A study of helix formation in polydimethysiloxane by Raman scattering

Depolarization ratios ρ of the Raman bands due to CH₃ stretching at 2907 cm^?1 and the Si? O skeletal mode at 491 cm^?1 have been measured in polydimethylsiloxane gum as a function of temperature from 100°C to ?45°C. Below 0°C the changes in p have been interpreted in terms of the formation of helical regions in the gum. The enthalpy of helix formation ΔH has been determined as 3200 ± 600 cal/mole. An upper limit on the entropy change, ΔS, of 16 ± 3 e.u./mole and minimum values of helix content at different temperatures have been found. The Raman spectrum of crystalline polydimethylsiloxane is presented.     

Deformation mechanisms of constrained polymer chains. II. Deformation energetics of tie molecules

Energy-deformation characteristics for the primary T, S, and U conformational units of tie molecules were obtained from the analysis of data generated from a constrained minimization algorithm. Energy-deformation profiles (covering the range from compact equilibrium defect structures to the fully extended chain) are reported for the S₀ and S₁ members of the S_λ family and for the U₀₀ member of the U_mn family. Estimates of the energy content V⁰ and the elastic modulus E were obtained from the computed energy-deformation data in the vicinity of the equilibrium Structure—S₀ → {60°, 180°, ?60°}, V = 1.7 kcal/mole, E = 60 kcal/cm³ [250 × 10¹⁰ dyn/cm²];S₁ → {60°, 180°, 180°, 180°, ?60°}: V = 1.7 kcal/mole, E = 25 kcal/cm³ [100 × 10¹⁰ dyn/cm²]; and U₀₀ → {60°, 180°, 60°, 180°, 60°}: V = 2.7 kcal/mole, E = 80 kcal/cm³ [340 × 10¹⁰ dyn/cm²]. Although the elastic modulus of the U₀₀ unit is comparable to the elastic modulus of the fully extended chain, the highenergy content of this unit (V₀ = 2.7 Kcal/mole) prohibits a significant population and thereby mitigates an appreciable reinforcing effect from this rigid unit. A model for a surrogate force constant is introduced to generalize the results from this study to any member of the S_λ or U_mn family as well as any combination of S_λ and U_mn units. This generalization provides a basis for estimating the deformation characteristics of tie molecules comprised of various populations of these primary conformational building blocks.     

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.     

Pyrolytic production of Se (3P) from carbon diselenide. II. Rate of CSe2 decay

Pyrolytic decay of carbon diselenide was monitored by ultraviolet absorption spectroscopy in reflected shock waves in the temperature range of 1600–2600°K. The temperature dependence of the absorption coefficient of CSe₂ at 2308 Å was determined and was used to provide kinetic information along with a deconvolution procedure which accounted for and removed systematic distortions of the fast time-resolved absorbance profile. For temperatures of 1600–2600°K and argon densities of 1.5–7.0 × 10^?5 mol/cm³ dilute (1.0–9.0 × 10^?9 mol/cm³) CSe₂ pyrolyzed with measured first-order decay rates in the range of log₁₀ k₁ (sec^?1) = 3.0?5.7; at midrange (2100°K and 4.3 × 10^?5 mol/cm³ in Ar) k₁ ≈ 3 × 10⁴ sec^?1. The decay probably occurs via a unimolecular low-pressure process, first order in both CSe₂ and Ar, for which k₂ ± 10⁹ cm³/mol·sec at 2100°K. The deconvoluted data yield Arrhenius activation energies of 53.2 kcal/mol under second-order treatment, but the activation energy is less reliable than the general magnitude of the rate constant. A comparison of CSe₂ with other molecules which are isoelectronic in their valence shells (CO₂, CS₂, OCS, and N₂O) is made.     

γ-radiation-initiated polymerization of bulk styrene at high pressure

The polymerization of styrene in bulk at pressures up to 273 MPa and temperatures between 3 and 49°C with the use of γ-radiation as the initiator has been studied. The polymerization rate and the molecular weight of the polymer increased with increasing pressure; the molecular weight increased at a slightly faster rate. The difference in the rate is a theoretical expectation which has not previously been observed because chain-transfer reactions obscure the effect in chemically initiated systems. A small but significant retardation of the initiation reaction occurs as the pressure is increased. The results of previous workers are critically reviewed. Chain transfer at 25°C for pressures below 220 MPa is negligible when γ-radiation is the initiator. The activation energy for bulk polymerization decreased with increasing pressure from 28.1 kJ/mole at 0.1013 MPa to 22.3 kJ/mole at 203 MPa. Volumes of activation at 25°C for 0.1013 < p < 273 MPa were calculated to be Initiation, +4.0 < ΔV < +4.4 cm³/mole; polymerization; ?Δ = ?20.9 cm³/mole; degree of polymerization; ΔV = ?25.3 cm³/mole; propagation/termination; ?ΔV = ?22.7 cm³/mole.     

A flash photolysis–resonance fluorescence kinetics study of the reactions of ground-state sulfur atoms. V. Rate parameters for the reaction of S(3P)with cis-2-butene and tetramethylethylene

Rate parameters for the reaction of ground-state atomic sulfur, S(³P), with the olefins cis-2-butene and tetramethylethylene have been determined over a temperature range of ∽280°K. A major finding of this study was that the rate constants for both reactions showed negative temperature dependencies. When k is expressed in the form of an Arrhenius equation, this necessarily leads to negative activation energies: k₁ = (4.68 ± 0.70) × 10^?12 exp ^{(+0.23 ± 0.09 kcal/mole)/RT} (219°-500°K) k₂ = (4.68 ± 1.70) × 10^?12 exp ^{(+1.29 ± 0.23 kcal/mole)/RT} (252°-500°K) Units are cm³ molec^?1s^?1. When a threshold energy of 0.0 kcal/mole is assumed for reaction (2), the temperature dependence of the preexponential term has a value of T^?2. Making the usual simplifying assumptions, neither collision theory nor transition state theory leads to a preexponential factor with a strong enough negative temperature dependence. A comparison of these results with those derived from studies of the reactions of atomic oxygen, O(³P), with the same olefins shows that in both studies simple bimolecular processes were being examined. Also discussed are the possible experimental and theoretical ramifications of these new results.     

Complexation with diol host compounds. 13. Crystal structures and thermal analyses of inclusion compounds of 1,1,6,6-tetraphenylhexa-2,4-diyne-1,6-diol with cyclohexane,O-xylene,m-xylene and p-xylene

Abstract

It has been shown that host compound 1,1,6,6-tetraphenylhexa-2,4-diyne-1,6-diol is able to include polar guests and now we report on its ability to form clathrate compounds with apolar guests. The structures of this host with cyclohexane (1) and the ortho (2), meta (3) and para (4) xylenes have been determined and are discussed. Crystal data: (1) 2C₃₀H₂₂O₂C₆H₁₂, M _r = 913.20 g mol^?1, mono-clinic, C2/c, a = 22.851(6), b = 14.010(2), c = 17.076(6) Å, β = 108.71(3)°, V = 5178(2) ų, Z = 4, D _c = 1.17g cm^?3, N = 3326, R = 0.092. (2) 2C₃₀H₂₂O₂1 ½C₈H₁₀, M _r = 1976.5 g mol^?1, triclinic, P 1, a = 13.185(3), b = 15.466(3), c = 16.573(2) Å, α = 96.39(13)°, β = 106.96(15)°, γ = 114.94(18)°, V = 2822(2) ų, Z = 2, D _c = 1.16 g cm^?3, N = 6152, R = 0.075. (3) 2C₃₀H₂₂O₂1 ½C₈H₁₀, M _r = 1976.5 g mol^?1, triclinic, P 1, a = 13.267(5), b = 15.453(3), c = 16.654(5) Å, α = 97.12(2)°, β = 107.09(3)°, γ = 114.68(3)°, V = 2843(2) ų, Z = 2, D _c = 1.15 g cm^?3, N = 6505, R = 0.083. (4) 2C₃₀H₂₂O₂1 ½C₈H₁₀, M _r = 1976.5 g mol^?1, triclinic, P 1, α = 13.070(2), b = 15.348(3), c = 16.776(3) Å, α = 67.88<2)°, β = 74.27(1)°, γ = 65.29(1)°, V = 2817(1) ų, Z = 2, D _c = 1.15 g cm^?3, N = 6711, R = 0.050. Thermal analysis studies were also performed in order to examine their stability and the strength with which the guest species are held in the crystal lattice.     

The kinetics of the reaction of O(3P) and D atoms with cyclohexane

The kinetics of the reactions of O(³P) and D atoms with cyclohexane have been investigated using fast-flow techniques. The rates of reaction were computed by monitoring changes in both atom and cyclohexane concentrations using electron spin resonance and mass spectrometric methods, respectively. The O(³P) + C₆H₁₂ reaction was studied over a temperature range of 344 to 513°K and we obtain a specific rate constant of (3.2±0.6) × 10¹⁴ exp (?4400±400/RT) cm³/mole˙sec for this reaction. The only reaction product detectable mass spectrometrically under flow conditions of excess oxygen atoms is formaldehyde. The D + C₆H₁₂ reaction was studied over a temperature range of 297 to 596°K. A specific rate constant of (4.1±1.0) × 10¹³ exp (?4000±300/RT) cm³/mole˙sec was obtained for this reaction. On the basis of the results obtained in these studies, the important primary process in both the O(³P) and D atom reactions is concluded to be abstraction of a hydrogen atom from the cyclohexane molecule.     

Structure and electrical conductivity of La0.84Sr0.16MnO3

X-Ray diffraction, density, and electrical conductivity measurements were performed on the perovskite-like mixed oxide La_0.84Sr_0.16MnO₃. A rhombohedral crystalline structure with lattice parameters a = 3.893 Å and α = 90°29′16″ was assigned to the powder prepared by standard ceramic technique. Its theoretical density is therefore 6.576 g/cm³, while the experimental density was determined as 6.48 g/cm³. The conductivity measured at 1000°C is 133 Ω^?1 cm^?1. The temperature dependence of the conductivity indicates that the charge carriers are small polarons. The activation energy of the mobility is 9.6 kJ/mole.     

Synthesis and Structural Characterization of Three New Inorganic ＂Host-gues＂ Polyoxomolybdates

Since two interesting inorganic “host‐guest” polyoxomolybdates 1 and 2 have been reported previously, we have now succeeded in selectively isolating three new acetated “host‐guest” polyoxomolybdates 3–5, which considerably extend the range of structures in the cyclic polyoxomolybdate catalogue. 3 crystallizes in the triclinic space group P‐1 with a = 1.22235(1) nm, b = 1.52977(2) nm, c = 1.54022(1) nm, a = 113.746(1)°, β = 96.742(1)°, γ = 101.564(1)°, V = 2.51892(4) nm³, Z =1, D_c = 2.568 g. cm^?3. 4 and 5 crystallize in the monoclinic system: P2(1)/n, a = 1.08298(2) nm, b = 1.54029(1) nm, c = 2.78893(5) nm, β =94.2730(10)°, V = 4.63929(12) nm³, Z = 2 and D_c = 2.671 g. cm^?3 for 4, and C2/c, a =2.59907(8) nm, b = 1.65992(3) nm, c = 2.28473(7) nm, β‐93.4370(10)°, V = 9.8392(5) nm³, Z = 4 and D_c = 2.556 g. cm^?3 for 5. The structures of 3, 4 and 5 consist of 18‐membered “host‐guest” polyoxoanions [ Na (X)₂| ∈ |(μ_3‐OH)₄Mo^y₈Mo^VI₁₀52(μ₂‐CH₃COO)₂]^?(R+9 (X = CH₃COO^?for 3, DMF for 4 and H₂O for 5), which are connected via Na* ions or hydrogen bonds into infinite extended frameworks.     

Crystal structure and fluorescence spectrum of the complex [Eu(III)(TTA)3(phen)]

The title complex Eu(III)(TTA)₃(phen) (where TTA = thenoyltrifluoroacetone monoanion; phen = 1,10-phenanthroline) has been synthesized in mixed solvents of acetone and ethanol (1:1 volume ratio) and its crystal structure has been determined by X-ray diffraction. The complex crystals are triclinic, space group P 1 (# 2) with cell dimensions of a = 1.32.41 (2) nm, b = 1.5278(4) nm, c = 0.9755(3) nm, α = 92.49 (2)°, β = 102.57(2)°, γ = 91.62(2)°, V = 1.9268(8) nm³, Z = 2, μ (Mo Ka)= 18.77 cm^?1, D_x=1.720 g/cm³. The coordination geometry of Eu atom is a distorted square antiprism, and the encapsulated structure that can meet the structural requirement of the typical europium luminescent sensor. The fluorescence spectrum suggests that the complex is a strong photoluminescent material.     

Shock initiated ignition in COS?O2?Ar mixtures

The ignition of COS + O₂ mixtures diluted in argon was studied behind reflected shocks in a single-pulse shock tube over the temperature range of 1100–1700°K. Ignition delay times and the distribution of reaction products before and after ignition were determined experimentally. From a total of 63 tests run at varying initial conditions, the following correlation for the induction times was derived: where β₁ = +0.30, β₂ = 1.12, and E = 16.9 kcal/mole. Using a reaction scheme of 14 steps, the following values were obtained by a computer modeling of the induction times: β₁ = +0.22, β₂ = 1.55, and E = 17.3 kcal/mole. The calculations showed that the reaction COS + S → CO + S₂ caused the inhibiting effect of the COS. The reaction COS → O ± CO₂ + S has a very strong accelerating effect, whereas the parallel channel COS + O → CO + SO shows the opposite effect. It was also shown that the reaction O + S₂ → SO + O is very slow and does not contribute to the overall oxidation reaction. It is suggested that the rate constant given to the four-center reaction COS + SO → CO₂ + S₂, that is, 10¹¹ cm³/mole · sec at 300°K is incorrect. This constant is not much higher than 10⁸ cm³/mole · sec at 1300°K.     

Temperature dependence of the activation volume in a nonlinear optical polymer: Evidence for chromophore reorientation induced by sub-Tg relaxations

Second harmonic generation (SHG) was used to measure the temperature dependence of the reorientation activation volume of 4-(diethylamino)-4′-nitrotolane (DEANT) in poly(methyl methacrylate) (PMMA). The decay of the SHG signal from films of DEANT/PMMA was recorded at hydrostatic pressures up to 3060 atm and at different temperatures between 25°C below the glass transition temperature to 35°C above it. The activation volume, ΔV*_αβ associated with the long range α-type motion of the polymer remained constant at 213 ± 10 ų between T_g − 25°C and T_g + 10°C. At higher temperatures, ΔV*_αβ decreased linearly with increasing temperature. The activation volume, ΔV*_αβ, associated with short range secondary relaxations was constant over the entire temperature range with a value of 77 ± 10 ų. The data suggest that above T_g chromophore reorientation is coupled to both the long range and local motions of the polymer; whereas, well below T_g chromophore reorientation is closely coupled to the local relaxations of the polymer. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 901–911, 1998     

The Structure of Hydrotris(imidazolyl)boratothallium(I) – the First Structurally Authenticated Tris(imidazolyl)borate Metal Complex

The title compound crystallizes in the monoclinic space group P2₁/n with a = 8.706(1), b = 9.192(1), c = 15.261(2) Å, β = 94.740(3)°, V = 1217.0(3) ų, Z = 4, D_calc = 2.278 g cm^–3. The tris(imidazolyl)borate ligand bridges between three thallium atoms. The structure consists of one‐dimensional twisted ladder‐like strands.     

Oxygen diffusion in poly(dimethyl siloxane) using fluorescence quenching. I. Measurement technique and analysis

The transport properties of oxygen in poly(dimethyl siloxane) have been measured using the method of quenching of fluorescence. This paper discusses the uniqueness of this method and its use in measuring the diffusion coefficient of oxygen in unfilled PDMS. The results show (1) a large value for the diffusion coefficient of oxygen in pure PDMS at 25°C, D = 3.55 × 10^?5 cm²/s, (2) a low value of the acitivation energy, E_D = 4.77 kcal/mole, which was not temperature dependent in the ranges evaluated, and (3) a large value of the preexponential term, D₀ = 0.115 cm²/s. The diffusion coefficient was found to be independent of both the oxygen concentration and fluorophor concentration in the pressure and temperature ranges used in these experiments. The import of these experiments lies in their application to a unique biomedical oxygen sensor which is fast, sensitive, and does not consume oxygen.     

High-Pressure,High-Resolution Nuclear Magnetic Resonance (HPHR-NMR.): A Tool in Chemical Kinetics

In recent years the volume of activation Δ V* has become a powerful tool in chemical kinetics. High resolution NMR. spectroscopy is now one of the most common techniques used in the study of the kinetics of labile chemical systems. In order to measure Δ V* by this technique, we have built a ¹H-probe-head, for a Fourier transform spectrometer, working up to 4 kbar and with a resolution of 0.6 Hz. The temperature is regulated and measured with an accuracy better than 0.2°. The high pressure probe-head has been tested on a chemical system showing a dissociative-associative crossover for the ligand substitution mechanism. It had been shown previously that the ligand exchange TaBr₅ · L + *L ? TaBr₅ · *L + L proceeds via a D mechanism when L=Me₂O, and via an I_a mechanism when L=Me₂S. As expected, ΔV is positive (+30.5 ± 2.0 cm³ mol^?1) for the dissociative process and negative (?12.6 ± 1.0 cm³ mol^?1) for the associative one.     

Die Konformeren des 5,5-Dimethyl-3,7-dithia-1,2-benzocycloheptens-(1) und ihre Umwandlungen Konformative Beweglichkeit flexibler Ringsysteme—XV. Untersuchungen mit Hilfe der Protonenresonanzspektroskopie

The temperature dependence of the NMR spectrum of 5,5-dimethyl-3,7-dithia-1,2-benzocyclo-heptene-( 1 ) is described and discussed. This compound occurs in two conformers with different topographies of the seven membered ring. From the chemical shift of the 2 geminal methyl groups (obtained by low-temperature spectra) it can be shown that the ring occurs in one case in the chair form and in the other case in the twist form. The free conformational energy of the twist form is only about 20 cal/mole. Two conformational changes can be distinguished: the conversion between chair and twist forms and the pseudorotation of the twist forms. The free activation enthalpies of the conversion (ΔG_V^≠) and the pseudorotation (ΔG_P^≠) are 12·6 ± 0·1 Kcal/mole (at ?12°C) and 8·3 ± 0·3 Kcal/mole (at ?95°C) respectively.     

ab initio calculations and thermochemical analysis on C1 atom abstractions of chlorine from chlorocarbons and the reverse alkyl abstractions: Cl2 + R· ↔ Cl· + RCl

Thermodynamic properties (ΔH°_f(298), S°₍₂₉₈₎ and Cp(T) from 300 to 1500 K) for reactants, adducts, transition states, and products in reactions of CH₃ and C₂H₅ with Cl₂ are calculated using CBSQ//MP2/6‐311G(d,p). Molecular structures and vibration frequencies are determined at the MP2/6‐311G(d,p), with single‐point calculations for energy at QCISD(T)/6‐311 + G(d,p), MP4(SDQ)/CbsB4, and MP2/CBSB3 levels of calculation with scaled vibration frequencies. Contributions of rotational frequencies for S°₍₂₉₈₎ and Cp(T)'s are calculated based on rotational barrier heights and moments of inertia using the method of Pitzer and Gwinn [1]. Thermodynamic parameters, ΔH°_f(298), S°₍₂₉₈₎, and C_P(T), are evaluated for C₁ and C₂ chlorocarbon molecules and radicals. These thermodynamic properties are used in evaluation and comparison of Cl₂ + R· → Cl· + RCl (defined forward direction) reaction rate constants from the kinetics literature for comparison with the calculations. Data from some 20 reactions in the literature show linearity on a plot of Ea_fwd vs. ΔH_rxn,fwd, yielding a slope of (0.38 ± 0.04) and intercept of (10.12 ± 0.81) kcal/mole. A correlation of average Arrhenius preexponential factor for Cl· + RCl → Cl₂ + R· (reverse rxn) of (4.44 ± 1.58) × 10¹³ cm³/mol‐sec on a per‐chlorine basis is obtained with Ea_Rev = (0.64 ± 0.04) × ΔH_rxn,Rev + (9.72 ± 0.83) kcal/mole, where Ea_Rev is 0.0 if ΔH_rxn,Rev is more than 15.2 kcal/mole exothermic. Kinetic evaluations of literature data are also performed for classes of reactions. Ea_fwd = (0.39 ± 0.11) × ΔH_rxn,fwd + (10.49 ± 2.21) kcal/mole and average A_fwd = (5.89 ± 2.48) × 10¹² cm³/mole‐sec for hydrocarbons: Ea_fwd = (0.40 ± 0.07) × ΔH_rxn,fwd + (10.32 ± 1.31) kcal/mole and average A_fwd = (6.89 ± 2.15) × 10¹¹ cm³/mole‐sec for C₁ chlorocarbons: Ea_fwd = (0.33 ± 0.08) × ΔH_rxn,fwd + (9.46 ± 1.35) kcal/mole and average A_fwd = (4.64 ± 2.10) × 10¹¹ cm³/mole‐sec for C₂ chlorocarbons. Calculation results on the methyl and ethyl reactions with Cl₂ show agreement with the experimental data after an adjustment of +2.3 kcal/mole is made in the calculated negative Ea's. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 548–565, 2000     

The Structure of Hydrotris(pyrazolyl)boratothallium(I)

The title compound crystallizes in the monoclinic space group P2₁ with a = 7.928(6), b = 9.306(4), c = 17.16(2) Å, β = 92.06(8)°, V = 1265(2) ų, Z = 2, D_calc = 2.191 g cm^–3. From two independent molecular units, metal-ligand strands are formed based on electrostatic interactions between the thallium centers and pyrazolyl π manifolds from neighboring molecules.     

Synthesis and study of the physicochemical properties of ammonium hydrogen hexaaquacobaltate(III) isopolyvanadate [(NH4)2][Co(H2O)6] · H[V10O28] · 8H2O

Ammonium hydrogen hexaaquacobaltate(III) isopolyvanadate [(NH₄)₂][Co(H₂O)₆] · H[V₁₀O₂₈] · 8H₂O was synthesized and studied by X-ray diffraction, thermogravimetry, and IR and NMR spectroscopy. The crystals are triclinic, space group P [`1]\bar 1, a = 8.216(2), b = 9.965(2), c = 11.796(2) ?, α = 77.71(3)°, β = 71.11(3)°, γ = 86.11(3)°, V = 867.0(3) ?³, ρ(calcd.)= 2.501 g/cm³, Z = 1.