Standard formation enthalpies for α-amino acids: L-serine,L-arginine,and L-tyrosine

The energies of combustion, Δ _c U ⁰, of three amino acids: L-serine (I), L-arginine (II) and L-tyrosine (III) in the crystalline state at 298.15 K were determined using a static-bomb isoperibolic calorimeter. From these data, the enthalpies of combustion, Δ _c H ⁰, and the enthalpies of formation, Δ _f H ⁰, are calculated. The Δ _f H ⁰ values are compared with the published data.     

A steered molecular dynamics simulation on the elastic behavior of adsorbed star polymer chains

<正>Elastic behavior of 4-branched star polymer chain with different chain length N adsorbed on attractive surface is investigated using steered molecular dynamics(SMD) simulation method based on the united-atom(UA) model for branched alkanes.The simulation is realized by pulling up the chain via a linear spring with a constant velocity v = 0.005 nm/ps.At the beginning,the chain lies extensionally on adsorbed surface and suffers continuous deformations during the tensile process.Statistical parameters as mean-square radii of gyration S~2_(xy),S~2_z,shape factor δ,describing the conformational changes,sectional density den which gives the states of the chain,and average surface attractive energy U_a,average total energy U,average force f probed by the spring,which characterize the thermodynamic properties, are calculated in the stimulant process.Remarkably,distinguishing from the case in linear chains that there only exists one long plateau in the curve of f,the force plateau in our study for star chains is multiple,denoting different steps of desorption,and this agrees well with the experimental results in essence.We find during the tensile process,there are three characteristic distances Z_c,Z_t and Z_0 from the attractive surface,and these values vary with N.When Z=Z_c,the chain is stripped from the surface,but due to the form of wall-monomer interaction,the surface retains weak influence on the chain till Z = Z_c.From Z=Z_t,parameters U_a,U and f respectively reach a stable value,while the shape and the size of the chain still need adjustments after Z_t till Z_0 to reach their equilibrium states.Specifically,for short chain of N= 41,Z_t and Z_0 are incorporated.These results may help us to deepen the knowledge about the elastic behavior of adsorbed star polymer chains.     

Electrochemiluminescence in the reduction of uranyl ions on platinum in sulfuric acid solutions

Electrochemiluminescence (ECL) accompanying the reduction of uranyl ions on a platinum cathode in sulfuric solutions in the presence of XeO₃ was observed and studied. At early stages of the electrolysis (at 0.34<φ<0.62 V), the chemiluminescence reaction U^V+XeO₃ contributes mainly to the luminescence intensity, whereas at φ<0.32 V, the contribution of the reaction U^IV+XeO₃ predominates. The transfer coefficient for the primary electrochemical stage, formation of U^V, was estimated: α=0.46−0.54. The high sensitivity of ECL to the state of the electrode surface is explained by the fact that the reduction of U^VI occurs on the most active sites only. the background chemiluminescence reactions, among which U^V+O₂ and U^V+H₂O₂ can be distinguished, were considered, and their contributions to ECL were determined. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1928–1934, October, 1999.     

Collapse of preformed cobalt stearate film on water surface

Preformed cobalt stearate (CoSt) molecules form a film on the water surface, which with barrier compression shows multilayers of different heights that are evidenced from the structures of the films deposited on hydrophilic silicon (0 0 1) substrates by using a horizontal deposition technique at different positions of the surface pressure (π)–specific molecular area (A) isotherm. In-plane morphology and out-of-plane structures are obtained from the atomic force microscopy (AFM) and X-ray reflectivity studies. Electron density profiles (EDPs), extracted from the reflectivity data, show that the monolayer coverage is maximum when π is far before the collapse point (π_c) but with barrier compression domains of multilayers start to form even before π_c. After π_c, two different bilayer repeat distances have been observed from the two different series of the Bragg peaks implying the formation of domains by both the tilted and untilted CoSt molecules. Far after π_c, reflectivity decreases rapidly and morphology of the deposited films changes totally. Structures before and after π_c of the CoSt film have also been obtained by changing the pH of the subphase water. From all the structural information it is clear that the preformed CoSt film collapses in a different way in comparison with the collapse of the standard cobalt stearate monolayer where cobalt stearate molecules were formed at the air–water interface. Reasons for obtaining different structures on the water surface with barrier compression have been proposed.     

Intermolecular core-potential depth versus critical temperature

Two constants, U₀ and ?₀, characterizing the convex-core potential of intermolecular forces are revised on the basis of a new compilation of virial coefficients of gases by Dymond and Smith. The mean potential depths U₀ for non-polar gases are compared with the critical temperatures. T_c.     

Ex‐Situ Kinetic Investigations of the Formation of the Poly‐Oxo Cluster U38

The ex‐situ qualitative study of the kinetic formation of the poly‐oxo cluster U₃₈, has been investigated after the solvothermal reaction. The resulting products have been characterized by means of powder XRD and scanning electron microscopy (SEM) for the solid phase and UV/Vis, X‐ray absorption near edge structure (XANES), extended X‐ray absorption fine structure (EXAFS), and NMR spectroscopies for the supernatant liquid phase. The analysis of the different synthesis batches, stopped at different reaction times, revealed the formation of spherical crystallites of UO₂ from t=3 h, after the formation of unknown solid phases at an early stage. The crystallization of U₃₈ occurred from t=4 h at the expense of UO₂, and is completed after t=8 h. Starting from pure uranium(IV) species in solution (t=0–1 h), oxidation reactions are observed with a U^IV/U^VI ratio of 70:30 for t=1–3 h. Then, the ratio is inversed with a U^IV/U^VI ratio of 25/75, when the precipitation of UO₂ occurs. Thorough SEM observations of the U₃₈ crystallites showed that the UO₂ aggregates are embedded within. This may indicate that UO₂ acts as reservoir of uranium(IV), for the formation of U₃₈, stabilized by benzoate and THF ligands. During the early stages of the U₃₈ crystallization, a transient crystallized phase appeared at t=4 h. Its crystal structure revealed a new dodecanuclear moiety (U₁₂), based on the inner hexanuclear core of {U₆O₈} type, decorated by three additional pairs of dinuclear U₂ units. The U₁₂ motif is stabilized by benzoate, oxalates, and glycolate ligands.     

Modeling of interaction properties of surfaces of phyllosilicates: A theoretical forecast of adsorption isotherms of noble gases at the talc surface

The electrostatic potential and the intensity of the electric field above the isolated layer of the phyllosilicate mineral talc (layer group symmetry C2/m) were computed using the semiempirical INDO /2 method. The electrostatic potential V_c and the intensity of electric field (OVERBAR)E_c, above the surface of semi-infinite crystal were obtained as the sum of the contributions of the infinite number of individual layers. The interaction energy U_int between a noble gas atom and the talc crystal was computed as (a) a pure Lennard-Jones energy U_LJ and (b) the sum of U_LJ and interaction energy with induced dipole moment: U_ind = α|(OVERBAR)E|², where α is the dipole polarizability of the noble gas atom. The one-particle configurational integral was calculated within the classical mechanics limit. Both the free and adsorbed gas phases were assumed to be ideal. The obtained results demonstrate that in the case of uncharged and nonpolar adsorbate and an uncharged surface the electrostatic part of the interaction energy should be included in the total interaction energy. © 1996 John Wiley & Sons, Inc.     

Synthesis of Nonsymmetric Dibenzoyldihydromonobenzotetraazacyclo[14]annulenes as Transition Metal Hosts: X-ray Structure of 13,14-Benzo-3,10-di(p-chlorobenzoyl)-5, 8-dihydro-2,4,9,11-tetramethyl-1,5,8,12-tetraazacyclotetradeca-1,3,9,11-tetraene

13,14-Benzo-5,8-dihydro-2,4,9,11-tetramethyl-1,5,8,12-tetraazacyclotetradeca-1,3,9,11-tetraene and 13,14-benzo-3,10-di(p-[X]benzoyl)-5,8-dihydro-2,4,9,11-tetramethyl-1,5,8,12-tetraazacyclotetradeca-1,3,9,11-tetraene, wherein X = CH₃, H, Cl, NO₂ and OCH₃, were synthesized and characterized. IR spectra of the benzoylated compounds showed an intense band assigned to the stretching modes of C==O in the region 1632 ~ 1638 cm⁻¹. Hammett plot of the energies of π → π^* for the compounds was linear with a positive slope (+0.923). ¹H NMR signals exhibited a deshielding effect due to benzoyl groups, while methyl protons only exhibited the shielding effect by the magnetic anisotropy of the group. The substituent effect on ¹³C NMR was similar to that on proton NMR. The cyclic voltammograms of the compounds mostly showed two one-electron irreversible oxidation peaks and one or two reduction waves between −0.280 and −2.500 V depending on the substituents. Hammett plots of 1st and 2nd oxidation potentials (E_op(1) and E_op(2)) were linear with slopes of +0.036 and +0.045, respectively. The structure of the compound 4 (monoclinic, C2/c, a = 26.0059 (19), b = 6.9058 (5), c = 33.082 (2), β = 109.5930°, Z = 8) was determined using X-ray diffraction method.     

Physical adequacy of parameters in statistical-mechanical calculations of the properties of strong electrolyte solutions

Calculations have been carried out on the distribution functions, free energy F, and heats of dilution H for a 1-1 electrolyte solution with a variety of parameters representative of noncoulombic short-range interionic interaction in water at 15–40°C using a theoretical equation which is strictly valid at c<0.1 moles-liter^–1 and gives a good approximation up to c=0.3. The main result is that agreement with experimental H data could be obtained by very small, physically highly reasonable variations of the interionic specific potentialU _ab(r) of the order of 1% per degree centigrade. Any change in the distance of specific interactionr _c gives an appreciable contribution to H only if it is taken in the form of its temperature derivative. These contributions are large enough to account for the experimental data if one takes simultaneously into account appropriate changes ofU _ab(r) nearr _c to conjugate the former smoothly with the coulomb potential atr=r _c. To fit the calculated H to experimental data for NaCl in H₂O at 25°C it is necessary to allow the solvation of ions to increase with increasing temperature, which is in agreement with experiment.     

Influence of the smectic A-nematic transitional order on the formation of the smectic phases of 4-n-hexyloxybenzylidene-4′-n-butyIaniline and 4-n-butyloxybenzylidene-4′-n-octyIaniline from an electrically deformed nematic phase

Abstract

The crucial role of the smectic A-nematic transitional order for the formation of the smectic A, B and G phases from an electrically deformed nematic phase of the liquid crystal 4-n-hexyloxy-benzylidene-4′-n-butylaniline (6O.4) with a typical smectic A-nematic first order transition and the formation of the smectic A and B phases from an electrically deformed nematic phase of the liquid crystal (4-n-butyloxy-benzylidene-4′-n-octylaniline (40.8) with a smectic A-nematic second order transition has been demonstrated. The nematic phase was deformed by an AC voltage of 2U,_th 5U _th and 10U _th, where U _th is the threshold voltage which causes the appearance of the Fréedericksz transition in the homeotropic nematic layer. The smectic textures have been observed on free cooling of the nematic phase or after the use of an oven. The smectic A phase of the liquid crystal 60.4 was observed with the formation of a clear smectic A-nematic phase boundary while the smectic A phase of the liquid crystal 40.8 has been formed from intermediate pretransitional stripes, observed by Cladis and Torza [1]. The homeotropic anchoring of the direction was crucial for the formation of the smectic phases of the liquid crystal 40.8 but not significant for the liquid crystal 60.4.     

Thermodynamic study on inclusion complex formation of riboflavin with hydroxypropyl-β-cyclodextrin in water

The inclusion complex formation of riboflavin (RF) with hydroxypropyl-β-cyclodextrin (HP-β-CD) in water was investigated by ¹H NMR, UV-vis spectroscopy, and solubility methods. A 1:1 stoichiometry and thermodynamic parameters of complex formation (K, Δ_c G ⁰, Δ_c H ⁰, and Δ_c S ⁰) were determined. Complexation was characterized by negative enthalpy and entropy changes due to prevalence of van der Waals interactions and hydrogen bonding between polar groups of the solutes. A partial insertion of RF into macrocyclic cavity was revealed on the basis of ¹H NMR data and molecular mechanics calculation. Location of benzene ring of RF molecule inside the hydrophobic cavity of HP-β-CD results in an increase of aqueous solubility of the former.     

Dynamic computer simulations of Cs incorporation in Si during low‐energy (0.2–3 keV) irradiation

The incorporation of Cs atoms in silicon was investigated by dynamic computer simulations using the Monte‐Carlo code T‐DYN that takes into account the gradual change of the target composition due to the Cs irradiation. The implantation of Cs atoms at normal incidence was studied for four energies (0.2, 0.5, 1, and 3 keV) and three different Cs surface‐binding energies U_Cs (0.4, 0.8, and 2.4 eV). The total implantation fluences were 2 × 10¹⁷ Cs cm^?2 for 0.2 keV, 1.5 × 10¹⁷ Cs cm^?2 for 0.5 keV, and 1 × 10¹⁷ Cs cm^?2 for 1 and 3 keV. At these values, a stationary state has been reached. The steady‐state Cs‐surface concentrations exhibit a pronounced dependence both on impact energy and U_Cs, varying between ～1 (at 0.2 keV and U_Cs = 2.4 eV) and ～0.13 (3 keV and U_Cs = 0.4 eV). Under equilibrium, the partial sputtering yield of Si, Y_Si, experiences little influence of U_Cs, but varies with the Cs energy: at U_Cs = 0.8 eV from 0.09 to 1.0 Si atoms/Cs projectile. For all irradiation conditions a strongly preferential sputtering of Cs atoms as compared to Si atoms is found, increasing from 1.8 (at 3 keV and U_Cs = 2.4 eV) to 13.3 (at 0.2 keV and U_Cs = 0.4 eV). Preferential sputtering of Cs increases with decreasing irradiation energy and decreasing U_Cs. Copyright © 2008 John Wiley & Sons, Ltd.     

Thermochemistry of copper complex of 6-benzylaminopurine

The copper(II) complex of 6-benzylaminopurine (6-BAP) has been prepared with dihydrated cupric chloride and 6-benzylaminopurine. Infrared spectrum and thermal stabilities of the solid complex have been discussed. The constant-volume combustion energy, Δ_c U, has been determined as −12566.92±6.44 kJ mol⁻¹ by a precise rotating-bomb calorimeter at 298.15 K. From the results and other auxiliary quantities, the standard molar enthalpy of combustion, Δ_c H _m ^θ, and the standard molar of formation of the complex, Δ_f H _m ^θ, were calculated as −12558.24±6.44 and −842.50±6.47 kJ mol⁻¹, respectively.     

Synthesis,Crystal Structure and Thermochemical Properties of a One‐Dimensional Chain Complex [Cd(succ)PIP]n

A new complex, [Cd(succ)PIP]_n (PIP=2‐phenyl‐imidazo[4,5‐f]1,10‐phenanthroline, H₂‐succ=succinate), was synthesized and characterized by X‐ray crystallography, elemental analysis, and TG‐DTG. The results show that the complex crystallizes in an orthorhombic space group Pcca; a=14.065(2) Å, b=9.901(8) Å, c=28.933(2) Å and Z=8. The structure of the complex is one‐dimensional chain [Cd(succ)PIP]_n, and each Cd²⁺ is five‐coordinated by two chelating nitrogen atoms from one PIP ligand, three oxygen atoms from three different succ dianionic ligands to form a distorted trigonal‐bipyramida geometry. The constant‐volume combustion energy of the complex, Δ_cU, was determined by an intelligent micro‐rotating‐bomb calorimeter (IMRBC‐type I) at 298.15 K. Then the standard molar enthalpy of combustion, Δ_cH_m^?, and the standard molar enthalpy of formation, Δ_fH_m^? have been calculated.     

Calorimetric study of thermal decomposition of lithium hexafluorophosphate

Enthalpy of formation of lithium hexafluorophosphate was calculated based on the differential scanning calorimetry study of heat capacity and thermal decomposition. It was found that thermal decomposition of LiPF₆ proceeds at normal pressure in the temperature range 450-550 K. Enthalpy of LiPF₆ decomposition is Δ_d H(LiPF₆, c, 298.15 K)= 84.27±1.34 kJ mole^-1. Enthalpy of formation of lithium hexafluorophosphate from elements in standard state is Δ_f H ⁰(LiPF₆,c, 298.15 K) = -2296±3 kJ mol^-1. This revised version was published online in July 2006 with corrections to the Cover Date.     

Unconventional Metal–Framework Interaction in MgSi5

The silicon‐rich cage compound MgSi₅ was obtained by high‐pressure high‐temperature synthesis. Initial crystal structure determination by electron diffraction tomography provided the basis for phase analyses in the process of synthesis optimization, finally facilitating the growth of single crystals suitable for X‐ray diffraction experiments. The crystal structure of MgSi₅ (space group Cmme, Pearson notation oS24, a=4.4868(2) Å, b=10.1066(5) Å, and c=9.0753(4) Å) constitutes a new type of framework of four‐bonded silicon atoms forming Si₁₅ cages enclosing the Mg atoms. Two types of smaller Si₈ cages remain empty. The atomic interactions are characterized by two‐center two‐electron bonds within the silicon framework. In addition, there is evidence for multi‐center Mg?Si bonding in the large cavities of the framework and for lone‐pair‐like interactions in the smaller empty voids.     

Anharmonic effects on theoretical IR line shapes of H-bonds

In the spirit of Y. Maréchal and A. Witkowski's [J. Chem. Phys. 48 (1968) 2697] work, one revisits, for weak H-bonds, the dependence of the angular frequency ω and of the equilibrium position q_e of the υ_X–H high frequency mode q, on the position coordinate Q of the low frequency υ_X–HY mode. One considers: ω=ω^o+bQ+cQ² and q_e=gQ+fQ². That leads to the anharmonic potential U: U=k₁q²+∑k_rQ^r+∑∑k_nmqⁿQ^m+k₁₅qQ⁵. Here k_r and k_mn (r=2–5, n=1,2 and m=1–4) are interrelated through b, c, g and f. By aid of the Hamiltonian involving U, we find the direct damped auto correlation function of υ_X–H, which, by Fourier transform, gives the IR spectral density (SD). When only b≠0, the SD is nothing but that given in a previous paper [P. Blaise, O. Henri-Rousseau, Chem. Phys. 243 (1999) 229]. When the adiabatic approximation is performed, this SD becomes that of N. Rösch and M. Ratner [J. Chem. Phys. 61 (1974) 3444] which reduces in turn to that of Maréchal and Witkowski in the absence of damping. With respect to b≠0, c produces a narrowing of the SD if c>0 and a subtle broadening if c<0. Besides, g induces the same narrowing for g>0 and g<0, while f gives subtle changes very sensitive to the sign of f and to the values of b, c and g. The situation b<0 and f>0 which is physically the most probable, leads to SDs which are the most evoking experimental profiles.     

GW-BSE Calculations of Electronic Band Gap and Optical Spectrum of ZnFe2O4: Effect of Cation Distribution and Spin Configuration

The G0W0, evGW0, evGW, and scGW0 approximations to many-body perturbation theory combined with the Bethe-Salpeter approach (BSE) are applied to calculate electronic and optical properties of the open-shell spinel ferrite ZnFe₂O₄. The effect of the various degrees of self-consistency is assessed by comparison to recent experimental results. Furthermore, the influence of the method for obtaining the ground-state wavefunction is studied, including the GGA functional PBE with and without an intermediate step using the COHSEX approximation, as well as PBE+U, where we try to minimize the influence of the Hubbard potential U. Best agreement for the optical band gap and the first maxima of the excitation spectrum is obtained with the evGW method based on a PBE+U wavefunction. This method is chosen and converged carefully to yield quantitative results for the optical spectra of four different magnetic structures and cation distributions of ZnFe₂O₄. With the results we provide a possible explanation for inconsistency in experimental results.     

Low‐Temperature Heat Capacities and Standard Molar Enthalpy of Formation of Gramine (C11H14N2)

Low‐temperature heat capacities of gramine (C₁₁H₁₄N₂) were measured by a precision automated adiabatic calorimeter over the temperature range from 78 to 401 K. A polynomial equation of heat capacities as a function of temperature was fitted by least squares method. Based on the fitted polynomial, the smoothed heat capacities and thermodynamic functions of the compound relative to the standard reference temperature 298.15 K were calculated and tabulated at 5 K intervals. The constant‐volume energy of combustion of the compound at T=298.15 K was measured by a precision oxygen‐bomb combustion calorimeter as Δ_cU=−(35336.7±13.9) J·g⁻¹. The standard molar enthalpy of combustion of the compound was determined to be Δ_cH_m⁰=−(6163.2±2.4) kJ·mol⁻¹, according to the definition of combustion enthalpy. Finally, the standard molar enthalpy of formation of the compound was calculated to be Δ;_cH_m⁰=−(166.2±2.8) kJ·mol⁻¹ in accordance with Hess law.     

1，1－二氨基2，2－二硝基乙烯（DADE）的热化学性质、热行为和热分解机理

The constant-volume combustion energy, △cU （DADE, s, 298.15 K）, the thermal behavior, and kinetics and mechanism of the exothermic decomposition reaction of 1,1-diamino-2,2-dinitroethylene （DADE） have been investigated by a precise rotating bomb calorimeter, TG-DTG, DSC, rapid-scan fourier transform infrared （RSFT-IR） spectroscopy and T-jump/FTIR, respectively. The value of △cHm （DADE, s, 298.15 K） was determined as （-8518.09±4.59） j·g^-1. Its standard enthalpy of combustion, △cU （DADE, s, 298.15 K）, and standard enthalpy of formation, △fHm （DADE, s, 298.15 K） were calculated to be （-1254.00±0.68） and （- 103.98±0.73） kJ·mol^-1, respectively The kinetic parameters （the apparent activation energy Ea and pre-exponential factor A） of the first exothermic decomposition reaction in a temperature-programmed mode obtained by Kissinger＇s method and Ozawa＇s method, were Ek=344.35 kJ·mol^-1, AR= 1034.50 S^-1 and Eo=335.32 kJ·mol^-1, respectively. The critical temperatures of thermal explosion of DADE were 206.98 and 207.08 ℃ by different methods. Information was obtained on its thermolysis detected by RSFT-IR and T-jump/FTIR.