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.     

Energy disposal in the three-centered elimination of df from 1,1,2-trifluoroethane-1-d1

The chemical activation data for three- and four-centered hydrogen fluoride elimination from CH₂FCDF₂ have been analyzed to assign the energy released to the olefin fragment in the three-centered process and to estimate the threshold energies for elimination channels. Based upon the cis–trans isomerization rates of CHF = CHF, 78% of the total available energy was released to the olefin fragment for the αα channel. The analysis suggests the existence of an appreciable barrier (～10 kcal/mole) for the reverse reaction, addition of the CH₂FCF carbene to DF. The threshold energies for αα, αβ, and βα elimination from 1,1,2-trifluoroethane-1-d₁ were assigned as 71, 68, and 68 kcal/mole, respectively. Analysis of the chemical activation data for 1,1,2,2,-tetrafluoroethane, without distinguishing between the three- and four-centered elimination channels, suggests a threshold energy of ～75 kcal/mole.     

Kinetics of the gas-phase reaction of acetone with iodine: Heat of formation and stabilization energy of the acetonyl radical

The kinetics of the gas-phase reaction CH₃COCH₃ + I₂ ? CH₃COCH₂I + HI have been measured spectrophotometrically in a static system over the temperature range 340–430°. The pressure of CH₃COCH₃ was varied from 15 to 330 torr and of I₂ from 4 to 48 torr, and the initial rate of the reaction was found to be consistent with \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_3 {\rm COCH}_3 + {\rm I}^{\rm .} \stackrel{1}{\rightarrow}{\rm CH}_{\rm 3} {\rm COCH} + {\rm HI} $\end{document} as the rate-determining step. An Arrhenius plot of the variation of k₁ with temperature showed considerable scatter of the points, depending on the conditioning of the reaction vessel. After allowance for surface catalysis, the best line drawn by inspection yielded the Arrhenius equation, log [k₁/(M^?1 sec^?1)] = (11.2 ± 0.8) – (27.7 θ 2.3)/θ, where θ = 2.303 R T in kcal/mole. This activation energy yields an acetone C? H bond strength of 98 kcal/mole and δH (CH₃CO?H₂) radical = ?5.7 ± 2.6 kcal/mole. As the acetone bond strength is the same as the primary C? H bond strength in isopropyl alcohol, there is no resonance stabilization of the acetonyl radical due to delocalization of the radical site. By contrast, the isoelectronic allyl resonance energy is 10 kcal/mole, and reasons for the difference are discussed in terms of the π-bond energies of acetone and propene.     

The dissociation of 3-chloro-1-butene and the resonance energy of the chloro-allyl radical

Methane is a primary product of pyrolysis of 3-chloro-l-butene at temperatures in the range 776–835°K, and from its rate of formation values have been obtained for the limiting high-pressure rate constant of the reaction These may be represented by the expression log [(k₁)_∞/sec^?1] = (16.7 ± 0.3) ? (71.5 ± 1.5)/θ, where θ = 2.303RT kcal/mole. Assuming a zero activation energy for the reverse reaction and that over the experimental temperature range the rates at which a methyl radical adds on to chlorobutene are comparable to those at which it abstracts hydrogen, the activation energy for the dissociation reaction leads to a value of 83.2 ± 1.9 ckal/mole for D(H? CHClCH:CH₂) at 298°K. Taking D(H? CHClCH₂CH ₃) = 95.2 ± 1.0 kcal/mole a value of 12.0 ± 2.1 kcal/mole is obtained for the resonance energy of the chloroallyl radical. This value in conjunction with resonance energies obtained in earlier work indicates that substitution of a hydrogen atom on the carbon atom adjacent to the double bond in the allyl radical leads to no significant variation in the allylic resonance energy.     

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.     

Broad line NMR studies of linear and branched polyethylene

Temperature dependence of the shape and linewidth of the broad-line NMR spectra of commercially available high-density polyethylene (PE/HD), low-density polyethylene (PE/LD with ～3 per cent CH₃), block-copolythene (PE/AA with ～3 per cent acrylic acid) and polyethylene single crystal (PE/SC) were investigated to obtain information on the effect of branching and structural changes on the glass-transition temperature (Tg) and activation energy of molecular motion (E). The relatively lower value of Tg ～- ?100° for PE/HD compared to Tg ～- ?85° and ?60° for PE/CH₃ and PE/AA, along with the estimated lower value of E ～- 2·1 kcal/mole for PE/HD compared to 2·6 and 5·1 kcal/mole for PE/CH₃ and PE/AA, respectively, were interpreted by a molecular reorientational process connected mainly with the sidegroups of the main polymeric chains.     

Ab initio studies of parts of the potential surface for the system C3H6 + HO2

The addition reactions between HO₂ and propene leading to the radical intermediates CH₃CHCH₂OOH and CH₃CHOHCH₂ have been studied by ab initio molecular orbital calculations using a 6-31G * basis set and including electron correlation through fourth-order Møller–Plesset calculations. The intermediates are predicted to have energies of about 5 kcal/mol below the total reactant energies, the complex resulting from the HO₂ attack on the central carbon of propene being slightly preferred. The activation energies for the addition to the terminal carbon and the central carbon are predicted to be 8.5 and 8.0 kcal/mol, respectively, at the highest level of calculation [MP 4(SDTQ )] with corrections for spin contamination. Spin contamination corrections are found to be very important in the calculation of these values. Referring to previous calculations at the same level for the addition of HO₂ to ethylene [12], we assume that the addition step is the rate-determining one in the reaction leading to HO and propene oxide. The observed activation energy for this reaction, 14.2 kcal/mol [2], is significantly higher than the predicted one for the addition step. The discrepancy found, 6.2 kcal/mol, is virtually the same as the one encountered in the ethylene case, 6.6 kcal/mol [12]. The barrier to intramolecular hydrogen migration leading to the intermediate radical CH₂CH₂CH₂OOH is found to be 42.6 kcal/mol at the highest level of calculation. Spin contaminiation corrections are not important for this energy.     

A kinetic study of the thermal elimination of hydrogen fluoride from 1,2-difluoroethane. Determination of the bond dissociation energies D(CH2F?CH2F) and D(CH2F?H).

The kinetics of the thermal elimination of HF from 1,2-difluoroethane have been studied in a static system over the temperature range 734–820°K. The reaction was shown to be first order and homogeneous, with a rate constant of where θ = 2.303RT in kcal/mole. The A-factor falls within the normal range for such reactions and is in line with transition state theory; the activation energy is similarly consistent with an estimate based on data for the analogous reactions of ethyl fluoride and other alkyl halides. The above activation energy has been compared with values of the critical energy calculated from data on the decomposition of chemically activated 1,2-difluoroethane by the RRKM theory and the bond dissociation energy, D(CH₂F? CH₂F) = 88 ± 2 kcal/mole, derived. It follows from thermochemistry that ΔH_f⁰(CH₂F) = -7.8 and D(CH₂F? H) = 101 ± 2 kcal/mole. Bond dissociation energies in fluoromethanes and fluoroethanes are discussed.     

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.     

Very low-pressure pyrolysis (VLPP) of 3,3-dimethylbut-1-yne (tert-butyl acetylene). The heat of formation and stabilization energy of the dimethylpropargyl radical

The unimolecular decomposition of 3,3-dimethylbut-1-yne has been investigated over the temperature range of 933°-1182°K using the technique of very low-pressure pyrolysis (VLPP). The primary process is C? C bond fission yielding the resonance stabilized dimethylpropargyl radical. Application of RRKM theory shows that the experimental unimolecular rate constants are consistent with the high-pressure Arrhenius parameters given by log (k/sec^?1) = (15.8 ± 0.3) - (70.8 ± 1.5)/θ where θ = 2.303RT kcal/mol. The activation energy leads to DH⁰[(CH₃)₂C(CCH)? CH₃] = 70.7 ± 1.5, θH⁰_f((CH₃)₂?CCH,g) = 61.5 ± 2.0, and DH⁰[(CH₃)₂C(CCH)? H] = 81.0 ± 2.3, all in kcal/mol at 298°K. The stabilization energy of the dimethylpropargyl radical has been found to be 11.0±2.5 kcal/mol.     

Pulsed NMR study of molecular motion in the uncured diglycidyl ether of bisphenol-A

The diglycidyl ether of bisphenol-A, an uncured epoxy resin, has been studied by pulsed NMR. Values of the proton relaxation times T₁, T_1p, and T₂ have been measured over the temperature range from ?160 to 200°C. The resin was studied in its monomeric form and in two mixtures containing higher oligomers. The relaxation times are interpreted in terms of the molecular motion in the resins. The motion responsible for relaxation in the solid monomer form is thought to be methyl group reorientation at low temperatures and general molecular motion at high temperatures. The motions are characterized by activation energies of 5 kcal/mole and 33 kcal/mole, respectively. The solid mixtures exhibit similar effects to the monomer, but an additional relaxation mechanism is observed which is attributed to segmental motion. This motion is characterized by an activation energy of 12–15 kcal/mole. The self-diffusion coefficient was measured in the liquid monomer, and the activation energy for self-diffusion is found to be 11 kcal/mole.     

Kinetic studies of the reactions CH2I2 + HI⇄CH3I + I2 and 2 CH3I⇄CH4 + CH2I2 the heat of formation of the iodomethyl radical

The rate of the reaction CH₂I₂ + HI ? CH₃I + I₂ has been followed spectrophotometrically from 201.0 to 311.2°. The rate constant for the reaction fits the equation, log (k₁/M^?1 sec^?1) = 11.45 ± 0.18 - (15.11 ± 0.44)/θ. This value, combined with the assumption that E₂ = 0 ± 1 kcal/mole, leads to ΔH (CH₂I, g) = 55.0 ± 1.6 kcal/mole and DH (H? CH₂I) = 103.8 ± 1.6 kcal/mole. The kinetics of the disproportionation, 2 CH₃I ? CH₄ + CH₂I₂ were studied at 331° and are compatible with the above values.     

Theoretical Study of the Structure and the Physico Chemical Properties of 1,2-Benzyne

The structure of 1,2-benzyne (I) has been optimized with respect to its total molecular energy using the MINDO/2 SCF-procedure. The results indicate a bond length of ～1.26 Å for the strained triple bond. The overall geometry suggests that I possesses considerable resonance energy. The calculated heat of formation (ΔHf(I) = 107 kcal/mole) is in good agreement with an estimate from mass spectrometric studies (ΔHf^exp(I) = 118 ± 5 kcal/mole). From model calculations for bent acetylene the strain energy of I is estimated to be about 60 kcl/mole. Some chemical reactions of I are discussed in the light of the results.     

A theoretical study of the photolytic decomposition of ethane ethylene via a triplet-state ethylidene intermediate

The complete geometrically optimized triplet state of ethane, using the MINDO/2 method, spontaneously dissociates into CH₃H; and H₂. The reaction paths for rearrangement of CH₃CH: to CH₂CH₂ in the triplet state is calculated. The activation energy was determined to be 19.4 kcal/mole. These results are discussed in the context of previously reported experimental results for the gas phase photolyses of alkanes.     

Ab initio calculations on small hydrides including electron correlation

The two lowest electronic states (³ B ₁ and ¹ A ₁) of the methylene radical (CH₂) are calculated both in SCF-approximation and with the IEPA-PNO method (including electron correlation). The influence of polarization functions and electronic correlation on the shape of the potential curves of the two states is discussed. The calculated equilibrium geometries agree very well with experiment, but the results for transition energies (e.g. ³ B ₁¹ A ₁ excitation energy=9.2 kcal/mole, total binding energy=187 kcal/mole) are more reliable than the existent experimental values.     

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.     

Isothermal and nonisothermal creep of lightly crosslinked cis-1,4 polybutadiene

A nonisothermal creep experiment has been analyzed to ascertain its suitability for determining the temperature dependence of low activation energy viscoelastic processes in elastomers far above T_g. The nonisothermal method was employed to determine the activation energy for creep near 35°C in a lightly crosslinked cis-1,4 polybutadiene elastomer at small strains within the linear viscoelastic region, and at various large deformations up to rupture. The observed activation energy was essentially independent of the level of strain, and the value of ΔH_a (～11 kcal/mole) determined via the nonisothermal creep method was in good agreement with the result (～12 kcal/mole) obtained via time-temperature superposition of isothermal linear viscoelastic creep data. The nonisothermal data allowed for an estimate of the volume of the “flow unit” associated with the controlling viscoelastic creep mechanism, attributed here to slippage of entanglements within the lightly crosslinked network.     

A theoretical study of C4H5N isomers: Comparison of cyano and isocyano as substituents

Ab initio molecular orbital calculations using a 3-21G basis set have been used to optimize geometries for pyrrole, CH₃(X)CCH₂, CH₃(H)CCHX (both cis and trans), c-C₃H₅X, and CH₂CHCH₂X, where X is CN and NC. In all the alkenyl derivatives methyl groups are found to adopt the conformation in which the methyl hydrogen eclipses the double bond. 6-31G*∥3-21G level calculations show the alkenyl cyanides to be of similar energy to pyrrole, but the isocyanides are ～20 kcal mol^?1 higher in energy. For both substituents the cyclopropyl derivatives are higher in energy by ～10 kcal mol^?1. At the 6-31G* level ring strain is 27.7 kcal mol^?1 for the cyanide and 30.6 kcal mol^?1 for the isocyanide. Data on the relative energies of RCN and RNC are compared when R is (i) a saturated hydrocarbon, (ii) an unsaturated hydrocarbon, (iii) an α-carbenium ion, (iv) an allyl cation, and (v) an α-carbanion.     

The MNDO study of the potential energy surface for 1,5-H-migration in the radicals ĊCl2(CH2)2X(Me)2Et (X = C,Si, Ge)

The optimal geometries of the radicals Cl₂(CH₂)₂X(Me)₂Et [R¹(X)] and CHCl₂(CH₂)₂XMe₂CHMe [R²(X)] (X = C, Si, Ge) and the transition state structures for 1,5-hydrogen migration in the radical R¹(X) (R¹(X) R²(X)) are determined by use of the MNDO method with unrestricted Hartree-Fock approximation. It is found that the activation energies of these reactions increase on going from C to Si by 1.40 kcal/mole and decrease on going from Si to Ge by 0.56 kcal/mole.Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No 1, pp. 98–100, January, 1993.     

Carbon–hydrogen bond strengths in methyl formate and the kinetics of the reaction with iodine

The gas phase reaction I₂ + HCOOCH₃ → HI + CH₃I + CO₂ has been studied spectrophotometrically in a static system over the pressure ranges I₂ (6–39 torr) and HCOOMe (28–360 torr). In the temperature range 293–356°, the initial rate of disappearance of I₂ is first order in [HCOOMe] and half-order in [I₂]. The rate determining step is where k₁ is given by where θ = 2.303 RT in kcal/mole. This activation energy gives a carbonyl C? H bond strength of 92.7 kcal/mole. At 356° there was no evidence of abstraction of a methoxy hydrogen, so a lower limit of 100 kcal/mole may be placed on this C? H bond strength. These ester C? H bond strengths are discussed in relation to comparable values in aldehydes and ethers.