models were calculated and compared. Desorption studies was also carried out and found that complete desorption of Cr(VI) took place at pH of 9.5. The data were also subjected to multiple regression analysis and a model was developed to predict the removal of Cr(VI) from wastewater.     

Quantitative description of the hydrophobic effect: the enthalpic contribution

A new method of experimental determination of the hydrophobic effect enthalpy is proposed. The method is based on regarding the hydration enthalpy as the sum of the nonspecific hydration enthalpy, specific hydration enthalpy, and the hydrophobic effect enthalpy. The hydrophobic effect enthalpies of noble and simple substance gases, alkanes, arenes, and normal aliphatic alcohols are determined. For the noble gases and alkanes, the hydrophobic effect enthalpy is found to be negative and independent of the size of molecule. For aromatic hydrocarbons, it is positive and grows up with the size of the hydrocarbon. The hydrophobic effect enthalpies of normal aliphatic alcohols are determined by assuming that the specific interaction enthalpies of alcohols in water and in methanol are equal. The hydrophobic effect enthalpy values for the aliphatic alcohols (-10.0 +/- 0.9 kJ.mol(-1)) were found to be close to the alkanes hydrophobic effect enthalpies (-10.7 +/- 1.5 kJ.mol(-1)).     

Lu(Et2dtc)3(phen)的生成反应焓变、摩尔热容和恒容燃烧能测定

A ternary solid complex Lu(Et₂dtc)₃(phen) has been obtained from the reaction of hydrated lutetium chloride with sodium diethyldithiocarbamate (NaEt₂dtc), and 1,10-phenanthroline (o-phen·H₂O) in absolute ethanol. IR spectrum of the complex indicates that Lu³⁺ binds with sulfur atom in the Na(Et₂dtc)₃ and nitrogen atom in the o-phen. The enthalpy change of liquid-phase reaction of formation of the complex, Δ_rH_m^Ө (l), was determined to be (-32.821 ± 0.147 ) kJ·mol^-1 at 298.15 K by an RD-496 Ⅲ type heat conduction microcalormeter. The enthalpy change of the solid-phase reaction of formation of the complex, Δ_rH_m^Ө (s), was calculated to be (104.160 ± 0.168) kJ · mol^-1 on the basis of an appropriate thermochemistry cycle. The thermodynamics of liquid-phase reaction of formation of the complex was investigated by changing the temperature of liquid-phase reaction. Fundamental parameters, such as the activation enthalpy (ΔH_≠^Ө), the activation entropy (ΔS_≠^Ө), the activation free energy (ΔG_≠^Ө), the apparent reaction rate constant (k), the apparent activation energy (E), the pre-exponential constant (A) and the reaction order (n), were obtained by combination the reaction thermodynamic and kinetic equations with the data of thermokinetic experiments. The molar heat capacity of the complex, c_m, was determined to be (82.23 ± 1.47) J·mol^-1·K^-1 by the same microcalormeter. The constant-volume combustion energy of the complex, Δ_cU, was determined as (-17 898.228 ± 8.59) kJ·mol^-1 by an RBC-Ⅱtype rotating-bomb calorimeter at 298.15 K. Its standard enthalpy of combustion, Δ_cH_m^Ө, and standard enthalpy of formation, Δ_fH_m^Ө, were calculated to be (-17 917.43 ± 8.11) kJ·mol^-1 and (-859.95 ±10.12) kJ·mol^-1, respectively.     

Sorption and diffusion of normal alkanes through poly(ethylene‐co‐vinyl acetate) membranes

The transport behavior of uncrosslinked and crosslinked poly(ethylene‐co‐vinyl acetate) membranes has been investigated using normal alkanes as probe molecules, in the temperature range of 30–60 °C. Benzoyl peroxide was used for crosslinking the matrix. It has been observed that, a critical concentration of crosslinker is necessary for maximum solvent uptake, followed by a decrease at higher concentration. The effect of free volume on liquid transport was investigated by positron annihilation lifetime spectroscopy. The mechanism of transport has been found to deviate from the regular Fickian behavior. The dependence of the transport coefficients on crosslink density, nature of penetrants, and temperature was studied. The polymer–solvent interaction parameter, enthalpy, and entropy of sorption have also been estimated from the transport data. The affine and phantom models for chemical crosslinks were used to predict the nature of crosslinks. Finally, the experimental sorption data were compared with theoretical predictions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2470–2480, 2007     

Thermodynamics and the structural aspects of the ternary complexes of Am(III), Cm(III) and Eu(III) with Ox and EDTA + Ox

The stability constants and the associated thermodynamic parameters of formation of the binary and the ternary complexes of Am(3+), Cm(3+) and Eu(3+) were determined by a solvent extraction to measure the variation in the distribution coefficient with temperature (0-60 degrees C) for aqueous solutions of I = 6.60 m (NaClO(4)). The formation of ternary complexes is favored by both the enthalpy (exothermic) and the entropy (endothermic) values. (13) C NMR, TRLFS and EXAFS spectral data was used to study the coordination modes of the ternary complexes. In the formation of the complex M(EDTA)(Ox)(3-), the EDTA retained all its coordination sites with Ox binding via two carboxylates and with one water of hydration remaining attached to the M(3+). In the complex M(EDTA)(Ox)(2)(5-), one carboxylate, either from EDTA or Ox, is not bounded to M(3+) and there were no water of hydration attached to these cations.     

A thermodynamic study of the complexation reaction for some amino acids with cerium(III) and yttrium(III)

The thermodynamic parameters for the complexation reaction of leucine, valine, proline and hydroxyproline with cerium(III) and yttrium(III) were determined potentiometrical1y in aqueous solution at 25, 35 °C and μ=0.1. The values for the formation constants have been reported. The values of enthalpy changes (ΔS) and entropy changes (ΔS) are positive for all systems. The chelation effect is believed to be essentially an entropy effect.     

Kinetics of thermal unfolding of phenylalanine hydroxylase variants containing different metal cofactors (FeII, CoII, and ZnII) and their isokinetic relationship

The kinetics of thermal unfolding of apo- and holo-Chromobacterium violaceum phenylalanine hydroxylase (cPAH) was investigated using circular dichroism (CD) over the temperature range 44-76 degrees C. In addition to the native cofactor (FeII), the unfolding kinetics of holo-cPAH was characterized using ZnII and CoII as cofactors. Kinetic profiles for apo- and holo-cPAH showed a single-phase exponential rise in the CD signal at lambda=222 nm and a first-order dependence on protein concentration. The extrapolated unfolding rate constants (ku) at ambient temperature followed the order apo>Fe>Zn>Co. Transition-state analysis of the activation parameters revealed an isokinetic correlation, which suggests a common mechanism for the enzyme variants. The values of the entropy of activation (DeltaS++) for apo- and Fe-cPAH were negative but small: -34+/-24 and -32+/-18 J mol(-1) K(-1), respectively. On the other hand, DeltaS++ values for Zn- and Co-cPAH were large and positive: 54+/-9 and 175+/-27 J mol(-1) K(-1), respectively. Therefore, at higher temperatures the unfolding rates of Zn- and Co-cPAH are affected significantly by entropy, while the unfolding rates of apo- and Fe-cPAH are dominated by enthalpy even at higher temperatures. The rate of unfolding of holo-cPAH did not depend on excess metal concentrations and maintained single-phase kinetic profiles, refuting the occurrence of adventitious metal binding and the notion that unfolding occurs via apo-cPAH exclusively. Isothermal titration calorimetry (ITC) was employed to measure cPAH binding affinities for Fe, Zn, and Co as well as the enthalpy of metal coordination. Dissociation constants (Kd) decreased in the order Fe>Zn>Co. The non-native metals, Zn and Co, were bound more tightly than Fe. The activation enthalpy for unfolding (DeltaH++) displayed a linear correlation with the enthalpy of metal binding obtained from ITC measurements (DeltaHITC). On this basis, a common mechanism (transition state) is suggested for this family of metal cofactors, and the varying enthalpy of activation arises from the differing stabilities of enzyme variants having different metal cofactors.     

Cage Effects in Dimethylaminyl Radical Formation

The rate of decomposition of 1,1,4,4-tetramethyl-2-tetrazene (TMT) was measured in various unbranched alkanes of varying viscosities. The observed rate constant for the decomposition of TMT decreases with the viscosity of the solvent. A compensating change was found in the enthalpy and entropy of activation. It was also found that the decomposition of TMT occurs by the scission of only one N-N single bond and undergoes cage return in the alkanes by recombination of radicals.