Transformation of three-dimensional three-connected silicon nets in SrSi2

Dimorphic SrSi₂ is the first compound for which the two simplest three-dimensional three-connected nets are found in its polymorphs. The cubic net of three-connected silicon atoms (SrSi₂ type of structure) can be transformed into the tetragonal one (α-ThSi₂ type of structure) by a high-pressure-high-temperature treatment. The tetragonal phase is quenchable. Heating of this phase to 600–700°C at ambient pressure results in transformation into the cubic one. At a heating rate of 20°C/min complete transformation can be achieved within 5 min in a DTA apparatus. The energy of transformation has been obtained from the peak areas of the DTA curves to ?1.6 ± 0.3 kcal/mole. Although the transformation between the three-dimensional three-connected sets in SrSi₂ must be formally classified as a reconstructive one, a relatively small entropy change (ΔS = 1 ·₁ cal/deg · mole) has been calculated from the change in molar volume and p-T equilibrium conditions. Therefore, structural relations between the cubic and the tetragonal nets are discussed.     

Thermogravimetric investigation of the dehydration thermodynamics of amorphous silica after its hydrothermal and thermovaporous treatment

After hydrothermal and thermovaporous treatment of chemically pure amorphous aqueous silicic acid in solutions of NaOH and NH₄OH and in water vapour it is possible, using complex thermal analysis, to detect the weight loss and heat effects corresponding to evaporation of various forms of combined water, and to estimate the heats of evaporation of these forms. From the obtained data, the following water forms have been identified: (1) at 200–300° capillary-condensed water formations of the cluster type evaporate;ΔH _deh is about 8 kcal/mole H₂O; (2) at 250–400°, molecules of water linked by hydrogen bonds with hydroxyl groups on the surface and in the volume of the particles;ΔH _deh. is about 5 kcal/mole H₂O; (3) at 350 600°, molecules of water coordinated to silicon atoms in the volume of the particles;ΔH _deh is approximately 1 kcal/mole H₂O. The total evaporation heat changes from 10 kcal/mole H₂O when water of form 1 predominates, to 5 kcal/mole H₂O when forms 2 and 3 predominate.     

Transformation of three-connected silicon in BaSi2

Solid-solid transitions in trimorphic BaSi₂ have been investigated up to 40 kbar and 1000°C by X-ray powder technique in quenched samples. All transformations between orthorhombic BaSi₂I with isolated Si-tetrahedra, trigonal BaSi₂II with corrugated Si-layers, and cubic BaSi₂III with a three-dimensional three-connected Si-net can be performed in approximately 5 min at high-pressure-high-temperature conditions. At ambient conditions the difference in molar volume between BaSI₂I and BaSi₂III is relatively large (ΔV_I–III = ?6.79 cm³/mole) and that between BaSi₂III and BaSi₂II very small (ΔV_III-II = ?0.05 cm³/mole). Consequently in the pressure-temperature phase diagram the boundary (I–III) shows a strong pressure dependence contrary to that of (III-II) which is less dependent on variation of pressure. The triple point between the three solid phases is near 11 kbar and 925°C. Substitution of divalent metal and quadrivalent metalloid can easily influence the phase relations in BaSi₂.     

Thermochemische und kinetische Untersuchungen der endothermen Umbildungsreaktionen des Epsomits (MgSO4 · 7H2O)

Heterogeneous decompositions of MgSO₄ · 7H₂O (Epsomite) monocrystals were studied with thermal (DTA, DSC, TG) and thermo-optical methods. The polythermal reaction is controlled by nucleation of the reactant. This process has been considered by the Avrami-Erofe'ev equation: $$kt = [ - \ln (1 - \alpha )]^{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 3}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{$3$}}} $$ The plots and the slope which give the activation energyE+=23.5 kcal/mole (760 Torr N₂, 50–100°), are obtained from the Freeman-Carroll equation. The DSC technique was used to determine the heat of decomposition (ΔH=42.3 kcal/mole, 760 Torr N₂, 50–100°). The heat of transformation for the reaction 39–47° $$MgSO_4 \cdot 7H_2 O\xrightarrow{{39 - 47^ \circ }}MgSO_4 \cdot 6H_2 O + H_2 O$$ wasΔH=2.8 kcal/mole. The isothermal reaction (20°, 10^?6 Torr) is controlled by first-order kinetic.     

Conformational analysis of intramolecular bonded amino alcohols : The conformational free energies of some intramolecular OH ⋯ N hydrogen bonds

Equilibrium positions between intramolecular OH ? N hydrogen bonded and free OH forms of some 3-piperidinols, decahydroisoquinolinols, a decahydroquinolinol, lupinine and N-methyl-3-piperidinemethanol have been determined from dilute solution IR spectral data at 33°. Conformational free energies of the H-bonds (ΔG°_OH?N, attractive) have been calculated. The results suggest a linear relationship between the apparent value of ΔG°_OH?N, as defined by the method of calculation, and the strength of the OH ? N bond expressed as Δν, within the limits of 0·5 ± 0·2kcal/mole per 100 cm^?1, from Δν 90 to 350 cm^?1. For cis-decahydroisoquinoline (N-Me or N-H) systems, a 0·4 kcal/mole difference has been calculated between the two possible ring-fused conformations, in favor of the so-called steroid form. For the corresponding cis-decahydroqumoline equilibrium, a 0·8 kcal/mole difference has been calculated, in favor of the nonsteroid form.     

Bestimmung der Dissoziationsenergien der gasförmigen Moleküle CuGe,AgGe und AuGe

The dissociation energies of the gaseous molecules CuGe, AgGe, AuGe, Ge₂, and Cu₂ have been determined by mass spectrometric investigations of the vapour phases above the liquid alloys Ge?Cu, Ge?Ag, and Ge?Au. The evaluation according to the third-law method leads to the following values for the dissociation energies:D ₀°(CuGe)=49,0±5 kcal/mole;D ₀°(AgGe)=40,8±5 kcal/mole;D ₀°(AuGe)=65,3±3,5 kcal/Mol;D ₀°(Ge₂)=64,5±5 kcal/Mol;D ₀°(Cu₂)=64,5±5 kcal/Mol.     

Cyclic polysilanes : VIII. The crystal structure of 1,2,3,4-tetra-tert-butyl-tetramethylcyclotetrasilane

The crystal structure of [Si(CH₃)(t-C₄H₉)]₄ has been determined by single crystal X-ray diffraction. The crystals are tetragonal, P4₂/n; a = b = 13.069(4), c = 7.880(2) Å, Z = 2. The structure was determined using 745 independent data and refined with anisotropic least-squares to a final unweighted R-value of 3.5%. Each tetrameric molecule was found to be arranged about a $\text{4}$ axis, with the independent crystallographic unit comprising one silicon atom, one methyl and one tert-butyl group. The four-membered ring of silicon atoms is nonplanar with an unusually large dihedral angle of 36.8°. The principal mean bond lengths are SiSi 2.377(1), SiC(methyl) 1.893(4), SiC(tert-butyl) 1.918(3) Å, and the SiSiSi bond angle is 86.99°. The SiSi bond length is somewhat longer than in other polysilanes.     

Complex Formation of Ytterbium (III) with Bromopyrogallol Red—A Spectrophotometric Study

The complex formation of Ytterbium (III) with Bromopyrogallol red has been studied. spectrophotometrically in an attempt to establish composition, stability, thermodynamic parameters and optimum conditions for determining small amounts of ytterbium. The violet complex of ytterbium has λ_max at 620nm against a reagent blank. The composition determined by different methods is 1:1 at pH 6.2±0.1. The mean value of log K, free energy (ΔG), the heat content (ΔH) and entropy (ΔS) changes, of the complex are found to be 6.0, —8.1.kcal/mole, —3.5 Kcal/mole and 15.53 e.U. respectively at 30°C. The net molar absorptivity and Sandell's sensitivity is 19850 and 0.0087 μg of ytterbium /cm². The effect of diverse ions was examined with thirteen cations and ten anions in the determination of ytterbium.     

Computer simulation of crystal structures applied to the solution of the superstructure of cubic silicondiphosphate

The superstructure of cubic SiP₂O₇ has a volume 27 times as large as the substructure of this compound. Because conventional methods failed in solving the superstructure, computer simulation of the structure was applied. Computer simulation depends on the accurate prediction of individual interatomic distances in a structure and on an appropriate weighting scheme. The predicted distances are treated as observations in a distance least-squares refinement, in which the positional coordinates of the atoms are varied until the calculated distances conform to the predicted values. Cubic SiP₂O₇ (a = 22.418Å, space group Pa3, Z = 108, D_x = 3.22) has 50 atoms in the asymmetric unit. The initial R-value for the simulated structure was 0.18, which dropped to 0.061 after three cycles of least squares refinement using F_obsd = 1382. In the substructure, all POP angles in the diphosphate groups are straight because of symmetry requirements, and the angles POSi are 164°. In the superstructure, the average angle POP is 150°; the average POSi angle is 149°. The tendency to decrease these angles may be responsible for the formation of the superstructure. However, two diphosphate groups retain, even in the superstructure, the 180° configuration. Such a feature is usually only observed in high temperature polymorphs and is explainable as a positional disorder of bent P₂O₇ groups, or by the assumption of a highly anisotropic motion of the bridging oxygen atom. The silicon atoms in cubic SiP₂O₇ are octahedrally coordinated, as they are in the two monoclinic polymorphs of SiP₂O₇. However, the three modifications are topologically distinct from each other as can be proved by considering the three-dimensional nets on which the three structures are based.     

The structure of the H3O+ (hydronium) ion

Near Hartree-Fock level ab initio molecular orbital calculations on H₃O⁺ and a minimum energy structure with θ(HOH) = 112.5° and r(OH) = 0.963 Å and an inversion barrier of 1.9 kcal/mole. By comparing these results to calculations on NH₃ and H₂O, where precise experimental geometries are known, we estimate the “true” geometry of isolated H₃O⁺ to have a structure with θ(HOH) = 110-112°, r(OH) = 0.97–0.98 Å and an inversion barrier of 2–3 kcal/mole. Our prediction for the proton affinity of water is ≈ 170 kcal/mole, which is somewhat smaller than the currently accepted value.     

Kinetics of the gas-phase reaction of cyclopropylcarbinyl iodide and hydrogen iodide. Heat of formation and stabilization energy of the cyclopropylcarbinyl radical

The rate of the gas phase reaction has been measured spectrophotometrically over the range 480°–550°K. The rate constant fits the equation where θ = 2.303RT in kcal/mole. This result, together with the assumption that the activation energy for the back reaction is 0 ± 1 kcal/mole, allows calculation of DH (Δ? CH₂? H) = 97.4 ± 1.6 kcal/mole and ΔH (Δ? CH₂·) = 51.1 ± 1.6 kcal/mole. These values correspond to a stabilization energy of 0.4 ± 1.6 kcal/mole in the cyclopropylcarbinyl radical.     

Solid state electrochemical study of the phase diagram and thermodynamics of the ternary system CuGeO

Coulometric titrations using solid zirconia ionic conductors have been employed to determine the phase diagram of the ternary system CuGeO in the temperature range from 750 to 950°C. CuGeO₃ was found to be the only existing ternary compound in the system. It is in equilibrium with Cu₂O, CuO, GeO₂, and oxygen of atmospheric pressure. Cu and Cu₂O may coexist with GeO₂. The standard Gibbs energy of formation of CuGeO₃ was found to be ΔG°_f (CuGeO₃) = ?424.5 kJ/mole at 900°C. The standard enthalpy and entropy of formation are ΔH⁰_f = ?756.8 kJ/mole and ΔS°_f = ?283 J/mole·K, respectively.     

Thermochemistry of the hydrates of k2CoCl4

A study of the dissociation pressure of crystalline K₂CoCl₄·2H₂O. The reactions can be summed up as K₂CoCl₄·nH₂O(c) = K₂CoCl₄·mH₂O(c)+(nm)H₂O (v). Below 50°C, n = 2 and 1, m = 1 and 0, above 50°C, n = 2 and m = 0. Below 50°C, the dihydrate is octahedral, the monohydrate and anhydrous compounds are tetrahedral. ΔH° and ΔS° are respectively 7.0 kcal and 14.2 e.u. for the loss of the first mole of water and 12.7 kcal and 32.0 e.u. for the loss of the last mole of water. Above 50°C, ΔH° and ΔS° are respectively 29.8 kcal and 77.6 e.u. for the loss of both waters. The changes in structure are discussed using the spectral and magnetic properties as indications for structural changes.     

Cyclic polysilanes : XIX. A temperature study of redistribution equilibria between permethylcyclopolysilanes

Equilibria among the cyclic compounds (Me₂Si)_n where n = 5, 6 and 7 have been studied between 30–58°C. Thermodynamic values for the redistribution reactions between pairs of compounds are, for n = 5 → 6, ΔH = ?18 kcal/mole, ΔS = ?20 cal/deg. mole; for n = 7 → 6, ΔH ?3, ΔS +33; for n = 7 → 5, ΔH +18, ΔS + 51. The enthalpies indicate that the stabilities of the rings increase in the order (Me₂Si)₅ < (Me₂Si)₇ < (Me₂Si)₆. The differences are smaller than corresponding differences among the cycloalkanes, probably because the silicon compounds are less affected by steric repulsions and angle strain.     

Zur konformation geschützter aminosäuren—I: NMR-untersuchungen von t-BOC-glycin

Z, E-Isomerism of the urethane bond of t-BOC-glycine was observed by ¹H- and ¹³C-NMR spectroscopy at various temperatures in several solvents. The special stabilization of the Z isomer at low temperatures in CDCl₃ has been explained by intra- and intermolecular H-bond forming a 7-membered ring. Thermodynamic data have been determined for the ground state (AH°= ?7 kcal/mol, Δ° = ?25 Clausius) as well asfor the barrier of interconversion (ΔG° = 15·4 kcal/mole for the deuterated title compound) in CDCl₃. The equilibrium between the Z and the E conformation is shifted towards the E conformation in more polar solvents (acetonitril-d₃, acetone-d₆, DMSO-d₆), in which cyclization of the Z conformation is not important.     

Copper(II) hypophosphite: the α‐ and β‐forms at 270 and 100 K,and the γ‐form at 270 K

Copper(II) hypophosphite has been shown to exist as several polymorphs. The crystal structures of monoclinic α‐, ortho­rhombic β‐ and ortho­rhombic γ‐Cu(H₂PO₂)₂ have been determined at different temperatures. The geometry of the hypophosphite anion in all three polymorphs is very close to the idealized one, with point symmetry mm2. Despite having different space groups, the structures of the α‐ and β‐polymorphs are very similar. The polymeric layers formed by the Cu atoms and the hypophosphite ions, which are identical in the α‐ and β‐polymorphs, stack in the third dimension in different ways. Each hypophosphite anion is coordinated to three Cu atoms. On cooling, a minimum amount of contraction was observed in the direction normal to the layers. The structure of the polymeric layers in the γ‐­polymorph is quite different. There are two symmetry‐independent hypophosphite anions; the first is coordinated to two Cu atoms, while the second is coordinated to four Cu atoms. In all three polymorphs, the Cu atoms are coordinated by six O atoms of six hypophosphite anions, forming tetragonal bipyramids; in the α‐ and β‐polymorphs, there are four short and two long Cu—O distances, while in the γ‐polymorph, there are four long and two short Cu—O distances.     

Equilibrium anionic polymerization of the isopropenyl monomers containing aromatic heterocycles

The anionic polymerization of three monomers, 2-isopropenyl-4,5-dimethyloxazole(I), 2-isopropenylthiazole(II), and 2-isopropenylpyridine(III), was studied in THF. These monomers produced red-colored living polymers on addition of sodium naphthalene or living α-methylstyrene tetramer as an initiator. It was observed that a considerable amount of monomer remained in the respective living polymer–monomer system, indicating that an equilibrium between the polymer and the monomer existed as in the case of α-methylstyrene. At lower temperatures, the conversion of the monomer to the polymer increased. The equilibrium monomer concentrations [M_e] were determined at different temperatures, and the heats (ΔH) and the entropies (ΔS°) of polymerization were obtained by plotting In(1/[M_e]) against 1/T as ΔH = ?9.4, ?6.8, and ?6.2 kcal/mole, ΔS°S = ?22.9, ?16.5, and ?16.6, eu for I, II, and III, respectively.     

Sättigungsdrucke,Bildungsenthalpien von WOBr4 und WO2Br2, die Reaktionsgleichgewichte 2WO2Br2 = WO3 + WOBr4 und 2WOBr4 + SiO2 = 2WO2Br2 + SiBr4

The saturation vapour pressures of WOBr₄ and WO₂Br₂ and their reaction equilibria have been determined by means of a membrane zero manometer and ampoule quenching experiments, respectively. From the pressuretemperature dependence the following sublimation data were estimated: Δ H° (subl., WOBr₄, 298) = 29.4 (± 1.0) kcal/mole; Δ H° (subl., WO₂Br₂, 298) = 36.6 (±1.5) kcal/mole; Δ S° (subl., WOBr₄, 298) = 50.1 (± 1) cl; Δ S° (subl. WO₂Br₂, 298) = 53.0 (±1.5) cl. For the decomposition reaction of solid WO₂Br₂ were obtained: Δ H° (s, 690) 37.5 (± 0.7) kcal/mole, Δ S° (s, 690) = 49.0 (± 0.5) cl; and for the decomposition of gaseous WO₂Br₂: Δ H° (g, 690) = ?29.6 (± 2.0) kcal/mole, Δ S°. (g, 690) = ?44.5 (± 1.5) cl.     

Kinetics of the I2-catalyzed isomerization of allyl chloride to cis- and trans-1-chloro-1-propene. The stabilization energy of the chloroallyl radical

The I₂-catalyzed isomerization of allyl chloride to cis- and trans- l-chloro-l-propene was measured in a static system in the temperature range 225–329°C. Propylene was found as a side product, mainly at the lower temperatures. The rate constant for an abstraction of a hydrogen atom from allyl chloride by an iodine atom was found to obey the equation log [k,/M^?1 sec^?1] = (10.5 ± 0.2) ?; (18.3 ± 10.4)/θ, where θ is 2.303RT in kcal/mole. Using this activation energy together with 1 ± 1 kcal/mole for the activation energy for the reaction of HI with alkyl radicals gives DH⁰ (CH₂CHCHCl? H) = 88.6 ± 1.1 kcal/mole, and 7.4 ± 1.5 kcal/mole as the stabilization energy (SE) of the chloroallyl radical. Using the results of Abell and Adolf on allyl fluoride and allyl bromide, we conclude DH⁰ (CH₂CHCHF? H) = 88.6 ± 1.1 and DH⁰ (CH₂CHCHBr? H) = 89.4 ± 1.1 kcal/ mole; the SE of the corresponding radicals are 7.4 ± 2.2 and 7.8 ± 1.5 kcal/mole. The bond dissociation energies of the C? H bonds in the allyl halides are similar to that of propene, while the SE values are about 2 kcal/mole less than in the allyl radical, resulting perhaps more from the stabilization of alkyl radicals by α-halogen atoms than from differences in the unsaturated systems.     

Ab initio valence-bond calculations of H2O

The first and second bond dissociation energies for H₂O have been calculated in anab initio manner using a multistructure valence-bond scheme. The basis set consisted of a minimal number of non-orthogonal atomic orbitals expressed in terms of gaussian-lobe functions. The valence-bond structures considered properly described the change in the molecular system as the hydrogen atoms were individually removed to infinity. The calculated equilibrium geometry for the H₂O molecule has an O-H bond length of 1.83 Bohrs and an HOH bond angle of 106.5°. With 49 valence-bond structures the energy of H₂O at this geometry was ?76.0202 Hartrees. The calculated equilibrium bond length for the OH radical was 1.86 Bohrs and the energy, using the same basis set, was ?75.3875 Hartrees. After correction for zero point energies the calculated bond dissociation energies are: H₂O → OH + H, D₁=75.38 kcal/mole and OH → O+H, D₂=54.79 kcal/mole.