Studies of the complex formation of silver (I) ion with 18-crown-6 in H2O-DMSO mixtures by calorimetry

The thermodynamic parameters (D_rG⁰, D_r H ⁰, TD_r S ⁰) of the reaction of [Ag18C6]⁺ complex formation were obtained for a wide range of H₂O-DMSO mixtures from the calorimetric data at 298.15 K. The relation between the thermodynamic parameters of complex formation and solvation of each reagent was investigated. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of background electrolyte concentration on the heat effects in the processes of formation of L-asparagine complexes with Cu<Superscript>2+</Superscript> ions in aqueous solution

A direct calorimetric method was used to measure the heat effects in the reactions of formation of Cu(II) complexes with L-asparagine in aqueous solutions at 298.15 K and ionic strength 0.5, 1.0, 1.5 (KNO₃). The standard thermodynamic characteristics (Δ_r H ⁰, Δ_r G ⁰, Δ_r S ⁰) of the processes of complex formation in the system under study were calculated.     

Nonlinear viscoelasticity of polystyrene solutions. I. Strain-dependent relaxation modulus

Strain-dependent relaxation moduli G(t,s) were measured for polystyrene solutions in diethyl phthalate with a relaxometer of the cone-and-plate type. Ranges of molecular weight M and concentration c were from 1.23 × 10⁶ to 7.62 × 10⁶ and 0.112 to 0.329 g/cm³. Measurements were performed at various magnitudes of shear s ranging from 0.055 to 27.2. The relaxation modulus G(t,s) always decreased with increasing s and the relative amount of decrease (i.e.,–log[G(t,s)/G(t,0)]) increased as t increased. However, the detailed strain dependences of G(t,s) could be classified into two types according to the M and c of the solution. When cM < 10⁶, the plot of log G(t,s) versus log t varied from a convex curve to an S-shaped curve with increasing s. For solutions of cM > 10⁶, the curves were still convex and S-shaped at very small and large s, respectively, but in a certain range of s (approximately 3 < s < 7) log G(t,s) decreased rapidly at short times and then very slowly; a peculiar inflection and a plateau appeared on the plot of log G(t,s) versus log t. The strain-dependent relaxation spectrum exhibited a trough at times corresponding to the plateau of log G(t,s). The longest relaxation time τ₁(s) and the corresponding relaxation strength G₁(s) were evaluated through the “Procedure X” of Tobolsky and Murakami. The relaxation time τ₁(s) was independent of s for all the solutions studied while G₁(s) decreased with s. The reduced relaxation strength G₁(s)/G₁(0) was a simple function of s (The plot of log G₁(s)/G₁(0) against log s was a convex curve) and was approximately independent of M and c in the range of cM <10⁶. This behavior of G₁(s)/G₁(0) was in agreement with that observed for a polyisobutylene solution and seems to have wide applicability to many polymeric systems. On the other hand, log G₁(s)/G₁(0) as a function of log s decreased in two steps and decreased more rapidly when M or c was higher. It was suggested that in the range of cM < 10⁶, a kind of geometrical factor might be responsible for a large part of the nonlinear behavior, while in the range of cM > 10⁶, some “intrinsic” nonlinearity of the entanglement network system might be important.     

Estimation of the agglomeration structure for conductive particles and fiber‐filled high‐density polyethylene through dynamic rheological measurements

A study on the correlation between electrical percolation and viscoelastic percolation for carbon black (CB) and carbon fiber (CF) filled high‐density polyethylene (HDPE) conductive composites was carried out through an examination of the filler concentration (?) dependence of the volume resistivity (ρ) and dynamic viscoelastic functions. For CB/HDPE composites, when ? was higher than the modulus percolation threshold (?_G ～ 15 vol %), the dynamic storage modulus (G′) reached a plateau at low frequencies. The relationship between ρ and the normalized dynamic storage modulus (G_c^′/G_p^′, where G_c^′ and G_p^′ are the dynamic storage moduli of the composites and the polymer matrix, respectively) was studied. When ? approached a critical value (?_r), a characteristic change in G_c^′/G_p^′ appeared. The critical value (G_c^′/G^′_p)c was 9.80, and the corresponding ?_r value was 10 vol %. There also existed a ? dependence of the dynamic loss tangent (tan δ) and a peak in a plot of tan δ versus the frequency when ? approached a loss‐angle percolation (?_δ = 9 vol %). With parameter K substituted for A, a modified Kerner–Nielson equation was obtained and used to analyze the formation of the network structure. The viscoelastic percolation for CB/HDPE composites could be verified on the basis of the modified equation, whereas no similar percolation was found for CF/HDPE composites. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1199–1205, 2004     

Thermochemical study of acid-base interactions in L-asparagine aqueous solutions

The heats of reactions between an L-asparagine solution and HNO₃ solutions were measured calorimetrically within different pH ranges at 298.15, 308.15, and 318.15 K and an ionic strength of 0.5, 1.0, and 1.5 (KNO₃ and LiNO₃). The heats of stepwise dissociation of L-asparagine were determined. The standard thermodynamic characteristics (Δ _r H ^o, Δ _r G ^o, δ _r S ^o, and ΔC _p ^⪧) of the acid-base interactions in aqueous solutions of the amino acid were calculated. A relationship between the thermodynamic characteristics of the dissociation of L-asparagine and its structure was considered.     

Cationic radius ratio and formation of K2NiF4-type compounds

A survey of known compounds which can assume K₂NiF₄-type structure at least over a certain P, T range was made. The cationic radius ratios r_A/r_B of this family of compounds were found to vary over a relatively narrow range of values around 2.00, in contrast to the much wider ranges known for other important A₂BX₄ structure groups, e.g., β-K₂SO₄, thenardite, olivine, etc., providing thereby a constraint on the crystal chemical variabilities in this group of compounds. Usefulness of the simple size parameter r_A(IX)/r_B(VI) in predicting formation of new K₂NiF₄-type compounds is discussed.     

Thermodynamics of mixed-ligand complexation of cerium group lanthanide ethylenediaminetetraacetates

The thermodynamic parameters (logK, Δ _r G ⁰, Δ _r H, and Δ _r S) for mixed-ligand complexation of LnEdta⁻ (Ln³⁺ = La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Gd³⁺) with iminodiacetate and nitrilotriacetate ions in aqueous solution at 298.15 K and ionic strength I = 0.5 (KNO₃) were determined by calorimetry and pH-metry. The variation of the thermodynamic parameters of the reactions over the lanthanide series was discussed.     

Thermodynamics of mixed-ligand complexation of mercury(II) ethylenediaminetetraacetate with histidine and lysine in aqueous solution

The formation of mixed-ligand complexes HgEdtaIm²⁻, HgEdtaL³⁻, HgEdtaHL²⁻, and (HgEdta)₂L⁵⁻ (L is histidine, lysine; Im is imidazole) was studied by calorimetry, pH-metry, and NMR spectroscopy. The thermodynamic parameters (logK, ΔrG ⁰, ΔrH, Δr _S) for the reactions of complex formation at 298.15 K and ion strength of 0.5 (KNO₃) were determined. The most likely coordination mode for the complexone and amino acid in the mixed complexes was identified.     

Theoretical study with vibration–rotation and dipole moment calculations of quartet states of the CrCl molecule

The potential energy curves have been investigated for the 10 lowest quartet electronic states in the ^2s+1Λ^± representation below 30,000 cm^?1 of the molecule CrCl via CASSCF and MRCI (singly and doubly excitation with Davidson correction) calculations. The harmonic frequency ω_e, the internuclear distance r_e, the rotational constant B_e, the electronic energy with respect to the ground state T_e, and the permanent dipole moment μ have been calculated. By using the canonical functions approach, the eigenvalues E_v, the rotational constant B_v, and the abscissas of the turning points r_min and r_max have been calculated for the considered electronic states up to the vibrational level v = 19. Seven electronic states have been studied here theoretically for the first time. The comparison of these values to the theoretical results available in the literature shows a good agreement. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Extended molecular symmetry groups

The group theoretical analysis of Longuet-Higgins molecular symmetry groupsG(m, n) andGB(m) and their torsional extensionsG ^q (m, n) andGB ²(m) of molecules consisting of two coaxially rotated parts is presented. The structure of the groups is described in terms of direct and semi-direct products. The groupsG ^q (m, n) andGB ²(m) are shown to be group extensions of the groupsG(m,n) andGB(m), respectively. All irreducible representations of the groupsG(m,n),GB(m), andGB ²(m) are derived using standard techniques of the extension or induction. A restriction method is proposed for derivation of irreducible representations of the groupG ^q (m,n). The class structure of the groups is determined and the character tables are given in the most general case.     

Estimation of flocculation structure in filled polymer composites by dynamic rheological measurements

 The frequency and concentration dependences of the storage modulus (G ^′) for carbon black and short-carbon-fiber-filled polymer composites were investigated by means of dynamic rheological measurements. It was found that G ^′ at low frequencies and amplitudes could be used as a sensitive experimental parameter for detecting the flocculation structure of the ultra-fine-particle-filled polymer composites. Correlation of electrical resistivity of the composites to the relative storage modulus, G ^′ _r(=G ^′ _c/ G ^′ _p), revealed that the three-dimensional interparticle networks start to construct through the matrix when G ^′ _r increases to 7 regardless of the composite systems. Quantitative calculations in order to determine the flocculation structure were carried out by means of the modified Kerner equation. A plot of the calculated value, defined as the floc index A, dependence of electrical resistivity for various systems was found to be a universal curve. Accordingly, we suggest that A might universally correspond to the flocculation structure of the filler, which is independent of the nature of the filler, the molecular weight, the chemical composition of the polymer and the temperature at which the measurement is made. This method is particularly effective for estimating the flocculation structure of ultra-fine-particle-filled polymer composites no matter whether the filler is conductive or not. Received: 26 May 1999/Accepted in revised form: 28 September 1999     

Thermodynamic characteristics of the acid dissociation of dopamine hydrochloride in water-ethanol solutions

Enthalpies of the interaction of protonated dopamine with a hydroxide ion in water-ethanol mixtures in the concentration range of 0–0.8 EtOH mole fractions are measured calorimetrically. The neutralization process of dopamine hydrochloride is shown to occur endothermally in solvents with an ethanol concentration of ≥0.5 mole fractions. Standard thermodynamic characteristics (Δ_r H ^○, Δ_r G ^○, and Δ_r S ^○) of the first-step acid dissociation of dopamine hydrochloride in solutions are calculated with regard to the autoprotolysis enthalpy of binary solvents. It is found that dissociation enthalpies vary within 9.1–64.8 kJ/mol, depending on the water-ethanol solvent composition.     

Extensions of the Marcus equation for the prediction of approximate transition state geometries in hydrogen transfer and methyl transfer reactions

The objective of this work is to propose a way to calculate approximate transition state geometries that can then be used as initial guesses in ab initio calculations. Transition state geometries are calculated for 26 hydrogen transfer reactions and 6 methyl transfer reactions at the MP2/6-31G* and MP2/6-311++G(d,p) levels. Selected cases are also done at other levels including CCSD(T)/6-311++G(d,p). The transition state geometry obeys an equation which arises from an extension of the Marcus equation proposed by Blowers and Masel [8]: In this equation, r B _,equ is the equilibrium bond length for the bond that breaks during the reaction, r F _,equ is the equilibrium bond length for the new bond which forms. r ^t _B and r ^t _F are the bond lengths at the saddle point in the potential energy surface. r ^t _B and r ^t _F are found to obey with an average error of 0.04 ?. In the last two equations above, ΔU is the heat of reaction, E ⁰ _A is the intrinsic barrier, and C _A is a constant that comes from the model of Blowers and Masel [8]. It is proposed that the above three equations are useful in generating initial guesses for transition state geometries in ab initio calculations. In the cases that were tried, rapid convergence was found when these guesses were used. Received: 23 February 2000 / Accepted: 13 May 2000 / Published online: 27 September 2000     

On the connectivity index of trees

The connectivity index χ₁(G) of a graph G is the sum of the weights d(u)d(v) of all edges uv of G, where d(u) denotes the degree of the vertex u. Let T(n, r) be the set of trees on n vertices with diameter r. In this paper, we determine all trees in T(n, r) with the largest and the second largest connectivity index. Also, the trees in T(n, r) with the largest and the second largest connectivity index are characterized. Mei Lu is partially supported by NNSFC (No. 10571105).     

Conformational Analysis. XIX properties and reactions of 1,3-oxathianes VIII A 1H NMR conformational study of methyl-substituted derivatives

The addition of thioacetic acid to unsaturated alcohols or acids was utilized to obtain mercaptoalkanols which were condensed with suitable carybonyl compounds to prepare 24 methyl-substituted 1,3-oxathianes. The ¹H NMR spectra of the 1,3-oxathiane products were recorded at 60, 100 and/or 300 MHz and fully analysed. The results are best explained by a chair form which is completely staggered in the C-4? C-5? C-6 moiety ψ₄₅ or (ψ₅₆=60±1°). 1,3-Oxathianes having syn-axial 2,4- (and/or 2,6-) methyl-methyl interactions exist appreciably, if not exclusively, in twist forms. The vicinal coupling constants lead to the conformational free energies of axial methyl groups at C-4, ΔG°=7.4±0.4 kJ mol^?1, and at C-5, ΔG°=3.7±0.3 kJ mol^?1, in good agreement with previous estimates. They also show that both r-4,cis-5,trans-6- and r-4,trans-5,trans-6- trimethyl-1,3-oxathianes greatly favour the chiar form where the methyl group at C-4 is axial. The chair-twist energy parameters are reestimated at ΔH°_CT 27.0 kJ mol^?1, ΔS°_CT 11.6J mol^?1K^?1, and ΔG°_CT(298) 23.5 kJ mol^?1 for a 2,5-twist form.     

Molecular structure of ethynylbenzene from electron diffraction and ab initio molecular orbital calculations

The molecular structure and benzene ring distortions of ethynylbenzene have been investigated by gas-phase electron diffraction and ab initio MO calculations at the HF/6-31G^* and 6-3G^** levels. Least-squares refinement of a model withC _2v, symmetry, with constraints from the MO calculations, yielded the following important bond distances and angles:r _g(C _i -C _o )=1.407±0.003 Å,r _g(C _o -C _m )=1.397±0.003 Å,r _g(C _m -C _p )=1.400±0.003 Å,r _g(Cr _i -CCH)=1.436 ±0.004 Å,r _g(C=C)=1.205±0.005 Å, C _o -C _i -C _o =119.8±0.4°. The deformation of the benzene ring of ethynylbenzene given by the MO calculations, including o-C_i-C_o=119.4°, is insensitive to the basis set used and agrees with that obtained by low-temperature X-ray crystallography for the phenylethynyl fragment, C₆H₅-CC-, in two different crystal environments. The partial substitution structure of ethynylbenzene from microwave spectroscopy is shown to be inaccurate in the ipso region of the benzene ring.     

Mixed complex formation of lead(II) and mercury(II) ethylenediaminetetraacetates with thiourea in an aqueous solution

The formation of mixed-ligand complexes HgEdtaThio²⁻, HgEdtaS₂O₃⁴⁻, PbEdtaThio²⁻, and Pb(Thio) _i ²⁺, i = 1, 2; Thio is thiourea) was studied by calorimetry, pH metry, and ¹H and ¹³C NMR spectroscopy. The thermodynamic parameters (logK, Δ _r G ⁰, Δ _r H, and Δ _r S) for the formation of the complexes at 298.15 K and the ionic strength I = 0.5(NaClO₄) were determined. The most probable coordination mode of the ligands in the mixed complex was considered.     

Simultaneous determination of aflatoxins B1, B2, G1, G2 in Fructus Bruceae by high‐performance liquid chromatography with online postcolumn photochemical derivatization

A simple, reliable, and highly sensitive method for the simultaneous determination of aflatoxin B₁, B₂, G₁, G₂ in Fructus Bruceae was developed using high‐performance liquid chromatography coupled to online postcolumn photochemical derivatization and fluorescence detection. Aflatoxins were first extracted by a methanol/water mixture and then cleaned up with an AflaTest? immunoaffinity column. Different clean‐up and derivatization methods were compared and optimized. The established method was extensively validated to show satisfactory performance of linearity (R² ≥ 0.9997), recovery (74.3–100.8%), and precision (RSDs ≤ 2.8%) for the investigated aflatoxins. This proposed method was also applied to 11 F. Bruceae samples and the results showed that 10 out of 11 were contaminated with aflatoxins ranging from 0.26 to 27.52 μg/kg and the occurrence of aflatoxin B₁, the most toxic one, was as high as 91% in all the samples, highlighting the severe contamination and the necessity to set legal limits for aflatoxins in F. Bruceae.     

Thermodynamics of mixed-ligand complexation of yttrium group lanthanide ethylenediaminetetraacetates

Mixed-ligand complexation of yttrium subgroup lanthanide ethylenediaminetetraacetates with asparaginate, iminodiacetate, and nitrilotriacetate ions in aqueous solution was studied by calorimetry and pH-metry. The full set of thermodynamic parameters (logK, Δ _r G ₀, Δ _r H, Δ _r S) of the addition reactions of Asp²⁻, Ida²⁻, and Nta³⁻ to LnEdta⁻ (Ln³⁺ = Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺) was determined at 298.15 K and ionic strength I = 0.5(KNO₃). The change in the thermodynamic parameters of the reactions over the series of lanthanides was discussed.     

Preparation,Structural Characterization and the Molecular Structure of 2,3,5-Trinitro-<Emphasis Type="Italic">p</Emphasis>-xylene

2,3,5-trinitro-p-xylene (TPX) is synthesized by nitration of p-xylene in mixed acid of nitric and sulfuric acid. Single crystal of TPX is cultured from DMF solution using a slow cooling method. The compound is characterized by FT-IR, ¹H NMR and MS techniques. The crystal structure is determined by X-ray⁴ single-crystal diffraction analysis. The crystal belongs to the monoclinic system with space group P_n. Its unit cell parameters are as follows: a = 0.8271(2), b = 0.6011(1), c = 1.0487(2) nm, β = 105.42(2)^∘, V = 0.50263(2) nm³, D_c = 1.593 g/cm³, Z = 2, F(0 0 0) = 248. The molecular structures of TPX have been calculated at the B3LYP/6-31G^** and B3LYP/6-311G^** levels of theory, and its frequencies analysis have also been accomplished at the same level of theory. The thermal decomposition process of the compound was studied using DSC and TG-DTG techniques. The predicted nitro group vibrations with B3LYP/6-311G^** calculation considerably agree with the observed frequencies. The calculated results propose that the structural parameters from the theory are close to those of the crystal structure from the experiments.