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1.
Anna Rita Campanelli Fabio Ramondo Aldo Domenicano István Hargittai 《Structural chemistry》1999,10(1):29-40
The molecular structure and conformation of p-bis(trimethylsilyl)benzene have been investigated by gas-phase electron diffraction, ab initio MO calculations at the HF/6-31G*, MP2(f.c.)/6-31G*, and B3LYP/6-31G* levels, and MM3 molecular mechanics calculations. The calculations indicate the syn- and anti-coplanar conformations, with two
bonds in the plane of the benzene ring, to be energy minima. The perpendicular conformations, with two
bonds in a plane orthogonal to the ring plane, are transition states. The two coplanar conformers have nearly the same energy with a low interconversion barrier, 0.3–0.5 kJ mol–1. The calculated lengths of the
and
bonds differ by only a few thousandths of an angstrom, in agreement with electron diffraction results from molecules containing either
or
bonds. The geometrical distortion of the benzene ring in p-bis(trimethylsilyl)-benzene may be described by superimposing independent distortions from each of the two SiMe3 groups. The electron diffraction intensities from a previous study (Rozsondai, B.; Zelei, B.; Hargittai, I. J. Mol. Struct.
1982, 95, 187) have been reanalyzed, imposing constraints from the theoretical calculations, and using a model based on a 1:1 mixture of the two coplanar conformers. The effective torsion angles of the SiMe3 groups may indicate nearly free rotation. Important geometrical parameters from the present electron diffraction analysis are
, and
. While the mean bond lengths are virtually the same from the previous and present analyses, the new ipso angle is in better agreement with the MO calculations [HF, 116.9° MP2(f.c.), 117.1° B3LYP, 116.9°]. 相似文献
2.
The standard molar Gibbs free energy of formation of ZnRh2O4(s) has been determined using an oxide solid-state electrochemical cell wherein calcia-stabilized zirconia (CSZ) was used
as an electrolyte. The oxide cell can be represented by: . The electromotive force was measured in the temperature range from 943.9 to 1,114.2 K. The standard molar Gibbs energy of
formation of ZnRh2O4(s) from elements in their standard state using the oxide electrochemical cell has been calculated and can be represented
by: . Standard molar heat capacity C
o
p,m(T) of ZnRh2O4(s) was measured using a heat flux-type differential scanning calorimeter in two different temperature ranges, from 127 to
299 and 307 to 845 K. The heat capacity in the higher temperature range was fitted into a polynomial expression and can be
represented by: . The heat capacity of ZnRh2O4(s), was used along with the data obtained from the oxide electrochemical cell to calculate the standard enthalpy and entropy
of formation of the compound at 298.15 K. 相似文献
3.
Dissociation Constants for Citric Acid in NaCl and KCl Solutions and their Mixtures at 25 °C 总被引:1,自引:0,他引:1
The constants for the dissociation of citric acid (H3C) have been determined from potentiometric titrations in aqueous NaCl and KCl solutions and their mixtures as a function of ionic strength (0.05–4.5 mol-dm–3) at 25 °C. The stoichiometric dissociation constants (Ki*)
were used to determine Pitzer parameters for citric acid (H3C), and the anions, H2C–, HC2–, and C3–. The thermodynamic constants (Ki) needed for these calculations were taken from the work of R. G. Bates and G. D. Pinching (J. Amer. Chem. Soc. 71, 1274; 1949) to fit to the equations (T/K):
The values of Pitzer interaction parameters for Na+ and K+ with H3C, H2C–, HC2–, and C3– have been determined from the measured pK values. These parameters represent the values of pK1*, pK2*, and pK3*, respectively, with standard errors of = 0.003–0.006, 0.015–0.016, and 0.019–0.023 for the first, second, and third dissociation constants. A simple mixing of the pK* values for the pure salts in dilute solutions yield values for the mixtures that are in good agreement with the measured values. The full Pitzer equations are necessary to estimate the values of pKi* in the mixtures at high ionic strengths. The interaction parameters found for the mixtures are Na-K – H2C = – 0.00823 ± 0.0009; Na-K – HC = – 0.0233 ± 0.0009, and Na-K – C = 0.0299 ± 0.0055 with standard errors of (pK1) = 0.011, (pK2) = 0.011, and (pK3) = 0.055. 相似文献
4.
The molecular structure of 1,2,4,5-tetracyanobenzene has been determined by gas-phase electron diffraction and by ab initio calculations at several levels of theory. The electron diffraction study indicates an elongation of the aromatic ring along the (H)CC(H) axis, characterized by angular deformation of the benzene ring and lengthening of the (NC)C—C(CN) bonds. The following bond lengths (r
g) and bond angles were obtained by electron diffraction:
. 相似文献
5.
Summary The mechanical properties of the most aluminium alloys depend strongly on their chemical composition, casting methods and the heat treatment. Alloys of the type G-AlMg5Si are known for good corrosion resistance and mechanical properties at elevated temperatures. Under the trade mark Hydronalium (Hy 511) they are used for the production of cylinder heads for air-cooled Diesel engines. To obtain better chemical characteristics, titanium is added to the alloy. This paper deals with the results obtained during investigations about the distribution of elements in the binary eutectic Mg2Si and the ternary eutectic
as well as with the distribution of titanium in samples of Hy 511, obtained during casting of cylinder heads. Studies of the distribution of the elements were performed using EDX/WDX spectrometers, and the distribution of titanium was studied also with Auger electron spectroscopy. 相似文献
6.
Raman spectroscopic measurements were performed on aqueous acid to neutral silica-bearing solutions (0.005 ≤ m Si ≤ 0.02, 0 ≤ pH ≤ 8) and Al–silica solutions at temperature from 20 to 150°C. At 20°C, the spectrum of silica-bearing solutions exhibits only the bands of water and a completely polarized band at 785 cm?1. This band is attributed to the ν1 band of the tetrahedral Si(OH)4 molecule. In ${\text{Si(OH)}}_{\text{4}} {\kern 1pt} {\kern 1pt} - {\kern 1pt} {\text{AlCl}}_3 {\kern 1pt} - {\kern 1pt} {\text{HCl}}$ solutions, the intensity of this band decreases with increasing Al concentration, temperature, and pH. This decrease can be explained by the formation of an inner sphere complex between Al3+ and Si(OH)4 according to the reaction: ${\text{Al}}^{{\text{3 + }}} {\text{ + H}}_{\text{4}} {\text{SiO}}_{\text{4}}^{\text{0}} ({\text{aq}}){\text{ }} \Leftrightarrow {\text{ AlH}}_{\text{3}} {\text{SiO}}_{\text{4}}^{{\text{2 + }}} {\text{ + H}}^{\text{ + }} $ The fraction of complexed silica deduced from raman spectroscopic measurements is in good agreement with that calculated for the similar solution compositions and temperatures using the complexation constant generated by Pokrovski et al. (23) from potentiometric measurements. At ambient temperature, the formation of aluminum silicate complex is weak and does not account for more than ca. 5 % of the total Al in most natural waters. As temperature increases, this complex becomes more significant and can dominate Al speciation in acid (pH ≤ 2) hydrothermal solutions. 相似文献
7.
Ab initio molecular orbital theory with the 6-31G(d), 6-31G(2d), 6-31+G(d), 6-31G(d,p), 6-31+G(d,p), and 6-311G(d,p) basis sets and the hybrid density functionals B3LYP, B3P86, and B3PW91 have been used to calculate the optimized geometries and relative energies of the chair, half-chair, sofa, twist, and boat structures of 2-thiaoxacyclohexane (1,2-oxathiane). The values of the energy difference (E, kcal/mol) between the chair and 3,6-twist structures of 1,2-oxathiane were 4.92 (HF), 4.73 (MP2), and 4.66 (DFT). The HF chair–twist energy difference (G
c–t
o) for 1,2-oxathane was 5.16 kcal/mol. Intrinsic reaction coordinate (IRC) calculations connected a transition state (TS-A) between the chair conformation and the less stable 2,5-twist form and connected two transition states (TS-B, TS-C) between the chair conformation and the more stable 3,6-twist conformer. The DFT energy differences between the chair and TS-A, TS-B, and TS-C were 11.4, 10.8, and 12.6 kcal/mol, respectively. Hyperconjugative stereoelectronic interactions were observed in the chair (n
o
and
) and 3,6-twist (n
S
and n
O
) conformers of 1,2-oxathiane. The chair conformation of 1,2-oxthiane is 9.6 and 10.0 kcal/mol, respectively, less stable than the chair conformations of 3-thiaoxacyclohexane (1,3-oxathiane) and 4-thiaoxacyclohexane (1,4-oxathiane, thioxane). 相似文献
8.
The integral equation method was used to study structure formation in concentrated aqueous solutions of lithium chloride in the near-critical region (P = 20 MPa, T = 298-623 K). It is found that when the LiCl–H2O system passes from a concentrated state to a melt-like one (
), some of its structural characteristics change oppositely depending on temperature, leading to anomalous structural properties of the most concentrated solution. Analyzing the temperature dependence of coordination numbers we established the temperature range and the steps of structural changes in the LiCl–H2O system. 相似文献
9.
Fifteen unique energy minima and thirteen transition states on the C 2H2B2 potential surface have been located and optimized at the MP2 level of theory with the 6-311G(d,p) basis set. The planar four-membered ring isomer
, 1, an analog of cyclobutadiene, is a transition state lying 37 kcal/mol above the nonplanar four-membered ring
, 3. The planar
, 10, is the second most stable species found, lying 72.2 kcal/mol below 3. The nonplanar, butterfly-shaped
ring, 4, is a local minimum 33.7 kcal/mol more stable than 3. A four-membered ring isomer with alternating boron–carbon locations,
, 5, lies 67.0 kcal/mol below 3 and 33.3 kcal/mol below 4. The ring of 5 is planar with one hydrogen above and one below the plane (C
2h symmetry). The borylene-substituted boracyclopropene,
, 8, is a planar local minimum lying 36.0 kcal/mol above 5. The most stable C2H2B2 isomer found was the planar, four-membered ring system 22 (D
2h symmetry) composed of two BCC three-membered rings fused across the C-C bond. Structure 22 lies 22.2 kcal/mole below 10, 105.4 kcal/mol below 3, 71.7 kcal/mol below 4, and 38.2 kcal/mol below 5. Isomer 22 is the structural analog of the trialene form of C4H2. The most stable linear isomer, HB
BH, 26, was surprisingly 50.5 kcal/mol less stable than 22. The stabilities of the two most stable cyclic isomers 10 and 22 may be explained by aromaticity. 相似文献
10.
Some equilibria involving gold(I) thiomalate (mercaptosuccinate, TM) complexes have been studied in the aqueous solution at 25 °C and I?=?0.2 mol·L?1 (NaCl). In the acidic region, the oxidation of TM by \( {\text{AuCl}}_{4}^{ - } \) proceeds with the formation of sulfinic acid, and gold(III) is reduced to gold(I). The interaction of gold(I) with TM at nTM/nAu?≤?1 leads to the formation of highly stable cyclic polymeric complexes \( {\text{Au}}_{m} \left( {\text{TM}} \right)_{m}^{*} \) with various degrees of protonation depending on pH. In general, the results agree with the tetrameric form of this complex proposed in the literature. At nTM/nAu?>?1, the processes of opening the cyclic structure, depolymerization and the formation of \( {\text{Au}}\left( {\text{TM}} \right)_{2}^{*} \) occur: \( {\text{Au}}_{4} ( {\text{TM)}}_{4}^{8 - } + {\text{TM}}^{3 - } \rightleftharpoons {\text{Au}}_{ 4} ( {\text{TM)}}_{5}^{11 - } \), log10 K45?=?10.1?±?0.5; 0.25 \( {\text{Au}}_{4} ( {\text{TM)}}_{4}^{8 - } + {\text{TM}}^{3 - } \rightleftharpoons {\text{Au(TM)}}_{2}^{5 - } \), log10 K12?=?4.9?±?0.2. The standard potential of \( {\text{Au(TM)}}_{2}^{5 - } \) is \( E_{1/0}^{ \circ } = -0. 2 5 5\pm 0.0 30{\text{ V}} \). The numerous protonation processes of complexes at pH?<?7 were described with the use of effective functions. 相似文献
11.
The oxidation of H2NOH is first-order both in [NH3OH+] and [AuCl4
–]. The rate is increased by the increase in [Cl–] and decreased with increase in [H+]. The stoichiometry ratio, [NH3OH+]/[AuCl4
–], is 1. The mechanism consists of the following reactions.
The rate law deduced from the reactions (i)–(iv) is given by Equation (v) considering that [H+] K
a.
The reaction (iii) is a combination of the following reactions:
The activation parameters for the reactions (ii) and (iii) are consistent with an outer-sphere electron transfer mechanism. 相似文献
12.
Summary The oxidation of H2O2 by [W(CN)8]3– has been studied in aqueous media between pH 7.87 and 12.10 using both conventional and stopped-flow spectrophotometry. The reaction proceeds without generation of free radicals. The experimental overall rate law,
, strongly suggests two types of mechanisms. The first pathway, characterized by the pH-dependent rate constant k
s, given by
, involves the formation of [W(CN)8· H2O2]3–, [W(CN)8· H2O2·W(CN)8]6– and [W(CN)8· HO]3– intermediates in rapid pre-equilibria steps, and is followed by a one-electron transfer step involving [W(CN)8·HO]3– (k
a) and its conjugate base [W(CN)8·O]4– (k
b). At 25 °C, I = 0.20 m (NaCl), the rate constant
with H
a
=40±6kJmol–1 and S
a
=–151±22JK–1mol–1; the rate constant
with H
b
=36±1kJmol–1 and S
b
=–136±2JK–1mol–1 at 25 °C, I = 0.20 m (NaCl); the acid dissociation constant of [W(CN)8·HO]3–, K
5
=(5.9±1.7)×10–10
m, with
and
is the first acid dissociation constant of H2O2. The second pathway, with rate constant, k
f, involves the formation of [W(CN)8· HO2]4– and is followed by a formal two-electron redox process with [W(CN)8]3–. The pH-dependent rate constant, k
f, is given by
. The rate constant k
7
=23±6m
–1
s
–1 with
and
at 25°C, I = 0.20 m (NaCl). 相似文献
13.
R. Pankajavalli Ashish Jain R. Babu S. Anthonysamy K. Ananthasivan V. Ganesan K. Nagarajan 《Journal of Thermal Analysis and Calorimetry》2013,111(2):1609-1614
The standard Gibbs energy of formation of Pr2TeO6 $ (\Updelta_{\text{f}} G^{^\circ } \left( {{ \Pr }_{ 2} {\text{TeO}}_{ 6} ,\;{\text{s}}} \right)) $ was derived from its vapour pressure in the temperature range of 1,400–1,480 K. The vapour pressure of TeO2 (g) was measured by employing a thermogravimetry-based transpiration method. The temperature dependence of the vapour pressure of TeO2 over the mixture Pr2TeO6 (s) + Pr2O3 (s) generated by the incongruent vapourization reaction, Pr2TeO6 (s) = Pr2O3 (s) + TeO2 (g) + ½ O2 (g) could be represented as: $ { \log }\left\{ {{{p\left( {{\text{TeO}}_{ 2} ,\;{\text{g}}} \right)} \mathord{\left/ {\vphantom {{p\left( {{\text{TeO}}_{ 2} ,\;{\text{g}}} \right)} {{\text{Pa}} \pm 0.0 4}}} \right. \kern-0em} {{\text{Pa}} \pm 0.0 4}}} \right\} = 19. 12- 27132\; \left({\rm{{{\text{K}}}}/T} \right) $ . The $ \Updelta_{\text{f}} G^{^\circ } \;\left( {{ \Pr }_{ 2} {\text{TeO}}_{ 6} } \right) $ could be represented by the relation $ \left\{ {{{\Updelta_{\text{f}} G^{^\circ } \left( {{ \Pr }_{ 2} {\text{TeO}}_{ 6} ,\;{\text{s}}} \right)} \mathord{\left/ {\vphantom {{\Updelta_{\text{f}} G^{^\circ } \left( {{ \Pr }_{ 2} {\text{TeO}}_{ 6} ,\;{\text{s}}} \right)} {\left( {{\text{kJ}}\,{\text{mol}}^{ - 1} } \right)}}} \right. \kern-0em} {\left( {{\text{kJ}}\,{\text{mol}}^{ - 1} } \right)}} \pm 5.0} \right\} = - 2 4 1 5. 1+ 0. 5 7 9 3\;\left(T/{\text{K}}\right) .$ Enthalpy increments of Pr2TeO6 were measured by drop calorimetry in the temperature range of 573–1,273 K and heat capacity, entropy and Gibbs energy functions were derived. The $ \Updelta_{\text{f}} H_{{298\;{\text{K}}}}^{^\circ } \;\left( {{ \Pr }_{ 2} {\text{TeO}}_{ 6} } \right) $ was found to be $ {{ - 2, 40 7. 8 \pm 2.0} \mathord{\left/ {\vphantom {{ - 2, 40 7. 8 \pm 2.0} {\left( {{\text{kJ}}\,{\text{mol}}^{ - 1} } \right)}}} \right. \kern-0em} {\left( {{\text{kJ}}\,{\text{mol}}^{ - 1} } \right)}} $ . 相似文献
14.
Marcelo R. L. Oliveira José E. J. C. Graúdo Nivaldo L. Speziali Vito M. De Bellis 《Structural chemistry》1999,10(1):41-45
Potassium N-4-methylphenylsulfonyldithiocarbimate, K2(4-CH3C6H4SO2N=CS2), reacted with nickel(II) chloride hexahydrate and tetrabutylammonium bromide to form the bis(N-4-methylphenylsulfonyldithiocarbimate)nickelate(II) tetrabutylammonium salt (Bu4N)2[Ni(4-CH3C6H4S2C=NSO2)2]. The elemental analyses, IR and UV-Vis data obtained were consistent with the formation of a
diamagnetic planar complex. The 1H NMR and the 13C NMR spectra showed the expected signals for the dithiocarbimate moiety and the tetrabutylammonium cation. The single-crystal structure analysis showed that this substance crystallizes in the triclinic
space group with a = 10.474(3) Å, b = 10.767(3) Å, c = 13.657(3) Å and = 81.54(2)°, = 80.44(2)°, = 67.63(2)°, V = 1398.5(8) Å3, and Z = 2. The nickel atom is coordinated to four sulfur atoms. 相似文献
15.
J. Heidberg E. Kampshoff M. Kandel R. Kühnemuth D. Meine O. Schönekäs M. Suhren H. Weiss 《Reaction Kinetics and Catalysis Letters》1993,50(1-2):123-129
Polarized infrared spectra of adsorbates on NaCl(100) and MgO(100) single crystal cleavage planes reveal Davydov splittings, structures with adsorbate orientations and/or 2D-phase transitions, in concert with diffraction studies. CO2 on NaCl shows the sharpest adsorbate spectra known (
). 相似文献
16.
Kinetics and equilibria for the formation of a 1:1 complex between palladium(II) and chloroacetate were studied by spectrophotometric measurements in 1.00 mol HClO4 at 298.2 K. The equilibrium constant, K, of the reaction
was determined from multi-wavelength absorbance measurements of equilibrated solutions at variable temperatures as log 0.006 with and , and spectra of individual species were calculated. Variable-temperature kinetic measurements gave rate constants for the forward and backward reactions at 298.2 K and ionic strength 1.00 mol as and , with activation parameters and , respectively. From the kinetics of the forward and reverse processes, and were derived in good agreement with the results of the equilibrium measurements. Specific Ion Interaction Theory was employed for determination of thermodynamic equilibrium constants for the protonation of chloroacetate () and formation of the PdL+ complex (). Specific ion interaction coefficients were derived. 相似文献
17.
Densities, ??, and viscosities, ??, of binary mixtures of 2-methyl-2-propanol with acetone (AC), ethyl methyl ketone (EMK) and acetophenone (AP), including those of the pure liquids, were measured over the entire composition range at 298.15, 303.15 and 308.15?K. From these experimental data, the excess molar volume $V_{\mathrm{m}}^{\mathrm{E}}$ , deviation in viscosity ????, partial and apparent molar volumes ( $\overline{V}_{\mathrm{m},1}^{\,\circ }$ , $\overline{V}_{\mathrm{m},2}^{\,\circ }$ , $\overline{V}_{\phi ,1}^{\,\circ}$ and $\overline{V}_{\phi,2}^{\,\circ} $ ), and their excess values ( $\overline{V}_{\mathrm{m},1}^{\,\circ \mathrm{E}}$ , $\overline{V}_{\mathrm{m,2}}^{\,\circ \mathrm{ E}}$ , $\overline {V}_{\phi \mathrm{,1}}^{\,\circ \mathrm{ E}}$ and $\overline{V}_{\phi \mathrm{,2}}^{\,\circ \mathrm{ E}}$ ) of the components at infinite dilution were calculated. The interaction between the component molecules follows the order of AP > AC > EMK. 相似文献
18.
The equilibrium constant for the hydrolytic disproportionation of I2
has been determined at 25°C and at ionic strength 0.2 M(NaClO4) in buffered solution. The reaction was followed in the pH range where the equilibrium concentration of I2, I–, and IO3
–are commensurable, i.e., the fast equilibrium
is also established. The equilibrium concentrations of I2and I3
–were determined spectrophotometrically, and the concentrations of all the other species participating in process (1) were calculated from the stoichiometric constraints. The constants determined are \log K_1 = -47.61\pm 0.07 and \log K_2 = 2.86 \pm 0.01. 相似文献
19.
New experimental vapor pressures and vaporization enthalpies of the ionic liquids \( [ {\text{C}}_{2} {\text{mim][CF}}_{3} {\text{CO}}_{2} ] \) and \( [ {\text{C}}_{4} {\text{mim][CF}}_{3} {\text{CO}}_{2} ] \) have been measured by the QCM method. The solution enthalpies of these ionic liquids were measured by using high-precision solution calorimetry and were used for calculation the aqueous enthalpy of formation \( \Delta_{\text{f}} H_{\text{m}}^{ \circ } ({\text{CF}}_{ 3} {\text{CO}}_{2}^{ - } ,_{{}} {\text{aq}}) \) of the anion for combination with quantum-chemical results. The solubility parameters of the ILs under study have been derived from experimental \( \Delta_{\text{l}}^{\text{g}} H_{\text{m}}^{ \circ } \)(298.15 K) values and were used for estimation of miscibility of some common solutes with \( [ {\text{C}}_{n} {\text{mim][CF}}_{3} {\text{CO}}_{2} ] \). 相似文献
20.
The enhanced influence of charge-assisted hydrogen bonds upon the resulting crystal structure in the presence of competing noncovalent interactions has been examined using single-crystal X-ray diffraction. The crystal structures of six organic salts are presented: 4-chlorobenzylammonium 2,4-dichlorobenzoate, 3-chlorobenzylammonium 2,4-dichlorobenzoate, 4-methylbenzylammonium pentafluorobenzoate, 4-chlorobenzylammonium pentafluorobenzoate, 3-methylbenzylammonium benzoate, and 4-chlorobenzylammonium benzoate monohydrate. All structures are dominated by charge-assisted
hydrogen bonds that generate either one-dimensional ribbons or two-dimensional sheets. The structural influence of weaker intermolecular interactions is limited as there are very few short, halogen-halogen, -, or
distances in this series of compounds. 相似文献