Molecular structures,vibrational assignments and barriers to pseudorotation of gaseous niobium pentachloride and tantalum pentachloride from electron diffraction data

The pentachlorides of niobium and tantalum have trigonal bipyramidal structures (D_3hsymmetry) with thermal average axial bonds, r_α, longer than the equatorial ones by 0.097(9) and 0.142(5) Å, respectively. The equatorial bonds are r(Nb—Cl) = 2.241(4) and r(Ta—Cl) = 2.227(3) Å. Standard deviations are given. Calculated amplitudes of vibration for the e' type of bending frequencies assigned as v₆ (equatorial in-plane bend) < v₇ (axial—equatorial bend) agree significantly better with the experimental vibrational amplitudes than do amplitudes computed for the opposite assignment. Assuming an analytical quartic-harmonic potential for the pseudorotational motion of the molecules, barriers to pseudorotation of 1.5(0.7) and 1.2(0.6) kcal mol^?1 are estimated from the electron diffraction data for NbCl₅ and TaCl₅, respectively. Effects from interatomic multiple scattering are included in the theoretical intensities, and are found to be of some importance to the results.     

Lattice vibration spectra. Part XCV. Infrared spectroscopic studies on the iron oxide hydroxides goethite (α), akaganéite (β), lepidocrocite (γ), and feroxyhite (δ)

Infrared spectra (IR, FIR, DRIFT, 90 and 295 K) and DSC measurements of the various polymorphs of iron oxide hydroxide, viz. goethite (α), akaganéite (β), lepidocrocite (γ), and feroxyhite (δ), and of deuterated specimens are reported. They are discussed with respect to the crystal structures proposed in the literature, the hydrogen bonds present, the energies of the OH stretching, OH bending (librational), and translational modes, and their thermal decomposition. From the two space groups proposed for β- and γ-FeO(OH), the groups I4/m and Cmc2₁, respectively, seem to be more reliable. The disorder of the OH⁻ ions of γ-FeO(OH) has not been confirmed in contrast to that of δ-FeO(OH). The intraionic O(H,D) distances of γ- and δ-FeO(OH) derived from neutron powder diffraction studies have to be doubted. The greater strength of the OHOH hydrogen bonds of lepidocrocite, for example, compared to that of the OHO hydrogen bonds of goethite despite the larger hydrogen bond acceptor capability of O²⁻ is due to the strong cooperativity of the hydrogen bonds of the γ-polymorph. The extremely different strength of the hydrogen bonds of isostructural α-AlO(OH) (v_OH = 2950 cm⁻¹, 295 K), α-MnO(OH) (v_OH = 2686 cm⁻¹), and α-FeO(OH) (v_OH = 3130 cm⁻¹) is caused by the different synergetic effect of the metal ions involved, especially that of Mn³⁺ due to its Jahn-Teller behaviour. The decomposition temperatures and heats of the various FeO(OH) modifications as well as the halfwidths of the DSC peaks evidence a much faster decomposition rate of akaganéite than those of the other polymorphs. This is obviously due to the Cl⁻ ion impurities present in this compound.     

Thermal decomposition and non-isothermal decomposition kinetics of carbamazepine

The thermal stability and kinetics of isothermal decomposition of carbamazepine were studied under isothermal conditions by thermogravimetry (TGA) and differential scanning calorimetry (DSC) at three heating rates. Particularly, transformation of crystal forms occurs at 153.75°C. The activation energy of this thermal decomposition process was calculated from the analysis of TG curves by Flynn-Wall-Ozawa, Doyle, distributed activation energy model, ?atava-?esták and Kissinger methods. There were two different stages of thermal decomposition process. For the first stage, E and logA [s^?1] were determined to be 42.51 kJ mol^?1 and 3.45, respectively. In the second stage, E and logA [s^?1] were 47.75 kJ mol^?1 and 3.80. The mechanism of thermal decomposition was Avrami-Erofeev (the reaction order, n = 1/3), with integral form G(α) = [?ln(1 ? α)]^1/3 (α = ～0.1–0.8) in the first stage and Avrami-Erofeev (the reaction order, n = 1) with integral form G(α) = ?ln(1 ? α) (α = ～0.9–0.99) in the second stage. Moreover, ΔH ^≠, ΔS ^≠, ΔG ^≠ values were 37.84 kJ mol^?1, ?192.41 J mol^?1 K^?1, 146.32 kJ mol^?1 and 42.68 kJ mol^?1, ?186.41 J mol^?1 K^?1, 156.26 kJ mol^?1 for the first and second stage, respectively.     

Über Polygermane. II [1]. Die Modifikationen des Dodecaphenylcyclohexagermans [2]

On Polygermanes. II. Modifications of Dodecaphenylcyclohexagermane Dodecaphenylcyclohexagermane (1) can be obtained from solutions pure (1a) , with 2 und 7 moles of crystal benzene (1b, 1c) and with 2 moles of crystal toluene (1d). Mass spectrum, ¹³C-nmr spectrum, assigned vibrational spectra (v_{s, Ge? Ge? Ge} = 220–235 cm^?1, v_{as, Ge? Ge? Ge} = 245–260 cm^?1) and crystal data are given. The crystal structure of 1c has been determined and refined to a R of 0.059. 1 forms a flattened Ge₆-chair (Ge-Ge distance 245.7(1) pm) with 6 axial and equatorial substituents respectively. The molecules of 1 approach spheres with a packing midway between a simple cubic and a cubic closest-packed arrangement.     

Non-isothermal decomposition kinetics of diosgenin

The thermal stability and kinetics of isothermal decomposition of diosgenin were studied by thermogravimetry (TG) and Differential Scanning Calorimeter (DSC). The activation energy of the thermal decomposition process was determined from the analysis of TG curves by the methods of Flynn-Wall-Ozawa, Doyle, ?atava-?esták and Kissinger, respectively. The mechanism of thermal decomposition was determined to be Avrami-Erofeev equation (n = 1/3, n is the reaction order) with integral form G(α) = [?ln(1 ? α)]^1/3 (α = 0.10–0.80). E _a and logA [s^?1] were determined to be 44.10 kJ mol^?1 and 3.12, respectively. Moreover, the thermodynamics properties of ΔH ^≠, ΔS ^≠, and ΔG ^≠ of this reaction were 38.18 kJ mol^?1, ?199.76 J mol^?1 K^?1, and 164.36 kJ mol^?1 in the stage of thermal decomposition.     

Electrochemical Responses and Sensitivities of Films Based on Multi-Walled Carbon Nanotubes in Aqueous Solutions

Novel films consisting of multi-walled carbon nanotubes (MWCNTs) were fabricated by means of chemical vapor deposition with decomposition of either acetonitrile (ACN) or benzene (BZ) using ferrocene as catalyst. The electrochemical responses of MWCNT-based films towards the ferrocyanide/ferricyanide, [Fe(CN)₆]^3?/4? redox couple were probed by means of cyclic voltammetry and electrochemical impedance spectroscopy at 25.0?±?0.5?°C. Both MWCNT-based films exhibit Nernstian response towards [Fe(CN)₆]^3?/4? with some slight kinetic differences. Namely, heterogeneous electron transfer rate constants lying in ranges of 2.69?×?10^?2?C1.7?×?10^?3 and 9.0?×?10^?3?C2.6?×?10^?3?cm·s^?1 were obtained at v?=?0.05?V·s^?1 for MWCNT_ACN and MWCNT_BZ, respectively. The detection limit of MWCNT_ACN, estimated to be about 4.70?×?10^?7?mol·L^?1 at v?=?0.05?V·s^?1, tends to become slightly poorer with the increase of the scan rate, namely at v?=?0.10?V·s^?1 the detection limit of 1.70?×?10^?6?mol·L^?1 was determined. Slightly poorer response ability was exhibited by MWCNT_BZ; specifically the detection limits of 1.57?×?10^?6 and 4.35?×?10^?6?mol·L^?1 were determined at v?=?0.05 and v?=?0.10?V·s^?1, respectively. The sensitivities of MWCNT_ACN and MWCNT_BZ towards [Fe(CN)₆]^3?/4? were determined as 1.60?×?10^?7 and 1.51?×?10^?7?A·L·mol^?1·cm^?2, respectively. The excellent electrochemical performance of MWCNT_ACN is attributed to the presence of incorporated nitrogen in the nanotube??s structure.     

Syntheses and conformational analyses of mono- and trans-1,4-dialkoxy substituted cyclohexanes—the steric substituent/skeleton interactions

Mono- and trans-1,4-dialkoxy substituted cyclohexanes (alkyl=Me, Et, i-Pr, t-Bu) were prepared using the solvomercuration-demercuration (SM-DM) procedure. The axial?axial and axial,axial?equatorial, equatorial conformational equilibria of the products were studied by low temperature ¹H and ¹³C NMR spectroscopy in CD₂Cl₂. The structures and relative energies of the participating conformers were calculated at both the B3LYP (6-311G^∗//6-311+G^∗) and MP2 (6-311+G^∗//6-311G^∗) levels of theory. In the case of DFT, good correlations of ΔG^o_calcd versus ΔG^o_exptl were obtained. Both the structures and the energy differences of the conformers have been discussed with respect to established models of conformational analysis, viz. steric and hyperconjugative interactions. In addition, ¹J_H,C coupling constants were considered with respect to the hyperconjugation present.     

Preparation and characterization of microcapsulated red phosphorus and kinetic analysis of its thermal oxidation

Microcapsulated red phosphorus (MRP) with aluminum hydroxide/phenolic resin coating layer was prepared by a two-step coating process. The results of Fourier-transform infrared spectroscopy and scanning electron microscopy show that red phosphorus (RP) is coated by aluminum hydroxide and phenolic resin. MRP absorbs less water and is more thermally stable than RP. The thermal oxidation kinetics of MRP was investigated by TG/DTG/DTA under air atmosphere using non-isothermal experiments with the heating rates ranging from 10 to 25°C/min. The values of the apparent activation energy E _a were 168 ± 9 kJ/mol as determined by the isoconversional Ozawa–Flynn–Wall method and 164 ± 12 kJ/mol according to the Kissinger?Akahira?Sunose method. Based on Málek’s procedure the kinetic reaction follows the ?esták–Berggren model with f(α) = α0.34(1 ? α)^0.94 (α is RP conversion) and pre-exponential factor A = 3.11 × 10¹² s^–1. The simulated curves were fitted with experimental curves constructed by plotting dα/dt vs temperature at different heating rates.     

Molecular structure and conformational preferences of 1-bromo-1-silacyclohexane, CH2(CH2CH2)2SiH-Br, as studies by gas-phase electron diffraction and quantum chemistry

The molecular structure of axial and equatorial conformer of the 1-bromo-1-silacyclohexane molecule, CH₂(CH₂CH₂)₂SiH-Br, as well as thermodynamic equilibrium between these species are investigated by means of gas-phase electron diffraction and quantum chemistry on the MP2(full)/SDB-AUG-cc-PVTZ level of theory. It is revealed that according to electron diffraction data, the compound exists in the gasphase as a mixture of conformers possessing the chair conformation of the six-membered ring and C _s symmetry and differing in the axial and equatorial position of the Si-Br bond (ax. = 80(5) mol %, eq. = 20(7) mol %) at 352 K, that corresponds to the value of A = (G _ax ^?? ? G _eq ^?? ) = ?0.82(32) kcal/mol. It is found that observed data agree well with theoretical ones. Using Natural Bond Orbital (NBO) analysis it is revealed that axial conformer of 1-bromo-1-silacyclohexane molecule is an example of the stabilization of the form that is unfavorable from the point of view of steric effects and effects of conjugations. It is concluded that stabilization is achieved due to electrostatic interactions.     

A crystal chemical study of stishovite

The crystal structure of stishovite, SiO₂, a = 4.1773(1), c = 2.6655(1)Å, space group $\text{P4}{}_2\text{mnm}$, with Z = 2, has been refined with 217 graphite-monochromatized MoKα data (2θ max = 120°) to R = 0.012 (R_w = 0.014). Electron deformation density maps show a modest accumulation of charge density ascribed to partial covalent bonding in both the equatorial and axial bonds together with a delocalization of density in and around the shared octahedron edges with the shorter equatorial bonds showing higher peaks (0.47 eÅ^?3) than the longer axial bonds (0.29 eÅ^?3). Net atomic charges for Si and O determined by a population and κ refinement by varying the occupancy and shape of their valence shells are +1.71 and ?0.86(15) e, respectively. The observed structural distortions and charge distributions conform with the results of ab initio molecular orbital calculations undertaken on molecules designed to mimic the local geometry of the structure of stishovite. The SiO bond length (1.73 Å) calculated for a neutral molecule with 6-coordinated Si is slightly shorter than that observed on the average (1.76 Å) for silicates and molecular crystals with 6-coordinated Si. In addition, the calculated geometrical parameters of two edge-sharing octahedra agree to within ～5% of the observed geometry of stishovite. The calculated charge on Si is indicated to increase with coordination number and bond length in agreement with the larger net charge recorded for stishovite as compared with that recorded for α-quartz, Q(Si) = +1.0 e. Observed deformation maps are compared with theoretical ones calculated in the equatorial plane of the 6-coordinated molecule using molecular orbital theory with a split valence s,p basis supplemented with d-type polarization functions on Si.     

Zur synthese von siloxanen: II. Sterische substituentenkonstanten für siloxyreste

The acetolysis reactions of compounds of the type XSiMe₂Cl (I) with acetic acid in the presence of acetic anhydride were studied kinetically by means of ¹H NMR spectroscopy. We found these reactions exclusively influenced by steric effects (? = 0)The steric susceptibility constant (δ) of the acetolysis reaction, using alkylchlorodimethylsilanes was found to be 1.3. In this investigation the Taft E_s values showed a better correlation than the E_s(Si) values of Cartledge.From the rate constants of the acetolysis reactions with I (X = Cl, acetoxy or siloxy) we could then obtain for the first time E_s values for Cl, acetoxy and a series of siloxy groups at silicon.     

Nucleophilic reactivities of Schiff base derivatives of amino acids

Treatment of α-imino esters derived from glycine esters and benzophenone or benzaldehydes with potassium tert butoxide in DMSO gave persistent solutions of carbanions at 20?°C. The kinetics of their reactions with quinone methides and benzylidene malonates (reference electrophiles) have been followed photometrically under pseudo-first order conditions. The reactions followed second-order rate laws. Since addition of 18-crown-6 ether did not affect the reaction rates, the measured rate constants correspond to the reactions of the non-paired carbanions. Plots of the second-order rate constants against the electrophilicity parameters E of the electrophiles are linear, which allowed us to derive the nucleophile-specific parameters N and s_N, according to the linear Gibbs energy relationship lg k₂(20?°C)?=?s_N(N + E). The Ph₂C?=?N- and PhCH?=?N- groups act as very weak electron acceptors with the consequence that Ph₂C?=?N-CH^–-CO₂R and PhCH?=?N-CH^–-CO₂R have a similar nucleophilicity as Ph-CH^–-CO₂Et, the anion of ethyl phenylacetate.     

The conformation and vibrational spectra of isocyanato and isothiocyanatocyclohexane

The IR spectra of isocyanato and isothiocyanatocyclohexane (C₆H₁₁NCX) as liquids and as amorphous and crystalline solids at low temperatures have been recorded in the region 4000-50 cm^?1. High pressure (0–30 kbar) IR spectra of the neat samples were obtained in a diamond anvil cell and various high-pressure solid phases were studied. Raman spectra of the compounds as liquids and as low-temperature solids were obtained.Isocyanatocyclohexane crystallized directly as anisotropic solids containing equatorial molecules at low temperature and axial molecules at high pressure. Isothiocyanatocyclohexane formed a possibly plastic phase between 225 and 260 K where both equatorial and axial conformers are present. A solid high-pressure phase (1–3 kbar) at ambient temperature appeared anisotropic and contained both the e and a conformers. Below 225 K (atmospheric pressure) and above 10 kbar (ambient temperature) anisotropic crystals were formed which both contained equatorial conformers only.Normal coordinate analyses were carried out for the equatorial and axial conformers of the two molecules with different orientations (C_s and C_l symmetries) of the side chain. Force constants were transferred from various halo and pseudohalocyclohexanes. Tentative assignments of the fundamentals belonging to both the e and a-conformers are presented.     

Theoretical study on the effect of α-substituents on the properties of vinyl radicals

The effect of α-substituents on the structures, inversion barriers (E_inv) and isotropic ¹³C_α hyperfine splitting constants (α(¹³C_α)) of vinyl radicals CH₂=C(?)-Y has been determined by ab initio calculations. The degree of bending at the central carbon decreases slightly with decreasing electronegativity of σ-type α-substituents, the optimum CCY bond angle θ being in the range 130÷150° at the UMP2/TZP level. On the other hand, E_inv and α(¹³C_α) decrease strongly as the electronegativity of α-substituents decreases. The linear structure is favoured in the trisilylvinyl radical in accord with ESR evidences. This configuration is not found to be the more stable when Y contains a π-system capable of delocalizing the unpaired electron such as in the-CN≡N and-C(=O)OH groups as previously suggested. However, the inversion barrier is computed to be small (E_inv=1.4 and 2.8 kcal mol^?1, respectively, at the UQCISD/TZP//UMP2/TZP level). This finding is in accord with recent studies of stereoselectivity.     

Description of the shape of thermoanalytical curves : Part 3. A Method for estimating kinetic constants from parameters characterizing peak shape

The relationships between empirical parameters characterizing the shape of thermo-analytical curves and the constants of the kinetic equation, dα/dt = A exp (?E/RT)(1 ? α)ⁿ, are studied. A procedure is developed for the estimation of the three constants of this equation from the empirical parameters. The efficiency of this method is compared to that of model fitting. In evaluation of the confidence intervals of estimated constants and in the case of identification, least-squares (or similar) fitting is shown to be inferior because of its low sensitivity to properties other than the position of the peak along the temperature axis. This lack of sensitivity may be a major cause of the apparent kinetic compensation effect often encountered in the field of thermal analysis.     

Hydrogen bonds in solid hydroxides, a bond valence approach

The bond valences s_OH due to the O-H bonds of OH⁻ ions in solids have been calculated indirectly from intermolecular H?O distances, viz. those within the Wigner Seitz cell around the respective hydrogen atom, by using the equation s_OH=1−∑s_H?O. The bond valences thus derived are an excellent measure of the strength of O-H bonds [J. Mol. Struct. 351 (1995) 205]. This is shown by their almost linear correlation with the wave numbers of the stretching modes of matrix isolated OD⁻ ions observed with IR or Raman experiments. In the case of very weak or lacking hydrogen bonds, this correlation fails because then other interionic bonding phenomena than hydrogen bonds as metal-oxygen interactions and hydrogen-hydrogen repulsion etc. gain in importance or dominate finally and, hence, partly or fully determine the wave numbers of the OD stretching modes, which, however, still remain a measure of the respective bond strengths. The relation of the distances r_OH, the bond valences s_OH, and the stretching modes ν_OD of both free, gaseous OH⁻ ions and H₂O molecules and those embedded in crystalline matrices is discussed.     

Application of the energy gap law to the decay of charge transfer excited states,solvent effects

Values of non-radiative decay rate constants (k_nr) and emission energies (E_cm) have been obtained for Os(Phen₃)²⁺ in a series of solvents and the results are consistent with the energy gap law. For hydroxylic solvents like water or methanol related studies suggest the existence of strong, specific contributions to the vibrational trapping energy of the solvent.     

Photodissociation of H2O2 at 248 nm: translational anisotropy and oh product state distributions

Ground-state OH(X²Π) fragments, from the photodissociation of H₂O₂ at 248 nm, have been probed by laser-induced fluorescence. Nascent product rotational distributions and polarisation-dependent Doppler lineshapes are reported. The high translational anisotropy (β = −1 at low N″) indicates a prompt repulsion along the O-O axis. Polarisation-dependent Doppler lineshapes are interpreted as a correlation between the fragment recoil velocity v and angular momentum J_OH, the product rotation being aligned along v. Product rotation is generated by an axial torsion about the O-O axis, consistent with a change in dihedral angle in the upper state.     

The triplet state of photosynthetic pigments. I. Pheophytins

ZFS constants (in cm^?1) and decay rate constants for the lowest triplet state of pheophytins have been determined by ESR: pheophytin a: D = 341 ± 3,E = 33 ± 3, K_T = 1050 s^?1; pheophytin b: D = 358 ± 8, E = 46 ± 5, K_T = 630 s^?1; bacteriopheophytin: D = 256 ± 4, E = 54 ± 5/37 ± 5, K_T ≈ 4000 s^?1. In addition values for the decay rate constants and relative populating rates of the individual spin levels have been obtained; these numbers turn out to be appreciably different from those for the corresponding chlorophylls. For the series pheophytin a, b and bacteriopheophytin we find parallel behaviour with the corresponding chlorophylls. The effects of side group substitution and pyrrole ring reduction on the ZFS constant D can be understood by including configuration interaction between the excited states using the 4-orbital model. The change of the mean triplet decay constant K_T upon side group substitution and pyrrole ring reduction follows an energy gap law. Substitution of the central Mg-ion by two protons, however, causes K_T to increase; this is attributed to the introduction of an extra promoting mode - of the NH-group - and/or to the presence of low lying nπ^* states in pheophytins.     

Application of iso-temperature method of multiple rate to kinetic analysis

A new method of the multiple rate iso-temperature was used to define the most probable mechanism g(α) of a reaction; the iterative iso-conversional procedure has been employed to estimate apparent activation energy E _a, the pre-exponential factor A was obtained on the basis of E _a and g(α). In this new method, the thermal analysis kinetics triplet of dehydration of calcium oxalate monohydrate is determined, which apparent activation energy E _a is 82.83 kJ mol^-1, pre-exponential factor A is 1.142·10⁵-1.235·10⁵ s^-1, the most probable mechanism belongs to phase boundary reaction R_n with integral form g(α)=1-(1-α)ⁿ and differential form f(α)=n(1-α)^1-(1/n), where accommodation factor n=2.40-1.40.