Chain transfer in ethylene polymerization. V. The effect of temperature

In order to determine the effect of temperature on the chain-transfer reaction in the free-radical polymerization of ethylene, chain-transfer constants were measured for sixteen transfer agents at 130°C and 200°C at 1360 atm. The results were interpreted as ΔE*, the activation energy of the chain-transfer constant. This value is equal to the difference in activation energy between the transfer step (hydrogen abstraction) and the propagation step (addition to the monomer double bond): ΔE* = E_s* ? E_p*. Excellent agreement was found between measured ΔE* values determined at 1360 atm pressure and (E_s* ? E_p*) data for ethyl radical determined in vacuum gas-phase reactions. Apparently, the ethyl radical is a good model for polyethyl radical. The chain-transfer constant of ethylbenzene was found to be insensitive to temperature changes, indicating that E_p* = E_s* for this compound.     

Thioether glycidyl resins. VI. Reaction products of opening of oxirane ring of phenylthioetherglycidyl resin and of some derivatives under the influence of temperature

The process of oxirane ring opening of thioether glycidyl resins under various temperatures has been described. Reaction rate constants (k) and the activaton parameters (E_α, ΔH*, ΔS*) for epoxy group loss of 1,2-epoxy-3-(phenylthio)propane, 1,2-epoxy-3-(p-tolythio)propane, and 1,2-epoxy-3-(p-chloro-phenylthio)propane using classical kinetic methods were determined. The reaction products were separated and analyzed by means of chromatography and the structure of the compounds was determined by means of the spectral analyses: IR, ¹H-NMR, and ¹³C-NMR     

Steric Effects on Reaction Rates. VI. Application of a New MM2 Force-Field for Carbenium Ions to Solvolysis Rates of Secondary p-Toluenesulfonates

An empirical force-field for carbenium ions has been incorporated in Allinger's MM2 programme. Structural parameters of secondary carbenium ions calculated by this method are compared with those obtained with Schleyer's BIGSTRN calculations. The strain changes occurring upon solvolysis of secondary p-toluenesulfonates are evaluated by means of this force-field and correlated with the rate constants for solvolysis. The equation for correlation of acetolysis, relative to cyclohexyl p-toluenesulfonate, of 28 k_c substrates is ΔG = 0.67 ΔE_st - 0.20 (r = 0.958).     

Kinetics and mechanisms in carbocationic polymerization: The quest for true rate constants

This article is a critical analysis of kinetic dataavailable on carbocationic polymerizations. A survey of published propagation rate constant (k_p) data revealed several orders of magnitude differences. In this article, an explanation of this apparent discrepancy is offered with a case study involving the carbocationic polymerization of 2,4,6‐trimethylstyrene (TMS). With the polymerization mechanism originally proposed for this system, k_p = 1.35 × 10⁴ L mol^?1 s^?1 was extracted from experimental data with the Predici polyreaction package. The alternative mechanism yielded k_p = 1.01 × 10⁷ L mol^?1 s^?1, close to that predicted by Mayr's Linear Free Energy Relationship (LFER). We propose that true rate constants can only be obtained from direct competition experiments or from kinetic interpretation based on independently proven mechanisms. The second part of this review discusses critical analysis of the temperature and concentration dependence of various living IB systems. Comparison of the temperature dependence in systems initiated with 2‐ chloro‐2,4, 4‐ trimethylpentane (TMPCl)/TiCl₄ from various laboratories yielded of ΔH ～?25 and ?34.5 kJ/mol for high and low TMPCl/TiCl₄ ratios, respectively. Aromatic (cumyl‐type) initiators show ΔH ～ ?40 kJ/mol, whereas H₂O/TiCl₄ in the presence of the strong electron‐ pair donor dimethylacetamide gave ΔH = ?12 kJ/mol. The significant differences indicate different underlying mechanisms with complex elementary reactions. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5394–5413, 2005     

Densities,speeds of sound and refractive indices for binary and ternary mixtures of {diethylcarbonate (1) + p-chloroacetophenone (2) + 1-hexanol (3)} at 303.15 K for the liquid region and at ambient pressure

Densities (ρ), speeds of sound (u) and refractive indices (n_D ), of the ternary mixture (diethylcarbonate + p-chloroacetophenone + 1-hexanol) and the involved binary mixtures (diethylcarbonate + p-chloroacetophenone, diethylcarbonate + 1-hexanol, and p-chloroacetophenone + 1-hexanol) have been measured over the whole composition range at 303.15 K for the liquid region and at ambient pressure. The data obtained are used to calculate isentropic compressibilities k_s , isentropic compressibility deviations Δk_s and refractive index deviations Δn_D , of the binary and ternary mixtures. The data of isentropic compressibility deviations and refractive index deviations of the binary systems were fitted to the Redlich–Kister equation while the best correlation method for the ternary system was found using the Cibulka equation. The experimental data of the constitute binaries and ternaries are analysed to discuss the nature and strength of intermolecular interactions in these mixtures.     

Synthesis and Diels-Alder Reactivity of 2,3,5,6-Tetramethylidenenorbornane

Pd-catalyzed double carbomethoxylation of the Diels-Alder adduct of cyclo-pentadiene and maleic anhydride yielded the methyl norbornane-2,3-endo-5, 6-exo-tetracarboxylate ( 4 ) which was transformed in three steps into 2,3,5,6-tetramethyl-idenenorbornane ( 1 ). The cycloaddition of tetracyanoethylene (TCNE) to 1 giving the corresponding monoadduct 7 was 364 times faster (toluene, 25°) than the addition of TCNE to 7 yielding the bis-adduct 9 . Similar reactivity trends were observed for the additions of TCNE to the less reactive 2,3,5,6-tetramethylidene-7-oxanorbornane ( 2 ). The following second order rate constants (toluene, 25°) and activation parameters were obtained for: 1 + TCNE → 7 : k₁ = (255 + 5) 10^?4 mol^?1 · s^?1, ΔH≠ = (12.2 ± 0.5) kcal/mol, ΔS≠ = (?24.8 ± 1.6) eu.; 7 + TCNE → 9 , k₂ = (0.7 ± 0.02) 10^?4 mol^?1 · s^?1, ΔH≠ = (14.1 ± 1.0) kcal/mol, ΔS≠ = ( ?30 ± 3.5) eu.; 2 + TCNE → 8 : k₁ = (1.5 ± 0.03) 10^?4 mol^?1 · s^?1, ΔH≠ = (14.8 ± 0.7) kcal/mol, ΔS≠ = (?26.4 ± 2.3) eu.; 8 + TCNE → 10 ; k₂ = (0.004 ± 0.0002) 10^?4 mol^?1 · s^?1, ΔH≠ = (17 ± 1.5) kcal/mol, ΔS≠ = (?30 ± 4) eu. The possible origins of the relatively large rate ratios k₁/k₂ are discussed briefly.     

Prediction of ortho substituent effect in alkaline hydrolysis of phenyl esters of substituted benzoic acids in aqueous acetonitrile

The second-order rate constants k for the alkaline hydrolysis of phenyl esters of meta-, para- and ortho-substituted benzoic acids, X-C₆H₄CO₂C₆H₅, in aqueous 50.9% acetonitrile have been measured spectrophotometrically at 25°C. The log k values for meta and para derivatives correlated well with the Hammett σ_m,p substituent constants. The log k values for ortho-substituted phenyl benzoates showed good correlations with the Charton equation, containing the inductive, σ_I, resonance, σ^○ _R, and steric, E _s ^B, and Charton υ substituent constants. For ortho derivatives the predicted (log k _X)_calc values were calculated with equation (log k _ortho)_calc = (log k ^H _AN)_exp + 0.059 + 2.19σ_I + 0.304σ^○ _R + 2.79E _s ^B ? 0.0164ΔEσ_I — 0.0854ΔEσ^○ _R, where DE is the solvent electrophilicity, ΔE = E _AN — E _H20 = ?5.84 for aqueous 50.9% acetonitrile. The predicted (log k _X)_calc values for phenyl ortho-, meta- and para-substituted benzoates in aqueous 50.9% acetonitrile at 25°C precisely coincided with the experimental log k values determined in the present work. The substituent effects from the benzoyl moiety and aryl moiety were compared by correlating the log k values for the alkaline hydrolysis of phenyl esters of substituted benzoic acids, X-C₆H₄CO₂C₆H₅, in various media with the corresponding log k values for substituted phenyl benzoates, C₆H₅CO₂C₆H₄-X.     

A reliable and efficient first principles‐based method for predicting pKa values. 4. organic bases

The ionization (dissociation) constant (pK_a) is one of the most important properties of a drug molecule. It is reported that almost 68% of ionized drugs are weak bases. To be able to predict accurately the pK_a value(s) for a drug candidate is very important, especially in the early stages of drug discovery, as calculations are much cheaper than determining pK_a values experimentally. In this study, we derive two linear fitting equations (pK_a = a × ΔE + b; where a and b are constants and ΔE is the energy difference between the cationic and neutral forms, i.e., ΔE = E_neutral?E_cationic) for predicting pK_as for organic bases in aqueous solution based on a training/test set of almost 500 compounds using our previously developed protocol (OLYP/6‐311+G**//3‐21G(d) with the the conductor‐like screening model solvation model, water as solvent; see Zhang, Baker, Pulay, J. Phys. Chem. A 2010 , 114, 432). One equation is for saturated bases such as aliphatic and cyclic amines, anilines, guanidines, imines, and amidines; the other is for unsaturated bases such as heterocyclic aromatic bases and their derivatives. The mean absolute deviations for saturated and unsaturated bases were 0.45 and 0.52 pK_a units, respectively. Over 60% and 86% of the computed pK_a values lie within ±0.5 and ±1.0 pK_a units, respectively, of the corresponding experimental values. The results further demonstrate that our protocol is reliable and can accurately predict pK_a values for organic bases. © 2012 Wiley Periodicals, Inc.     

邻-甲氧基苯酚和α-,β-环糊精包合现象的理论与实验研究

通过紫外可见光谱法考察了水溶液中邻-甲氧基苯酚(o-Mop)和α-与β-环糊精(CD)的分子间相互作用, 利用Hildebrand-Benesi方程给出了两个包合物的稳定常数(K_s). 采用半经验PM3方法研究了α-,β-CD和o-Mop及其类似物苯酚(Phe)、丁香酚(Eug)之间的包络作用, 阐述了这些主客体包合作用过程中体系能量随主客体相对位置改变而变化的细节, 据此推断出主-客体包合物可能的分子结构, 计算了包合物的稳定化能(ΔE_s). 研究结果表明, 本文所选主客体体系而言, 当客体和同一种主体分子作用时, 超分子包合物的ΔE_s随着客体分子苯环上取代基团数目的增多而增加. 基于PM3方法优化得到的主-客体包合物在真空中的分子结构和通过实验方法在水溶液中测定的结构一致.     

The gas-phase decomposition of methylsilane. Part III. Kinetics

The homogeneous gas-phase decomposition kinetics of methylsilane and methylsilane-d₃ have been investigated by the comparative-rate-single-pulse shock-tube technique at total pressures of 4700 torr in the 1125–1250 K temperature range. Three primary processes occur: CH₃SiH₃ → CH₃SiH + H₂ (1), CH₃SiH₃ → CH₄ + SiH₂ (2), and CH₃SiH₃ → CH₂ = SiH₂ + H₂ (3). The high-pressure rate constants for the primary processes in CH₃SiH₃ obtained by RRKM calculations are log (k₁ + k₃) (s^?1) = 15.2 - 64,780 Cal/θ and log k₂ (s^?) = 14.50 - 67,600 → 2800 Cal/θ. For CH₃SiD₃ these same rate constants are log k₁ (s^?) = 14.99 - 64,700 cal/θ log k₂ (s^?) = 14.68 – 66,700 → 2000 cal/θ, and log k₃ (s^?) = 14.3 ? 64,700 cal/θ.     

Preparation and Diels-Alder Reactivity of 2,3,5,6,7,8-Hexamethylidenebicyclo [2.2.2]octane (‘[2.2.2]Hericene’). Force-field calculations of exocyclic dienes as a moiety of bicyclic skeletons

The [2.2.2]hericene ( 6 ), a bicyclo[2.2.2]octane bearing three exocyclic s-cis-butadiene units has been prepared in eight steps from coumalic acid and maleic anhydride. The hexaene 6 adds successively three mol-equiv. of strong dienophiles such as ethylenetetracarbonitrile (TCE) and dimethyl acetylenedicarboxylate (DMAD) giving the corresponding monoadducts 17 and 20 (k₁), bis-adducts 18 and 21 (k₂) and tris-adducts 19 and 22 (k₃), respectively. The rate constant ratio k₁/k₂ is small as in the case of the cycloadditions of 2,3,5,6-tetramethylidene-bicyclo [2.2.2]octane ( 3 ) giving the corresponding monoadducts 23 and 27 (k₁) and bis-adducts 25 and 29 (k₂) with TCE and DMAD, respectively. Constrastingly, the rate constant ratio k₂/k₃ is relatively large as the rate constant ratio k₁/k₂ of the Diels-Alder additions for 5,6,7,8-tetramethylidenebicyclo [2.2.2]oct-2-ene ( 4 ) giving the corresponding monoadducts 24 and 28 (k₁) and bis-adducts 26 and 30 (k₂). The following second-order rate constants (toluene, 25°) and activation parameters were obtained for the TCE additions: 3 +TCE→ 23 : k₁ = 0.591±0.012 mol^?1·l·s^?1, ΔH^≠=10.6±0.4 kcal/mol, and ΔS^≠ = ?24.0±1.4 cal/mol·K (e.u.); 23 +TCE→ 25 : k₂=0.034±0.0010 mol^?1·l·s^?1, ΔH^≠ = 10.6±0.6 kcal/mol, and ΔS^≠ = ?29.7±2.0 e.u.; 4 +TCE→ 26 : k₁ = 0.172±0.035 mol^?1·l·s^?1, ΔH^≠ 11.3±0.8 kcal/mol, and ΔS^≠ = ?24.0±2.8 e.u.; 24 +TCE→ 26 : k₂ = (6.1±0.2)·10^?4 mol^?1·l·s^?1, ΔH^≠ = 13.0±0.3 kcal/mol, and ΔS^≠ = ?29.5±0.8 e.u.; 6 +TCE→ 17 : k₁ = 0.136±0.002 mol^?1·l·s^?1, ΔH^≠ = 11.3±0.2 kcal/mol, and ΔS^≠ = ?24.5±0.8 e.u.; 17 +TCE→ 18 : k₂ = 0.0156±0.0003 mol^?1·l·s^?1, ΔH^≠ = 10.9±0.5 kcal/mol, and ΔS^≠ = ?30.1 ± 1.5 e.u.; 18 +TCE→ 19 : k₃=(5±0.2) · 10^?5 mol^?1 mol^?1 ·l·s^?1, ΔH^≠ = 15±3 kcal/mol, and ΔS^≠ = ?28 ± 8 e.u. The following rate constants were evaluated for the DMAD additions (CD₂Cl₂, 30°): 6 +DMAD→ 20 : k₁ = (10±1)·10^?4 mol^?1 · l·s^?1; 20 +DMAD→ 21 : k₂ = (6.5±0.1) · 10^?4 mol^?1 ·l·^?1; 21 +DMAD→ 22 : k₃ = (1.0±0.1) · 10^?4 mol^?1 ·l·s^?1. The reactions giving the barrelene derivatives 19, 22, 26 and 30 are slower than those leading to adducts that are not barrelenes. The former are estimated less exothermic than the latter. It is proposed that the Diels-Alder reactivity of exocyclic s-cis-butadienes grafted onto bicycle [2.2.1]heptanes and bicyclo [2.2.2]octanes that are modified by remote substitution of the bicyclic skeletons can be affected by changes inthe exothermicity of the cycloadditions, in agreement with the Dimroth and Bell-Evans-Polanyi principle. Force-field calculations (MMPI 1) of 3, 4, 6 and related exocyclic s-cis-butadienes as a moiety of bicyclo [2.2.2]octane suggested single minimum energy hypersurfaces for these systems (eclipsed conformations, planar dienes). Their flexibility decreases with the degree of unsaturation of the bicyclic skeleton. The effect of an endocyclic double bond is larger than that of an exocyclic diene moiety.     

A qualitative scale for the electron withdrawing effect of substituted phenyl groups and heterocycles

The electron withdrawing effect of a variety of differently substituted phenyl groups can be classified on the basis of the CO or CN distance of the corresponding phenolate or aryl amide ions (Ar-O⁻ and Ar-NH⁻), respectively, which are reliably accessible by DFT calculations on B3LYP/6-311 + G(2d,p) level of theory. An increasing electron withdrawing effect of the aromatic group leads to a shortened CO and CN distance of the corresponding ions. Within the presented model it is also possible to characterize the electronic nature of different kinds of six membered heterocycles.The defined constants Δ(E)_m,p - the difference of the E-C distances of the substituted derivatives X-C₆H₄-E and the non-substituted phenyl derivative, C₆H₅-E - exhibit the same tendency as the corresponding Hammett constants. The values of Δ(E)_m,p strongly depend on the nature of E. With E = F, the resulting values Δ(F)_m,p are found to be accidentally close to the corresponding Hammett constants. Therefore, the values of Δ(E)_m,p, which can be easily determined by DFT calculations, are useful tools to classify the electronic nature of different kinds of substituents.The different electronic nature of the groups E, for example O⁻, NH⁻ or F, gives rise to varying electronic interactions with the connected aromatic ring system. These interactions influence on their part the special interaction of a given substituent X. This is the reason why the constants Δ(E)_m,p exhibits different values, which vary depending on the different electronic nature of the groups E. As a consequence, the values Δ(E)_m,p open the possibility to classify the electronic nature of substituents X in relation to any neutral or even charged group E.     

Pyrolysis of 2-phenylethylamines heats of formation of aminomethyl radicals R2NCH2 · (R = H,CH3)

A kinetic study of the very low-pressure pyrolysis of ethylbenzene (I), 2-phenylethylamine (II), and N,N-dimethyl 2-phenylethylamine (III) above 900 K yields the heats of formation of aminomethyl (A) and N,N-dimethylaminomethyl (B) radicals: ΔH_{?, 300 K}(A) = 30.3 and ΔH_{?, 300 K}(B) = 27.5 kcal/mol. The difference of stabilization energies E_s, (relative to methyl radicals): Δ = E_s(B) ? E_s(A) = (2 ± 1) kcal/mol, conforms to similar effects in methyl substituted alkyl and amino free radicals.     

1，1－二氨基2，2－二硝基乙烯（DADE）的热化学性质、热行为和热分解机理

The constant-volume combustion energy, △cU （DADE, s, 298.15 K）, the thermal behavior, and kinetics and mechanism of the exothermic decomposition reaction of 1,1-diamino-2,2-dinitroethylene （DADE） have been investigated by a precise rotating bomb calorimeter, TG-DTG, DSC, rapid-scan fourier transform infrared （RSFT-IR） spectroscopy and T-jump/FTIR, respectively. The value of △cHm （DADE, s, 298.15 K） was determined as （-8518.09±4.59） j·g^-1. Its standard enthalpy of combustion, △cU （DADE, s, 298.15 K）, and standard enthalpy of formation, △fHm （DADE, s, 298.15 K） were calculated to be （-1254.00±0.68） and （- 103.98±0.73） kJ·mol^-1, respectively The kinetic parameters （the apparent activation energy Ea and pre-exponential factor A） of the first exothermic decomposition reaction in a temperature-programmed mode obtained by Kissinger＇s method and Ozawa＇s method, were Ek=344.35 kJ·mol^-1, AR= 1034.50 S^-1 and Eo=335.32 kJ·mol^-1, respectively. The critical temperatures of thermal explosion of DADE were 206.98 and 207.08 ℃ by different methods. Information was obtained on its thermolysis detected by RSFT-IR and T-jump/FTIR.     

On the correlation between photoelectron and electronic spectra in trans- azomethane

A possibility of correlating electronic and photoelectron spectra is discussed, using trans-azomethane as an example. The Coulomb and exchange integrals required were obtained by three semi-empirical SCF-methods: MINDO/2, CNDO/2, and a modified CNDO method. The orbital energies were taken as minus the corresponding experimental ionization potentials. The sequence of the transition energies ΔE (n_s → π*) Δ E (n_a → π*) < ΔE (π → π*) is found to be different from the ionization potential sequence IP (n_s) < IP (π) < IP (n_a), in agreement with previous spectroscopic studies; the results support the latest view that the π → π* transition of the azo group occurs at around 12 eV.     

A generalized formula for the energies of alternant molecular orbitals. II. Heteronuclear molecules

Using the method of alternant molecular orbitals (AMO ), it is shown that the energies of AMOS (E_kσ) for an arbitrary heteronuclear alternant system, having a singlet ground state, are connected with the energies of MOS (e_{k(k )}) obtained by means of the conventional Hartree–Fock (HF ) method (SCF -LCAO -MO -PPP ) via the formula: In the general case, the determination of the correlation corrections δ_i,kσ is connected with the solving of a complicated system of integral equations, which is considerably simplified if the Hubbard approximation is accepted for the electron interaction. The energy spectrum of a chain with two atoms in the elementary cell (AB)_n is considered as an example. It is shown that if nontrivial solutions exist (δ_i,kσ ≠ 0), the correlation correction for AMOS of different spin are different (δ_i,kσ ≠ δ_i,kβ), from which it follows, that the width of the energy gap ΔE_∞ for AMOS with different spin is different: ΔE_∞,α ≠ ΔE_∞,β.     

Thermodynamic and transport properties of binary liquid mixtures of cyclohexanol with isomeric chlorotoluenes

Densities (ρ) at different temperatures from 303.15 to 318.15 K, speeds of sound (u) and viscosities (η) at 303.15 K were measured for the binary mixtures of cyclohexanol with 2-chlorotoluene, 3-chlorotoluene and 4-chlorotoluene over the entire range of composition. The excess volumes (V^E) for the mixtures have been computed from the experimental density data. Further, the deviation in isentropic compressibilities (Δκ_s) and deviation in viscosities (Δη) for the binary mixtures have been calculated from the speed of sound and viscosity data, respectively. The V^E values and Δκ_s values were positive and Δη data were negative for all the mixtures over the whole range of composition at the measured temperatures. The calculated excess functions V^E, Δκ_s and Δη were fitted to Redlich–Kister equation. The excess functions have been discussed in terms of molecular interactions between component molecules of the binary mixtures.     

1‐[(E)‐2‐Arylethenyl]‐2,2‐diphenylcyclopropanes: Kinetics and Mechanism of Rearrangement to Cyclopentenes

Kinetic measurements for the thermal rearrangement of 2,2‐diphenyl‐1‐[(E)‐styryl]cyclopropane ( 22a ) to 3,4,4‐triphenylcyclopent‐1‐ene ( 23a ) in decalin furnished ΔH =31.0±1.2 kcal mol^?1 and ΔS =?6.0±2.6 e.u. The lowering of ΔH^≠ by 20 kcal mol^?1, compared with the rearrangement of the vinylcyclopropane parent, is ascribed to the stabilization of a transition structure (TS) with allylic diradical character. The racemization of (+)‐(S)‐ 22a proceeds with ΔH =28.2±0.8 kcal mol^?1 and ΔS =?5±2 e.u., and is at 150° 106 times faster than the rearrangement. Seven further 1‐(2‐arylethenyl)‐2,2‐diphenylcyclopropanes 22 , (E)‐ and (Z)‐isomers, were synthesized and characterized. The (E)‐compounds showed only modest substituent influence in their k_rac (at 119.4°) and k_isom (at 159.3°) values. The lack of solvent dependence of rate opposes charge separation in the TS, but a linear relation of log k_rac with log p.r.f., i.e., partial rate factors of radical phenylations of ArH, agrees with a diradical TS. The ring‐opening of the preponderant s‐trans‐conformation of 22 gives rise to the 1‐exo‐phenylallyl radical 26 that bears the diphenylethyl radical in 3‐exo‐position, and is responsible for racemization. The 1‐exo‐3‐endo‐substituted allylic diradical 27 arises from the minor s‐gauche‐conformation of 22 and is capable of closing the three‐ or the five‐membered ring, 22 or 23 , respectively. The discussion centers on the question whether the allylic diradical is an intermediate or merely a TS. Quantum‐chemical calculations by Houk et al. (1997) for the parent vinylcyclopropane reveal the lack of an intermediate. Can the conjugation of the allylic diradical with three Ph groups carve the well of an intermediate?     

A Frustrated Phosphane–Borane Lewis Pair and Hydrogen: A Kinetics Study

The energy profile of a frustrated Lewis pair (FLP) dihydrogen splitting system was determined by a combined experimental kinetic and DFT study. A trimethylene‐bridged phosphane–borane FLP was converted into its endothermic H₂‐cleavage product by sequential H⁺/H^? addition. The system could be handled at low temperature, and the kinetics of the H₂ elimination were determined to give a rate constant of k_HH,exp(299 K)=(2.87±0.1)×10^?4 s^?1 in solution. The primary kinetic isotope effects were determined; for example, (k_HH/k_DD)_exp=3.19. The system was accurately analyzed by DFT calculations.     

High‐temperature dissociation of ethyl radicals and ethyl iodide

The decomposition of ethyl iodide and subsequent dissociation of ethyl radicals have been investigated behind incident shock waves in a diaphragmless shock tube by laser‐schlieren (LS) densitometry (1150–1870 K, 55 ± 2 Torr and 123 ± 3 Torr). The LS density‐gradient profiles were simulated assuming that the initial dissociation of C₂H₅I proceeded by 87% C–I fission and 13% HI elimination. Excellent agreement was found between the simulations and experimental profiles. Rate coefficients for the C–I scission reaction were obtained and show strong falloff. Gorin model RRKM (Rice, Ramsperger, Kassel, and Marcus) calculations are in excellent agreement with the experimental data with E₀ = 55.0 kcal/mol, which is in very good agreement with recent thermochemical measurements and evaluations. However, E₀ is approximately 2.7 kcal/mol higher than previous estimates. First‐order rate coefficients for dissociation of C₂H₅I were determined to be k_55Torr = 8.65 × 10⁶⁸ T^?16.65 exp(?37,890/T) s^?1, k_123Torr = 3.01 × 10⁶⁹ T^?16.68 exp(?38,430/T) s^?1, k_∞ = 2.52 × 10¹⁹ T^?1.01 exp(?28,775/T) s^?1. Rates of dissociation for ethyl radicals were also obtained, and these are in very good agreement with theoretical predictions (Miller J. A. and Klippenstein S. J. Phys Chem Chem Phys 2004, 6, 1192–1202). The simulations show that at low temperatures ethyl radicals are consumed through recombination reactions as well as dissociation, whereas at high temperatures, dissociation dominates. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 433–443, 2012