Crosslinking,filler, or transition constraint of polymer networks. I

The basic theory of modulus/swelling is developed to allow for limited extensibility, filler reinforcement or transition effects, and steric hindrance of aligned segments by extended chains or filler particles. Filler forms an effective hard fraction C_h per cubic centimeter of compound with v_c a new (compound) index of swelling. For 1/M_c + σ fix points having ratio φ to gum values 1/F₀(v_r) and with F(v_c) replacing the Flory function F(v_r): where σ denotes entanglement. Linkage reinforcement φ does not vary with sulfur crosslinking of SBR. Vacuoles invalidate φ from mass-increment F₀(v_r)/F(v_r) for inert fillers. Then, or for Graphon, with negligible φ ≈ 1: The effective C_h includes rubber stretched hard on Graphon by swelling or trapped inside hard aggregates. Only the right-hand equation fits normal blacks. In theory, C_h can always be obtained from swollen moduli G by linear slopes (1 + 1.4C_h) relating F(v_c) and (1 ? C)ρRT/G. For filler fractions C ≥ 0 cm^?3 and low strains α = 1.5?2.0 below prestretch the modulus G is given a new basic definition: Here C₂* ≈ 0.7 corresponds to Mooney-Rivlin C₂ and the effective crosslinking 1/[M_c] = (ρRT)^?1G is equal to (1 ? C)(1/M_c + σ) for unswollen prestretched rubber (v_r = 1). For higher strains a hypothesis of strain hardening is proposed. This is distinct and opposite in character to the initial prestretch softening (Mullins effect). Nonlinear effects of crosslinks are expressed by a fractional stress-upturn Ω (1/M_c + σ), effective mesh wieght (1/M_c + σ)^?1 ? Ω, and hard fraction Ω(1/M_c + σ). For μ_h characterizing strain hardening up to the prestretch (α_h ? 1) their contribution is: The sixth-power refinement has J = j(α_b ? 1)^1/2 with j ≈ 0.4. The hard phase is augmented by filler and grows with increasing strain up to the prestretch.     

Thermodynamic analysis of polymer‐solid adhesion: Sticker and receptor group effects

Adhesion of dense linear polymer chains containing a small number of randomly distributed sticker groups (?_X) to a solid substrate containing receptor groups (?_Y) has been analyzed by a single‐chain scaling approach. An entanglement sink probability (ESP) model motivated by vector percolation explains the nonmonotonic influences of sticker concentration (?_X), receptor concentration (?_Y), and their interaction strength (χ) on the adhesion strength G_IC of the polymer‐solid interface. The ESP model quantifies the degree of interdigitation between adsorbed and neighboring chains on the basis of the adsorbed chain domain with an extension of the scaling treatment of de Gennes. Here, the adsorbed chain domain changes thermodynamically with respect to the energy of interaction parameter, r = χ?_X?_Y. This model considers the situation of a blend consisting of a small volume fraction of adhesive molecules as a compatibilizer at the interface, where these molecules promote adhesion by adsorbing to the surface via sticker‐receptor interactions. The percolation model scales solely with r = χ?_X?_Y, and this parameter can be related to both the adhesive potential (G_A) and the cohesive potential (G_C). G_A describes adhesive failure between adsorbed chains and the solid surface and linearly behaves as G_A ～ r = χ?_X?_Y. The cohesive strength between adsorbed and neighboring chains corresponds to G_C ～ r^?0.5～?1.0 = (χ?_X?_Y)^?0.5～?1.0. When the fracture stresses for cohesive and adhesive failure are equal, the model predicts maximum adhesion strength at an optimal value of r* = (χ?_X?_Y)*. Thus, for a given χ value, optimal values ? and ? exist for the sticker and receptor groups, above or below which the fracture energy will not be optimized. Alternatively, if the X‐Y interaction strength χ increases, then the number of sticker groups required to achieve the optimum strength decreases. Significantly, the optimum strength is not obtained when the surface is completely covered with receptor groups (?_Y = 1) but is closer to 30%. For polybutadiene, the optimum value of r* was determined experimentally (Lee, I.; Wool, R. P. J Adhesion 2001, 75, 299), and typically ? ≈ 1–3%, ? ≈ 25–30%. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2343–2353, 2002     

Structural Studies of New Sulfito Mercurates(II) with General Formula xM[XHgSO3]middot;yHgX2·zMX·nH2O (M = NH4, K; X = Cl,Br)

Several solid phases with the general formula xM[XHgSO₃]·yHgX₂·zMX·nH₂O were obtained from aqueous solutions during phase formation studies in the systems M₂SO₃/HgX₂ (M = NH₄, K; X = Cl, Br). All phases were structurally characterized on the basis of single crystal X‐ray diffraction data and adopt new structure types. Compounds with x, y, z = 1 and n = 0 are isostructural (structure type I ) and crystallise with two formula units in space group P2₁/m and lattice parameters of a ≈ 9.7, b ≈ 6.2, c ≈ 10.4Å, β ≈ 111°. Compounds with x, y = 1 and z, n = 0 (structure type II ) crystallize in space group Cmc2₁ with four formula units and lattice parameters of a ≈ 5.9, b ≈ 22.0, c ≈ 6.9Å. The structures with x = 2, y, z = 1 and n = 0 are likewise isostructural (stucture type III ) and consist of four formula units in space group Pnma with lattice parameters of a ≈ 22.2, b ≈ 6.1, c ≈ 12.4Å. K[HgSO₃Cl]·KCl·H₂O is the only representative where x = 1, y = 0, z = 1 and n = 1 (structure type IV ). It is triclinic (space group ) with four formula units and lattice parameters of a = 6.1571(8), b = 7.1342(9), c = 10.6491(14) Å, α = 76.889(2), β = 88.364(2), γ = 69.758(2)°. Characteristic for all structures types is the segregation of the M⁺ cations and the anions and/or HgX₂ molecules into layers. The [XHgSO₃]⁻ anions are present in all structures and have m symmetry, except for K[HgSO₃Cl]·KCl·H₂O with 1 symmetry (but very close to m symmetry). The different [XHgSO₃] units exhibit very similar Hg‐S distances (average 2.372Å) and are more or less bent with ∠(X‐Hg‐S) angles ranging from 159.7 to 173.7°. The molecular HgX₂ entities present in structure types I ‐ III deviate only slightly from linearity with ∠(X‐Hg‐X) angles ranging from 174 to 179°. The structures are stabilised by interaction of the K⁺ or NH₄⁺ cations that are located between the anionic layers or in the vacancies of the framework, by K‐O contacts or, in case of ammonium compounds, by medium to weak hydrogen bonding interactions of the type N‐H···O.     

Protonenresonanzuntersuchungen an Aminoboranen—II: Einflüsse von para-Phenylsubstituenten auf die Rotationsbarriere um die N?B-Bindung

The electronic influence of substituents on the free enthalpy of rotation around the N? B bond in aminoboranes was investigated in two series of compounds: (a) (CH₃)₂N?BCl (phenyl-p-X), containing the para-phenyl substituent at the boron atom, and (b) (p-X-phenyl)CH₃N?B(CH₃)₂, containing the para-phenyl substituent at the nitrogen atom of the N? B linkage (X = ? NR₂, ? OCH₃, ? C(CH₃)₃, ? Si(CH₃)₃, ? H, ? F, ? Cl, ? Br, ? I, ? CF₃ and ? NO₂). By comparing the rotational barriers in corresponding compounds of both series, a reverse effect of the substituents could be observed. Electron-withdrawing substituents in the para position of the phenyl ring increase the ΔG_c^≠ if the phenyl group is attached to the boron atom; on the other hand, a lower ΔG_c^≠ is observed if the phenyl ring is bonded to the nitrogen atom of the N? B system. Substitution of the phenyl ring with electron-donating substituents in the paraposition exerts the opposite effect. Within each series of compounds, the differences of ΔG_c^≠ values [δ(ΔG_c^≠) = ΔG_c^≠ (X) ? ΔG_c^≠ (X = H)] between substituted and unsubstituted compounds can be explained in terms of inductive and mesomeric effects of the ring substituents and can be correlated with the Hammett σ constant of each substituent. A comparison of the slopes of the plotted lines shows that the influence of the ring substituents is more pronounced in compounds with N-phenyl-p-X than in those with B-phenyl-p-X.     

New manganites NdM3Sr3Mn4O12 and NdM3Ba3Mn4O12 (M = Li, Na, K): Synthesis and X-ray diffraction characteristics

Manganites NdM₃Sr₃Mn₄O₁₂ and NdM₃Ba₃Mn₄O₁₂ (M = Li, Na, K) were synthesized by a ceramic method from the corresponding oxides and carbonates. The X-ray diffraction analysis showed that all the compounds crystallized in the tetragonal crystal system with the following lattice parameters: NdLi₃Sr₃Mn₄O₁₂: a = 10.88 ?, c = 9.52 ?, V ^o = 1126.9 ?³, Z = 4, ρ_X = 4.95 g/cm³, ρ_pycn = 4.87 ± 0.05 g/cm³; NdNa₃Sr₃Mn₄O₁₂: a = 10.73 ?, c = 10.66 ?, V ^o = 1227.3 ?³, Z = 4, ρ_X = 4.80 g/cm³, ρ_pycn = 4.73 ± 0.07 g/cm³; NdK₃Sr₃Mn₄O₁₂: a = 10.87 ?, c = 11.71 ?, V ^o = 1382.6 ?³, Z = 4, ρ_X = 4.50 g/cm³, ρ_pycn = 4.43 ± 0.09 g/cm³; NdLi₃Ba₃Mn₄O₁₂: a = 10.97 ?, c = 10.34 ?, V ^o = 1244.3 ?³, Z = 4, ρ_X = 5.33 g/cm³, ρ_pycn = 5.23 ± 0.09 g/cm³; NdNa₃Ba₃Mn₄O₁₂: a = 10.99 ?, c = 11.15 ?, V ^o = 1346.7 ?³, Z = 4; ρ_X = 5.11 g/cm³, ρ_pycn = 5.05 ± 0.06 g/cm³; NdK₃Ba₃Mn₄O₁₂: a = 10.997 ?; c = 13.80 ?, V ^o = 1668.9 ?³, Z = 4, ρ_X = 4.32 g/cm³, ρ_pycn = 4.26 ± 0.07 g/cm³. Original Russian Text ? B.K. Kasenov, E.S. Mustafin, M.A. Akubaeva, S.T. Edil’baeva, Sh.B. Kasenova, Zh.I. Sagintaeva, S.Zh. Davrenbekov, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 3, pp. 424–427.     

Electrophoretic flow of interacting polymer chains: Effects of temperature,polymer concentration,and porosity

Using a Monte Carlo simulation in three dimensions, we studied the variation of the root-meansquare (rms) displacement (R_rms) of polymer chains with time and the rates of their mass transfer (j) as a function of biased field (B), polymer concentration (p), chain length (L_c), porosity (p_s), and temperature (T). In homogeneous/annealed system, the rms displacement of the chains shows a drift-like behavior, R_rms ∼ t, in the asymptotic time regime preceded by a subdiffusive power-law (R_rms ∼ t^k, with k < 1/2) at high p. The subdiffusive regime expands on increasing L_c and p but reduces on increasing T or B. In quenched porous media, the drift-like behavior of R_rms persists at low barrier concentration (p_b) and high T. However, at high p_b and/or low T, chains relax into a subdrift and/or subdiffusive behavior especially with high p or long L_c. Flow of chains is measured via an effective permeability (σ) using a linear response assumption. In annealed system, σ increases monotonically with B at high T and low p but varies nonmonotonically at low T, high p and high L_c. We find that σ decays with L_c, σ ∼ L, where α depends on B, p and T with a typical value a α ∼ 0.43−0.64 for p = 0.1-0.3 at B = 0.5. Further, σ decays with p, σ ∼ − Cp with a decay rate C sensitive to T and B. In quenched porous media, even at low pb and high T, σ varies nonmonotonically with bias, i.e., the increase of σ is followed by decay on increasing the bias beyond a characteristic value (B_c). This characteristic bias seems to decrease logarithmically with barrier concentration, B_c ∼ −klnp_b. The prefactor k depends on the chain length, k ≈ 0.35 for shorter chains (L_c = 20, 40) and ≈ 0.15 for longer chains (L_c = 60). Scaling dependence of σ on L_c similar to that in annealed system is also observed in porous media with different values of exponent α. The current density shows a nonlinear power-law response, j ∼ B^σ, with a nonuniversal exponent δ ≈ 1.10−1.39 at high temperatures and low barrier concentrations.     

Resonance-enhanced multiphoton ionization of molecular hydrogen via the E,F1Σg+ state: Photoelectron energy and angular distributions

Photoelectron energy and angular distributions are measured for the 2+1 multiphoton ionization process H₂ X¹Σ_g⁺ (ν = 0,J) + 2hv → E,F¹Σ_g⁺(ν_E,J_E = J) + hν → H₂⁺X²Σ_g⁺ (ν⁺) + e^?, for ν_E = 0, 1, or 2 and for J_E = 0 or 1 of the inner well of the double-minimum E,F state. Although a strong preference is found for ν⁺ = ν_E, the detailed H₂⁺ vibrational distribution does not exhibit Franck-Condon behavior, and the photoelectron angular distributions vary markedly with both the J_E value of the intermediate state and the ν⁺ value of the ion.     

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     

Etude structurale des composés MxTiSe2 (M = Fe,Co, Ni)

New compounds M_xTiSe₂ have been prepared with M = Fe (x ? 0.66), M = Co or Ni (x ? 0.50). The metal M is located in vacant octahedral sites of the TiSe₂ host lattice (hexagonal unit cell a′, c′). An ordering of vacancies occurs if x ? 0.20. With M = Co or Ni (x = 0.50) and with M = Fe (0.25 ? x ? 0.66) isotypic compounds of Ti₃Se₄ can be obtained (M₃ □ X₄ type; monoclinic unit cell a ≈ a′ √3, b ≈ a′, c ≈ 2c′). The compounds Fe_0.38TiSe₂ and Co_0.38TiSe₂ (hexagonal unit cell a ≈ a′ √3, c ≈ 2c′) are of the M₂ □ X₃ type, variety 2c′. The Fe_0.25TiSe₂ and Co_0.25TiSe₂ monoclinic unit cells (a ≈ 2a′ √3, b ≈ 2a′, c ≈ 2c′) allow us to assume, for these two compounds, a structure of the M₅ □ ₃X₈ type, variety 2c′, identical to the Ti₅Se₈ one. The compound Ni_0.25TiSe₂ has an hexagonal unit cell (a ≈ 2a′, c ≈ 3c′); it belongs to a so-called 3c′ variety of the M₅ □ ₃X₈ type.     

Bismut(II)-chalkogenometallate(III) Bi2M4X8, Verbindungen mit Bi24+-Hanteln (M = Al,Ga; X = S,Se)

Bismuth(II) Chalcogenometallates(III) Bi₂M₄X₈, Compounds with Bi₂⁴⁺ Dumbbells (M = Al, Ga and X = S, Se) The ternary bismuth(II) chalcogenometallates(III) Bi₂M₄X₈ (with M = Al, Ga and X = S, Se) were synthesized from the binary chalcogenides M₂X₃ and Bi₂X₃ and elementary bismuth. All compounds are diamagnetic semiconductors with E_g (opt.) = 1.8–2.7 eV. The phases (except Bi₂Al₄Se₈) are thermodynamically stable and decompose peritectically above 965–1020 K. Bi₂Al₄Se₈ is metastable below 825 K and is obtained only by rapid quenching from T > 825 K. The isotypic compounds crystallize in a new tetragonal tP28 structure type (P4/nnc). The characteristic unit is the hitherto unknown clustercation Bi₂⁴⁺, with the mean bond length d(Bi–Bi) = 314.2 pm, the Raman frequency 102 cm^–1 ≤ ν_s ≤ 108 cm^–1, and the mean force constant of f = 0.68 N · cm^–1. The Electron Localization Function, ELF, shows the covalent Bi–Bi bond, the lone electron pairs of the ψ-octahedrally coordinated Bi(II) cations, and the polar character of the Bi–X bonds.     

Diffuse interface between polymers: Structure and kinetics

The interfacial structure and diffusion kinetics of two compatible polymers, poly(methyl methacrylate) and poly(vinylidene fluoride) are studied in the melt. The interdiffusion rates of the two components are found to be unequal, giving unequal diffusion coefficients, a net mass flow across the interface, and an asymmetric interfacial composition profile. The structure and kinetics confirm the predictions of the reptation theory. The interfacial thickness d grows with t^1/2, and the interdiffusion coefficient is proportional to M^?2, where t is the time and M is the molecular weight. The scaling law for the interfacial thickness is therefore d ∝ M^?1t^1/2. The number of chains per unit area crossing the original interface reaches a constant value independent of diffusion time after a short induction time on the order of the tube disengagement time (about 0.1–10 s in the present cases depending on the molecular weights). The adhesive bond strength σ is scaled by σ ∝ t^1/4M^?1/2 and σ/σ∞ ∝ t^1/4M^?1/2 [1- (M_c/M)]^?1, where σ _∞is the σ at infinite molecular weight and M_c is the entanglement molecular weight.     

Vapour–liquid equilibria of butyl acetate with aromatic hydrocarbons at 298.15 K

Vapour pressures of butyl acetate?+?benzene or toluene or o- or m- or p-xylene were measured by static method at 298.15?±?0.01?K over the entire composition range. The activity coefficients and excess molar Gibb's free energies of mixing (G ^E) for these binary mixtures were calculated by fitting vapour pressure data to the Redlich–Kister equation using Barker's method of minimizing the residual pressure. The G ^E values for the binary mixtures containing benzene are positive; while these are negative for toluene, ortho, meta and para xylene system over the whole composition range. The G ^E values of an equimolar mixture for these systems vary in the order: benzene?>?m-xylene?>?o-xylene?>?p-xylene?>?toluene     

Copolymerization of aryldiazomethanes

Several aryldiazoalkanes (M₂) have been copolymerized with phenyldiazomethane (M₁) in toluene-methanol solution at 40°C, namely, the p-chloro-, p-methoxy-, p-mesyl-o and -p-methyl-, 2,4- and 3,4-dichlorophenyldiazoalkanes, and the α- and β- naphthyl-diazoalkanes. The copolymerization parameters r₁ and r₂ have been evaluated. By plotting 1/r₁ against the Hammett σ values a negative ρ values was found equal to ?0.88. From cationic copolymerizations carried out at ?78°C in the presence of boron trifluoride-diethyl ether as catalyst a similar plot of 1/r₁ against σ gives a value of ρ equal to ?0.82. The negative sign and the agreement between these ρ values demonstrates the cationie mechanism of the methanol polymerization and copolymerization of aryldiazoalkanes.     

Cinetique complexe de l'halogenation de cetones en milieu acide—II : Controle par la diffusion des vitesses d'halogenation des enols-consequences

The kinetics of the bromination and chlorination of acetone, diethylketone and di-isopropylketone (bromination only) have been studied at [X₂]_a^o ≈ 10^?7 to 10^?5 M; the apparent rate constants k_II^X₂ = K_Ek₂^X₂ (where K_E is the keto-enol equilibrium constant) for iodination, bromination and chlorination are approximately equal. This result is attributed to diffusion-controlled kinetics. The order of magnitude of such a limiting rate constant, 10⁹ M^?1s^?1 calculated from Smoluchowski's equation, leads to new values for K_E in solution (1·5 x 10^?8 for acetone) much smaller than those in the literature. The rate constants derived for enol ketonisation are then in good agreement with those from proton addition to the corresponding enol ethers.     

The effect of substituents in the ring on the reactivity of styrenes in copolymerization

Styrene (M₁) has been copolymerized with o-, m- and p-methyl-styrenes and p-methoxystyrene (M₂) at temperatures between 40 and 110°, using azoisobutyronitrile as initiator; the substituted styrenes were labelled with ¹⁴C in the β-position. The compositions of the copolymers were determined by liquid scintillation counting. Since [M₁] ? [M₂], a simplified form of the copolymer composition equation was used to determine reactivity ratios r₁. Arrhenius parameters of r₁ were found; they show that polar effects predominate when p-methoxystyrene copolymerizes whereas steric effects predominate for o-methylstyrene. Both polar and steric effects are very small for m-methylstyrene; for p-methoxystyrene, the predominance of polar and steric effects varies with the temperature. Values of (E₁₁ ? E₁₂) show good correlation with Hammett substituent constants.     

Viscoelasticity and birefringence of polyisoprene

The complex Young's modulus, E^*(ω), and the complex strain-optical coefficient, O^*(ω), which is the ratio of the birefringence to the strain, were measured for polyisoprene (PIP) over a frequency range of 1 ～ 130 Hz and a temperature range of 22 ～ ?100°C. The imaginary part of O^*, O″, was positive at low frequencies and negative at high frequencies. The real part, O′, was always positive and showed a maximum. The complicated behavior of O^* could be understood by the assumption that E^* = E_R^* + E_G^* and O^* = C_RE_R^* + C_GE_G^*, where E_R^* and E_G^* were complex quantities and C_R and C_G were constants. The C_R value, equal to the ordinary stress-optical coefficient measured in the rubbery plateau zone, was 2.0 × 10^?9 Pa^?1. The C_G value, defined as the ratio O″/E″ in the glassy zone, was ?1.1 × 10^?11 Pa^?1. The E_G^*, which was the major component of E^* in the glassy zone, showed almost the same frequency dependence as that of polystyrene and polycarbonate. The E_R^*, which was dominant in the rubbery zone, was described well by the bead-spring theory. The temperature dependence of the E_G^* was stronger than that of the E_R^*. This difference caused the breakdown of the thermorheological simplicity for E^* and O^* around the glass-to-rubber transition zone. © 1995 John Wiley & Sons, Inc.     

Zur kenntnis von [O2]22+[Ti7F30]2−

[O₂]²⁺₂[Ti₇F₃₀]^2? has been obtained by reaction of TiO₂ with a mixture of fluorine and oxygen (p_F2/O2 ≈ 300–3500 atm., t ≈ 300–450°C) either as colourless powder or in form of colourless, clear needles. From single crystal studies the spacegroup is P$\text{3}$ - C_3i¹ (No. 147) with a = 10.19₂, c = 6.50_o Å, Z = 1. The crystal structure has been refined to R = 0.086 [R_w = 0.058] (748 unique reflexions [F_o > 2σ(F_o)]). From the structure determination [O₂]²⁺₂[Ti₇F₃₀]^2? has isolated columns of partially distorted [TiF₆] octahedra (- column structure) which are connected only quite loosely by (disordered) O₂⁺ cations. ν_O2+ is at 1857 cm^?1, the magnetic moment μ_eff = 2.35 B.M. (295 K) is quite as expected for a ‘spin-only’ case.     

Conductibilité electrique aux basses températures des composés binaires Cr2X3 et Cr3X4 (X = S,Se ou Te)

Electrical resistivity of some binary compounds Cr₂X₃ and Cr₃X₄ (X = S, Se or Te) is studied on polycrystalline samples with the four point probe method, at temperatures between 4.2 and about 330 K. A metallic behavior is observed on Cr₂Te₃, Cr₃Te₄, Cr₃S₄ and the 3c′ form of Cr₂Se₃. Some other compounds are semiconductors: Cr₃Se₄ (E_300K ≈ 0.07 eV; E_4.2K = 2.07 × 10^?4 eV), the 2c′ form of Cr_2+εSe₃ (E_300K ≈ 0.074 eV; E_4.2K = 2.76 × 10^?4 eV) and the 3c′ form of Cr₂S₃ (E_275K ≈ 0.55 eV). The observed results seem to be closely related to the nature of the octahedral neighborhood of the cations.     

Conformational studies by nuclear magnetic resonance—IV: Restricted rotation in vinylogous thioamides

Enamino -thial and -thiones R¹C(S)CH?CHNR (R¹ = H or alkyl; R² = Me or Et) have been shown by NMR spectra to exist in two rotational forms, s-cis and s-trans, the populations of the latter being approximately the same as in the case of the parent oxa analogues. An increase of the order of 2 to 4 Kcal/mole in the heights of C? C and C? N rotation barriers (ΔG*) was found on comparing the title compounds with their oxa analogues. IR spectra failed as a tool to establish the rotational equilibrium. IR absorption bands of the ν_{C? C}, ν_{C? H} (in the NMe₂ group) and γ_HC?CH vibrations have been found, but the ν_C?S band could not be assigned unambiguously. Anomalies concerning the frequency and intensity of the ν_C?C band are discussed.     

Darstellung und spektroskopische Charakterisierung der Nona-halogenodiiridate(III), [Ir2X9]3−, X = Cl,Br

Preparation and Spectroscopic Characterization of Nonahalogenodiiridates(III), [Ir₂X₉]^3?, X = Cl, Br The pure nonahalogenodiiridates(III), A₃[Ir₂X₉] (A = K, Cs, tetraalkylammonium; X = Cl, Br) have been prepared. They are formed from the monomer hexahalogenoiridates(III) which are bridged to confacial bioctahedral complexes by ligand abstraction in less polar organic solvents. The IR and Raman spectra exhibit bands in three characteristic regions; at high wavenumbers stretching vibrations with terminal ligands ν(Ir?Cl_t): 360?300, ν(Ir?Br_t): 250?220; in a middle region with bridging ligands ν(Ir?Cl_b): 290?235, ν(Ir?Br_b): 205?190 cm^?1; the deformation bands are observed at distinct lower frequencies. The distance between ν(Ir?X_t) and ν(Ir?X_b) increases with decreasing size of the cations. The electronic spectra measured at thin films of the pure complex salts at 10 K show some intensive charge transfer transitions in the UV and one or two weak d? d bands in the visible region.