H2S-promoted thermal isomerization of butene-2 CIS to butene-1 or butene-2 trans around 500°C

H₂S increases the thermal isomerization of butene-2 cis (B_c) to butene-1 (B₁) and butene-2 trans (B_t) around 500°C. This effect is interpreted on the basis of a free radical mechanism in which buten-2-yl and thiyl free radicals are the main chain carriers. B₁ formation is essentially explainedby the metathetical steps: whereas the free radical part of B_t formation results from the addition–elimination processes: . It is shown that the initiation step of pure B_c thermal reaction is essentially unimolecular: and that a new initiation step occurs in the presence of H₂S: . The rate constant ratio has been evaluated: and the best values of k₁ and k_1', consistent with this work and with thermochemical data, are . From thermochemical data of the literature and an “intrinsic value” of E_?3 ? 2 kcal/mol given by Benson, further values of rate constants may be proposed: is shown to be E₄ ? 3.5 ± 2 kcal/mol, of the same order as the activation energy of the corresponding metathetical step.     

Kinetic study of the gas-phase reaction c-C5H8 + I2 ⇄ c-C5H6 + 2HI the heat of formation and the stabilization energy of the cyclopentenyl radical

The gas-phase dehydrogenation of cyclopentene to cyclopentadiene catalyzed by iodine in the range 178–283°C has been found to obey a rate law consistent with the slow rate-determining step, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm I} + {\rm c} - {\rm C}_5 {\rm H}_8 \stackrel{4}{\rightarrow}{\rm HI} + {\rm c} - {\rm C}_5 {\rm H}_7 $\end{document}, log [k₄/(1 mole^?1 sec^?1)] = 10.25 ± 0.08 - (12.26 ± 0.18)/θ, where θ = 2.303 R T in kcal/mole. Surface effects are not important. This value of E₄ leads to a value of DH = 82.3 ± 1 kcal/mole and ΔH_f298 = 38.4 ± 1 kcal/mole. From difference in bond strengths in the alkane and the alkene, the allylic resonance stabilization in the cyclopentenyl radical is 12.6 ± 1.0 kcal/mole, in excellent agreement with the value for the butenyl radical. Arrhenius parameters for the other steps in the mechanism are evaluated. The low value of A₄ (compared with A₄ for cyclopentane) suggests a “tighter” transition state for H-atom abstraction from alkenes than from alkanes.     

Photoactive Modified Hydroxyethylcellulose

A water‐soluble polymeric photosensitizer that contains naphthalene chromophores and absorbs light in the near UV region was obtained by modification of hydroxyethylcellulose. The excitation energy migrates along the naphthalene chromophores covalently attached to the polymer chain and can be used to induce photochemical reactions such as photoinduced electron transfer.

UV‐vis absorption (○), fluorescence emission (□), and fluorescence excitation (×) spectra of HENC in water at c_HENC = 0.232 g/L, and (+) emission spectrum of the lamps used for irradiations.     

Void Microstructuring in Lamellar Phase of Amphiphilic Macromolecules

The free volume (voids) distribution in the lamellae of the conventional symmetric and amphiphilic diblock copolymers is studied via Monte–Carlo simulation based on the standard bond fluctuation model. Both in the conventional and amphiphilic block copolymers the voids are found to concentrate on the interfaces between the incompatible units, the magnitude of the effect being unexpectedly significant. A crystalline‐like ordering of voids with increase of the incompatibility between the different repeated units in amphiphilic copolymers is first reported and implications of this peculiarity for the morphology and mechanical properties of the amphiphilic copolymers are discussed.

     

The gas-phase kinetics of the reaction of 1,1-difluoroiodoethane with hydrogen iodide: The C–I bond dissociation energy in 1,1-difluoroiodoethane

The kinetics of gas-phase reaction of CH₃CF₂I with HI were studied from 496 to 549K and have been shown to be consistent with the following mechanism: A least squares treatment of the data gave where θ = 2.303 RT kcal/mole. The observed activation energy E₁ was combined with E₂ = 0 ± 1 kcal/mole to yield The result, combined with data for several C? I bond dissociation energies, leads us to conclude that the C(sp³)? I bond is relatively insensitive to F for H substitution and that the C(sp²)–I bond has considerable double-bond character.     

Computational Studies of RAFT Polymerization–Mechanistic Insights and Practical Applications

Summary: Computational chemistry is a valuable complement to experiments in the study of polymerization processes. This article reviews the contribution of computational chemistry to understanding the kinetics and mechanism of reversible addition fragmentation chain transfer (RAFT) polymerization. Current computational techniques are appraised, showing that barriers and enthalpies can now be calculated with kcal accuracy. The utility of computational data is then demonstrated by showing how the calculated barriers and enthalpies enable appropriate kinetic models to be chosen for RAFT. Further insights are provided by a systematic analysis of structure‐reactivity trends. The development of the first computer‐designed RAFT agent illustrates the practical utility of these investigations.

     

Kinetics of the gas phase reaction of methyl benzoate with bromine and iodine: The methoxyl C?H bond strength of methyl esters

The gas phase reactions of PhCOOCH₃ with I₂ and Br₂ were studied spectrophotometrically in a static system over the temperature ranges 344–359° and 246–303°, respectively. For each system the initial rate was first order in PhCOOCH₃ and half order in halogen as the concentration of PhCOOCH₃ was varied from 1.4 to 15.2 torr, that of I₂ from 6.2 to 26.4 torr, and that of Br₂ from 3.0 to 13.6 torr. The rate-determining step is the extraction of a methoxyl hydrogen atom: Empirical assignment of A-factors for k₁ lead to for the I₂ system, and to for the Br₂ system, where ? = 2.303RT in kcal/mole. Combined with the assumption that E_–1 = 1 ± 1 kcal/mole and 2 ± 1 kcal/mole for HI and HBr, respectively, DH (PhCOOCH₂? H) calculated from the two systems shows excellent agreement at 100.2 ± 1.3 kcal/mole and 100.1 ± 1.3 kcal/mole. Using a value of δH (PhCOOMe) = –65.6 ± 1.5 kcal/mole obtained from group additivity estimates, δH_f,298⁰ (PhCOOCH₂) is calculated to be –16.7 ± 2.0 kcal/mole. Unimolecular decomposition of the Ph(CO)O°CH₂ radical was also observed: with a rate constant equal to The abnormally high methoxyl C? H bond strength is discussed in relation to the bonding in ethers, alkanes, and esters.     

KNUDSEN measurements of the sublimation and the Heat of Formation of GeSe

KNUDSEN effusion studies of the sublimation of polycrystalline GeSe have been performed employing mass spectrometry in a temperature range of about 530–730°K and vacuum microbalance techniques in the temperature range 601–796°K and at pressures ranging from about 10^?6–10^?4 atm. The results are concordant and demonstrate that GeSe vaporizes congruently under present experimental conditions according to the reaction GeSe(s) = GeSe(g). The mean values for the third law heat and second law entropy of reaction based on direct mass-loss data are ΔH = 42.0 ± 1.5 kcal/mole and ΔS = 42.3 ± 1.6 eu. From these data the standard heat of formation and absolute entropy of GeSe(s) were calculated to be ?10.1 ± 2.0 kcal/mole and 16.9 ± 2.0 eu, respectively.     

Oxazolidines versus schiff bases

The formation of oxazolidines from propionaldehyde and aliphatic β-aminoalcohols is complicated by the appearance of appreciable amounts of unsaturated Schiff bases. The simple Schiff base, often the dominant species when aromatic aldehydes react with amines, could not be detected in the present aliphatic systems. We conclude that in aliphatic systems the order of stability is and . The gem-dimethyl group α to nitrogen stablizes the heterocyclic ring remarkably.     

Kinetics and thermochemistry of the gas phase reaction of methyl ethyl ketone with iodine. I. The resonance energy of the methylacetonyl radical

The gas phase reaction of iodine (2.8–43.3 torr) with methyl ethyl ketone (MEK) (7.4–303.4 torr) has been studied over the temperature range 280–355°C in a static system. The initial rate of disappearance of I₂ is first order in MEK and half order in I₂. The rate-determining step is the abstraction of a secondary hydrogen atom by an iodine atom: where k₁ is given by and θ = 2.303RT in kcal/mole. This activation energy is equivalent to a secondary C? H bond strength of 92.3 ± 1.4 kcal/mole and ΔH of the methylacetonyl radical = -16.8 ± 1.7 kcal/mole. By comparison with 95 kcal/mole for the secondary C? H bond strength, when delocalization of the unpaired electron with a pi bond is not possible, the resonance stabilization of the methylacetonyl radical is calculated to be 2.7 ± 1.7 kcal/mole. This value is 10 kcal/mole less than the stabilization energy of the isoelectronic methylallyl radical. The difference in pi bond energies in the canonical forms of the methylacetonyl radical is shown to account for the variation in stabilization energies.     

Kinetic study of the gas-phase reaction c-C5H10 + I2 ⇄ c-C5H8 + 2HI the heat of formation of cyclopentyl radical

The kinetics of the gas-phase dehydrogenation of cyclopentane to cyclopentene is found to be consistent with a slow attack by an I atom (step 4, text) on cyclopentane in the range 282–382°C. The measured rate constants fit the Arrhenius equation, log k₄ = 11.95 ± 0.08 – (24.9 ± 0.23)/θ 1 mole^?1 sec^?1, where θ = 2.303 R T in kcal/mole. This leads to a value of ΔH = 24.3 ± 1 kcal/mole and a bond dissociation energy DH = 94.9 ± 1 kcal/mole. The latter value is identical with DH⁰(i-Pr-H) = 95 ± 1 kcal/mole and signifies that cyclopentane and the cyclopentyl radical have the same strain energy. Arrhenius parameters are deduced for all six steps in the reaction mechanism. Surface reactions are shown to be unimportant. Cyclopentyl iodide is an unstable intermediate in the reaction and the rate constant for its bimolecular formation from HI + cyclopentene is found to be log k₆ = 8.40 ± 0.29 - (26.9 ± 0.8)/θ 1 mole^?1 sec^?1. Together with the equilibrium constant, this yields for the unimolecular elimination of HI from cyclopentyl iodide, the rate constant, log k₅ = 13.3 ± 0.3 – (42.8 ± 1.2)/θ sec^?1.     

The kinetics and thermochemistry of the gas phase reaction of cyclopentadiene and iodine. Relevance to the heat of formation of cyclopentadienyl iodide and cyclopentadienyl radical

The rate of the reaction of cyclopentadiene with iodine has been followed spectrophotometrically over the temperature range 171.7° to 276.5°C. The reaction first proceeds almost to the point of equilibrium with cyclopentadienyl iodide and HI, although the final products are fulvalene and HI. Equilibrium constants obtained are those predicted by bond additivity. A third-law value of δH⁰_{f 298} (c-C₅H₅I,g) = 49 kcal/mole is obtained. Rate studies of the reaction up to the iodide equilibrium, yield values for the rate constant . Uncertainty in the Arrhenius parameters, as well as doubts as to the applicability of the usual assumption that E₃ = 1 ± 1 kcal/mole, make difficult an evaluation of total cyclopentadienyl stabilization energy (TSE) from these data. However, the value is probably 15 < TSE < 20.     

Decomposition of methylamino and aminomethyl radicals. The heats of formation of methyleneimine (CH2NH) and hydrazyl (N2H3) radical

A kinetic analysis of the thermal decomposition of methylamino and aminomethyl radicals into methyleneimine, reactions (1) and (2): leads to ΔH(CH₂?NH) = 25.0 ± 3 kcal/mol in excellent agreement with ion cyclotron resonance spectroscopy measurements and to a pi bond energy of E_π = 55.0 kcal/mol in CH₂?NH which is comparable but smaller than to the corresponding value in CH₂?CH₂ (63.7 kcal/mol). Assuming that E_π(CH₂?NH) = 0.5 [E_π(CH₂?CH₂) + E_π(NH?NH)] then requires that E_π(NH?NH) = 46.8 kcal/mol in diimine and BDE(N₂H₃-H) = 87.5 kcal/mol i.e. about 11.5 kcal/mol larger than current data for hydrazine but otherwise consistent with additional evidence. The entropy and heat capacity of methyleneimine, calculated from recent infrared and microwave spectroscopic data using the rigid rotor harmonic oscillator approximation, are also reported.     

Water‐Soluble Methyl Orange Fibrils as Versatile Templates for the Fabrication of Conducting Polymer Microtubules

One‐dimensional methyl orange fibrils can be easily prepared. They are stable in acidic aqueous solutions and soluble in neutral water. When used to synthesize conducting polymer microtubules, the fibrils act as “hard templates” formally but as “soft templates” effectively. Microtubular structures of polypyrrole, polyaniline, and poly(3,4‐ethylenedioxythiophene) have been achieved successfully via such water‐soluble versatile templates.

     

Low‐Band Gap Polymeric Cyanine Dyes Absorbing in the NIR Region

Two new, fully conjugated polymeric cyanine dyes based on trimethine and heptamethine moieties have been synthesized. Both polymers were characterized by gel permeation chromatography, UV‐vis and IR spectroscopy, elementary analysis and cyclic voltammetry. The structure of one material could be confirmed with NMR spectroscopy. Upon head‐to‐tail coupling of the dye moieties distinct bathochromic shifts up to 159 nm were observed for the polymers which absorb solely in the near infrared (NIR) region with maxima up to 1 002 nm and very high molar absorption coefficients. This highly efficient absorption in the NIR spectral domain combined with the strong electron accepting properties makes these dyes interesting candidates for many optical applications; investigations on photovoltaic devices based on polymeric cyanine dye/C₆₀ heterojunctions identify one of these possibilities.

     

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.     

Darstellung und Eigenschaften von Ti-substituierten N-Heterocyclen

Preparation and Properties of Ti-substituted N-Heterocycles The compounds (x = 2 to 6) have been prepared by transamination of Ti(NMe₂)₄ with the heterocyclic amines and have been characterised by elemental analyses and ¹H NMR and IR spectroscopy. The dependence on both x and n of the thermal decomposition has been studied for the series and . The results can be interpreted in terms of the steric strain of the R₂N and substituents. Apart from the piperidido groups none of the ligands exhibit protective group properties comparable to the R₂N groups.     

Very low-pressure pyrolysis (VLPP) of alkyl cyanides. II. n-propyl cyanide and isobutyl cyanide. The heat of formation and stabilization energy of the cyanomethyl radical

The very low-pressure pyrolysis (VLPP) technique has been used to study the pyrolysis of n-propyl cyanide over the temperature range of 1090–1250°K. Decomposition proceeds via two pathways, C₂? C₃ bond fission and C₃? C₄ bond fission, with the former accounting for >90% of the overall decomposition. Application of unimolecular reaction rate theory shows that the experimental unimolecular rate constants for C₂? C₃ fission are consistent with the high-pressure Arrhenius parameters given by where θ=2.303RT kcal/mole. The activation energy leads to DH₂₉₈⁰[C₂H₅? CH₂CN]=76.9±1.7 kcal/mole and ΔH(?H₂CN, g)=58.5±2.2 kcal/mole. The stabilization energy of the cyanomethyl radical has been found to be 5.1±2.6 kcal/mole, which is the same as the value for the α-cyanoethyl radical. This result suggests that DH[CH₂(CN)? H] ～ 93 kcal/mole, which is considerably higher than previously reported. The value obtained for ΔH_?⁰(?H₂CN) should be usable for prediction of the activation energy for C₂? C₃ fission in primary alkyl cyanides, and this has been confirmed by a study of the VLPP of isobutyl cyanide over the temperature range of 1011–1123°K. The decomposition reactions parallel those for n-propyl cyanide, and the experimental data for C₂? C₃ fission are compatible with the Arrhenius expression A significant finding of this work is that HCN elimination from either compound is practically nonexistent under the experimental conditions. Decomposition of the radical, CH₃CHCH₂CN, generated by C₃? C₄ fission in isobutyl cyanide, yields vinyl cyanide and not the expected product, crotonitrile. This may be explained by a radical isomerization involving either a 1,2-CN shift or a 1,2-H shift.