Iodine catalyzed pyrolysis of dimethyl sulfide. Heats of formation of CH3SCH2I,the CH3SCH2 radical,and the pibond energy in CH2S

A kinetic study has been made of the gas phase, I₂-catalyzed decomposition of (CH₃)₂S at 630–650 K. Some I₂ is consumed initially, reaching a steady-state concentration. The initial major products are CH₄ and CH₂S together with small amounts of CH₃SCH₂I, CH₃I, HI, and CS₂. The initial reaction corresponds to a pseudo-equilibrium: accompanied by: and which brings (I₂) into steady state and a final complex reaction: From the initial rate of I₂ loss it is possible to obtain Arrhenius parameters for the iodination: We measure k₁, (644 K) = 150 L/mol s and from both the Arrhenius plot and independent estimates A₁ (644 K) = 10^{11.2 ± 0.3} L/mol s. Thus, E₁ = 26.7 ± 1 kcal/mol. From the steady-state I₂ concentration, an assumed mechanism and the known rate parameters for the CH₃I/HI system. It is possible to deduce K_A (644) = 3.8 × 10^?2 with an uncertainty of a factor of 2. Using an estimated ΔS (644) = 4.2 ± 1.0 e.u. we find ΔH_A (644) = 7.0 ± 1.1 kcal. With 〈ΔC_PA〉644 = 1.2 this becomes: ΔH_A(298) = 6.6 ± 1.1 kcal/mol. Then ΔH (CH₃SCH₂I) = 6.3 ± 1 kcal/mol. Making the assumption that E_?1 = 1.0 ± 0.5 kcal/mol we find ΔH (644) = 25.7 ± 0.7 kcal/mol and with 〈ΔC_PI〉 = 1.2; ΔH = 25.3 ± 0.8 kcal/mol. This gives ΔH (CH₃S?H₂) = 35.6 ± 1.0 kcal/mol and DH (CH₃SCH₂? H) = 96.6 ± 1.0 kcal/mol. This then yields E_π(CH₂S) = 52 ± 3 kcal. From the observed rate of pressure increase in the system and the preceding data k₃, is calculated for the step CH₃SCH₂ → CH₃ + CH₂S. From an estimated A-factor E₃ is deduced and from the overall thermochemistry values for k_?3 and E_?3. A detailed mechanism is proposed for the I-atom catalyzed conversion of CH₂S to CS₂ + CH₄.     

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.     

Thermochemistry and kinetics of the reaction of methyl mercaptan with iodine

The gas-phase reaction CH₃SH + I₂ has been studied spectrophotometrically over the temperature range of 476–604 K. It was found that the reaction undergoes H abstraction by I at ≤575 K, leading to the formation of MeSI and followed by a secondary reaction which leads to the formation of MeSSMe: Taking into consideration the effect of reaction (2), the equilibrium constant K₁ (554 K) has been evaluated to be 0.025 ± 0.004. This value was combined with the estimated values S (CH₃SI, g) = 73.7 ± 1.0 eu and 〈ΔC〉 = 0.87 ± 0.3 eu to obtain ΔH = 4.03 ± 0.73 kcal/mol. This yields ΔH (CH₃SI, g) = 7.16 ± 0.73 kcal/mol when combined with known thermochemical values for CH₃SH, HI, and I₂. A kinetic study was vitiated by the concurrent heterogeneous reaction of MeSH and I₂ at lower temperatures and the rather complicated chemistry occurring at elevated temperatures. However, attempts at measuring rate constants at 554 K lead to a lower limit of ΔH (CH₃S·, g) ≥ 29.5 ± 2 kcal/mol when an estimated value of A = 10^{10.8 ± 0.2} L/mol·s for the reactionc is used. DH (CH₃S–I) is estimated to be 49.3 ± 1.7 kcal/mol. The bond strengths of some divalent sulfurs and the reaction mechanisms are discussed. A crude estimate of DH⁰(H–CH₂SH) = 96 ± 1 kcal has been obtained from the kinetic data.     

Unparalleled,Enormous Metal–Carbon Bond Strength in PdCH2I+

Thermalized Pd⁺ cations activate methyl iodide by selective cleavage of a C? H bond under formation of PdCH₂I⁺ and an H-atom. This finding implies that the interaction energy between the metal cation and the CH₂I fragment and thus the metal–carbon bond strength exceeds 103 kcal/mol. Theory predicts that the energetically most favorable isomer of this ion exhibits the Pd⁺? CH₂? I structure, which is stabilized by an unprecedented bridging interaction between the two heavy atoms Pd and I.     

Analysis of the kinetics of the thermal and chemically activated elimination of HF from 1,1,1-trifluoroethane: the C?C bond dissociation energy and the heat of formation of 1,1,1-trifluoroethane

The thermal, unimolecular elimination of HF from CH₃CF₃ was studied by three different groups over the temperature range 1000° to 1800°K. While the reported kinetic parameters varied greatly, it is shown here that these data may be satisfactorily correlated in terms of a four-center transition state. This correlation results in ΔE = 69.2 kcal/mol, and log (k/s^?1) = 14.6 – 72.6/θ. These results may then be combined with the kinetics of the chemically activated elimination of HF from CH₃CF₃ formed by the recombination of methyl and trifluoromethyl radicals. The data from three different laboratories are shown to be in excellent agreement. These data, combined with extant thermal data, yield as a best value DH(CH₃? CF₃) = 99.6 ± 1.1 kcal/mol. This gives the unexpectedly high value of DH₂₉₈°(CH₃? CF₃) = 101.2 ± 1.1 kcal/mol. It is suggested that dipoledipole interactions, primarily in CH₃CF₃, account for this surprisingly strong C? C bond dissociation energy. These results also yield δH(CH₃CF₃; g, 298) = ?178.6 ± 1.5 kcal/mol.     

The gas-phase kinetics of the reaction of 1,1,1-trifluoroethyl iodine with HI: The C?I bond dissocation energy in 2,2,2-trifluoroethyl iodide

The kinetics of the gas-phase reaction of 2,2,2-trifluoroethyl iodide with hydrogen iodide has been studied over the temperature range of 525°K to 602°K and a tenfold variation in the ratio of CF₃CH₂I/HI. The experimental results are in good agreement with the expected free radical-mechanism: An analysis of the kinetic data yield: where θ =2.303RT in kcal/mol. If these results are combined with the assumption that E₂ = 0 ± 1 kcal/mol, then one obtains DH (CF₃CH₂? I) = 56.3 kcal/mol. This result may be compared with DH(CH₃CH₂? I) = 52.9 kcal/mol and suggests that substitution of three fluorines for hydrogen in the beta position strengthens the C? I bond slightly.     

Spectroscopic studies on the dynamics of charge‐transfer interaction of pantoprazole drug with DDQ and iodine

Spectrophotometric method was used to study the kinetics of charge‐transfer (CT) complexes of pantoprazole with 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) and iodine. The reactions of DDQ and iodine with pantoprazole have been investigated in different solvents at three different temperatures. The products of the interactions have been isolated and characterized using UV–vis, GC‐MS, FT‐IR, and far‐IR spectral techniques. The rate of formation of the product has been measured and discussed as a function of solvents and temperature. The iodine complex indicates the formation of the tri‐iodide CT complex with a general formula [(PTZ)I]⁺I. The characteristic strong absorptions of I are observed around 360 and 290 nm in the electronic spectra, and the far‐IR spectrum exhibits three characteristic vibrations of I unit at 156, 112, and 69 cm^?1 assigned to ν_as(I‐I), ν_s(I‐I), and δ(I), respectively. The activation parameters (ΔG^#, ΔS^#, and ΔH^#) were obtained from the temperature dependence of the rate constants. The influence of relative permittivity of the medium on the rate indicated that the intermediate is more polar than the reactants, and this observation was further well supported by spectral studies. Based on the spectrokinetic results, plausible mechanisms for the interaction of the drug with the chosen acceptors, which proceed via the formation of CT complexes and its transformation into final products, have been proposed. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 787–799, 2009     

Nonlinear dynamics in the oxidations of substituted thioureas I: The reaction of 4‐methyl‐3‐thiosemicarbazide with acidic iodate [1]

The substituted thiourea, 4‐methyl‐3‐thiosemicarbazide, was oxidized by iodate in acidic medium. In high acid concentrations and in stoichiometric excess of iodate, the reaction displays an induction period followed by the formation of aqueous iodine. In stoichiometric excess of methylthiosemicarbazide and high acid concentration, the reaction shows a transient formation of aqueous iodine. The stoichiometry of the reaction is: 4IO + 3CH₃NHC(S)NHNH₂ + 3H₂O → 4I⁻ + 3SO + 3CH₃NHC(O)NHNH₂ + 6H⁺ (A). Iodine formation is due to the Dushman reaction that produces iodine from iodide formed from the reduction of iodate: IO + 5I⁻ + 6H⁺ → 3I₂(aq) + 3H₂O (B). Transient iodine formation is due to the efficient acid catalysis of the Dushman reaction. The iodine produced in process B is consumed by the methylthiosemicarbazide substrate. The direct reaction of iodine and methylthiosemicarbazide was also studied. It has a stoichiometry of 4I₂(aq) + CH₃NHC(S)NHNH₂ + 5H₂O → 8I⁻ + SO + CH₃NHC(O)NHNH₂ + 10H⁺ (C). The reaction exhibits autoinhibition by iodide and acid. Inhibition by I⁻ is due to the formation of the triiodide species, I, and inhibition by acid is due to the protonation of the sulfur center that deactivates it to further electrophilic attack. In excess iodate conditions, the stoichiometry of the reaction is 8IO + 5CH₃NHC(S)NHNH₂ + H₂O → 4I₂ + 5SO + 5CH₃NHC(O)NHNH₂ + 2H⁺ (D) that is a linear combination of processes A and B. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 193–203, 2000     

Kinetics and equilibria of the reaction between bromine and ethyl ether. The C?H bond dissociation energy in ethyl ether

The kinetics and equilibria of the reaction: have been studied in the temperature range 298–333 K by using the very low pressure reactor (VLPR) technique. Combining the estimated entropy change of reaction (1), ΔS = 8.1 ± 1.0 eu, with the measured ΔG, we find ΔH = 4.2 ± 0.4 kcal/mol; ΔH(CH₃CHOC₂H₅) = ?20.2 kcal/mol, and DH° [Et OCH(Me)-H] = 91.7 ± 0.4 kcal/mol. We find: where θ = 2.3 RT in kcal/mol. It has been shown that the reaction proceeds via a loose transition state and the “contact TS” model calculation gives a very good agreement with the observed value.     

The structure and stability of the acetylene dication

The dication C₂H has been investigated by ab initio molecular orbital theory. It is found to have a linear (D_∞h), structure with a triplet (³σ^?_g) ground state. Deprotonation to C₂H⁺ is exothermic by 9.8 kcal/mol, but this process is hindered by a large barrier of 65 kcal/mol.     

Carbodications. I. The structures and energies of C4H isomers

At high levels of ab initio theory (6-31G*//4-31G), the most stable C₄H isomer is indicated to be the nonplanar cyclobutadiene dication ( 1a ); the planar form, 1b , is indicated to be 7.5 kcal/mol less stable. The second most stable C₄H isomer, the methylenecyclopropene dication, is indicated to prefer the perpendicular ( 2a ) over the planar ( 2b ) arrangement by 7 kcal/mol. The “anti van't Hoff” cyclo-(HB)₂C?CH₂ system ( 4 ), isoelectronic with 2 , also prefers the perpendicular conformation ( 4a ), and retains the C?C double bond. The linear butatriene dication ( 3 ) is the least stable C₄H species investigated. The perpendicular (D_2d) arrangement ( 3a ), permitting double allyl cationlike conjugation, is preferred over the planar D_2h form ( 3b ) by 26 kcal/mol. The heat of formation of the most stable form of C₄H, 1a , is estimated to be 623–640 kcal/mol. This species should be thermodynamically stable toward dissociation into smaller charged fragments.     

Very low-pressure pyrolysis (VLPP) of but-1-yne the heat of formation and stabilization energy of the propargyl radical

The unimolecular decomposition of but-1-yne has been investigated over the temperature range of 1052° – 1152°K using the technique of very low-pressure pyrolysis (VLPP). The primary process is C? C bond fission yielding methyl and propargyl radicals. Application of RRKM theory shows that the experimental rate constants are consistent with the highpressure Arrhenius parameters given by where θ = 2.303 RT kcal/mol. The parameters are in good agreement with estimates based on shock-tube studies. The activation energy, combined with thermochemical data, leads to DH°[HCCCH₂? CH₃] = 76.0, ΔH(HCC?CH_2,g) = 81.4, and DH° [HCCCH₂? H] = 89.2, all in kcal/mol at 300°K. The stabilization energy of the propargyl radical SE° (HCC?CH₂) has been found to be 8.8 kcal/mol. Recent result for the shock-tube pyrolysis of some alkynes have been analyzed and shown to yield values for the heat of formation and stabilization energy of the propargyl radical in excellent agreement with the present work. From a consideration of all results it is recommended that ΔH(HCC?CH_2,g) = 81.5±1.0, DH[HCCCH₂? H] = 89.3 ± 1.0, and SE° (HCC?CH₂) = 8.7±1.0 kcal/mol.     

The reactions of polyacetylene with trimethyloxonium hexachloroantimonate. Covalent doping of (CH)x

Polyacetylene, (CH)_x, has been doped with trimethyloxonium hexachloroantimonate, (CH₃)₃O⁺SbCl(1), in dichloromethane and acetonitrile. The maximally doped (CH)_x films have moderate conductivities [σ_RT(CH₂Cl₂) = 10, σ_RT(CH₃CN) = 0.7 Ω^?1 cm^?1]. Reactions between 1 and (CH)_x CH₂Cl₂ or CH₃CN were followed in situ by ¹H nuclear magnetic resonance spectroscopy and x-band electron spin resonance spectroscopy. It was found that the reactions in the two solvents are different. In dichloromethane the dopant is SbCl₅, which forms from the decomposition of 1, and doping proceeds by electron removal from (CH)_x chains. Based on the ESR signal loss, an estimate can be made of the diffusion rate of SbCl₅, into the (CH)_x fibrils in CH₂Cl₂; it is found to be ca. 10^?17 cm²/s. In acetonitrile the dopant appears to be either CH₃CNCH, H⁺, CH, or a combination of one or more of these dopants. It is postulated that the CH₃CNCH, CH, and/or H⁺ dopant covalently binds to the (CH)_x chain. X-ray photoelectron spectra show that films doped with excess 1 in both solvents have approximately one SbCl per 33 CH units.     

The proton affinity and the structure of protonated species of methylsilane

An ab initio LCAO-MO-SCF calculation was made on the proton affinity (PA ) of methylsilane (CH₃SiH₃) by using STO -3G, MIDI -1, and MIDI -1* basis sets. Three types of protonated methylsilane are taken into account, and their geometrical parameters are optimized. The calculated PA of CH₃SiH₃ is 160.5 kcal/mol, which exceeds that of SiH₄ by 11.5 kcal/mol. The protonated species (I) which refers to Si—C bond protonation is shown to be most favorable, and to be a weak σ-complex between CH₄ and SiH. Other two species are also σ-complexes between H₂ molecule and SiH₃CH or CH₃SiH, and similar to CH, SiH, GeH, and C₂H.     

Reaction mechanism of platinum dimer cation with ammonia based on the relativistic density functional study

The gas‐phase reactions between Pt and NH₃ have been investigated using the relativistic density functional approach (ZORA‐PW91/TZ2P). The quartet and doublet potential energy surfaces of Pt + NH₃ have been explored. The minimum energy reaction path proceeds through the following steps: Pt(⁴Σ_u) + NH₃ → q‐1 → d‐2 → d‐3 → d‐4 → d‐Pt₂NH⁺ + H₂. In the whole reaction pathway, the step of d‐2 → d‐3 is the rate‐determining step with a energy barrier of 36.1 kcal/mol, and exoergicity of the whole reaction is 12.0 kcal/mol. When Pt₂NH⁺ reacts with NH₃ again, there are two rival reaction paths in the doublet state. One is degradation of NH and another is loss of H₂. In the case of degradation of NH, the activation energy is only 3.4 kcal/mol, and the overall reaction is exothermic by 8.9 kcal/mol. Thus, this reaction is favored both thermodynamically and kinetically. However, in the case of loss of H₂, the rate‐determining step's energy barrier is 64.3 kcal/mol and the overall reaction is endothermic by 8.5 kcal/mol, so it is difficult to take place. Predicted relative energies and barriers along the suggested reaction paths are in reasonable agreement with experimental observations. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

The activation energy of the skeletal isomerization in the radical cations of toluene and cycloheptatriene by mass spectrometry of their 2-phenylethyl derivatives

The Unimolecular mass spectrometric fragmentations of the molecular ions of 1,3-diphenylpropane, 1-(7-cycloheptatrienyl)-2-phenylethane and the 1-phenyl-2-tolylethanes and their [d₅]phenyl analogues have been investigated by metastable ion techniques and measurements of ionization and appearance energies. By comparing the formation of [C₇H₇]⁺, [C₇H₈]⁺?, [C₈H₈]⁺? and [C₈H₉]⁺ it is shown that the molecular ions of the four diaryl isomers do not undergo ring expansion reactions of the aromatic nuclei prior to these fragmentations. Conversely, the molecular ions of the cycloheptatrienyl isomer suffer in part a contraction of the 7-membered ring. From these results and from the measured ionization and appearance energies lower limits to the activation energies of these skeletal isomerizations have been estimated yielding E > 33±5 kcal mol^?1 formonoalkylbenzene, E > 20 2±5 kc mol^?1 for 7-alkylcycloheptatriene and E > 40±5 kcal mol^?1 for dialkylvbenzene positive radical ions. Upper limits can be deduced from literature evidence yielding E < 45 kcal mol^?1 for monoalkylbenzene and E < 53 kcal 4mol^?1 for dialkylbenzene positive radical ions. The activation energy thus estimated for monoalkylbenzene is in excellent agreement with the recently calculated value(s) for the toluene ion.     

Very low-pressure pyrolysis (VLPP) of pentynes. III. Pent-2-yne. Heat of formation and resonance stabilization energy of the 3-methylpropargyl radical

The thermal unimolecular decomposition of pent-2-yne has been studied over the temperature range of 988–1234 K using the technique of very low-pressure pyrolysis (VLPP). The main reaction pathway is C₄? C₅ bond fission producing the resonance-stabilized 3-methylpropargyl radical. There is a concurrent process producing molecular hydrogen and penta-1,2,4-triene presumably via the intermediate formation of cis-penta-1,3-diene. The 1,4-hydrogen elimination from cis-penta-1,3-diene is the rate-determining step in the molecular pathway. This is supported by an independent VLPP study of cis- and trans-penta-1,3-diene. RRKM calculations show that the experimental rate constants for C? C bond fission are consistent with the following high-pressure rate expression at 1100 K: where θ = 2.303RT kcal/mol and the A factor was assigned from the results of shock-tube studies of related alkynes. The activation energy leads to ΔH[CH₃C?C?H₂] = 70.3 and DH[CH₃CCCH₂? H] = 87.4 kcal/mol. The resonance stabilization energy of the 3-methylpropargyl radical is 10.6 ± 2.5 kcal/mol, which is consistent with previous results for this and other propargylic radicals.     

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.     

Properties and reactivity in groups of the periodic system: Ion-molecule reactions CH3X + CH3X+· (X = F through At)

The reactions indicated in the title have been studied in terms of direct processes and complex formation. Quantum-chemical methods have been applied to the passage of an acid (H⁺, CH, X⁺) from CH₃X^+· to CH₃X, and the abstraction of a radical (H^· CH, X^·) from CH₃X by CH₃X^+·. It has been shown that a complex represented by a dimer of a methyl-halide radical cation, (CH₃X), with a two-center three-electron bond X? X, has fairly high stability. These investigations were based on non-empirical quantum-chemical calculations, the results being systematically compared with experimental determinations. Some calculations included all electrons (X=F, Cl, Br), others were based on relativistic pseudopotentials (X=F through At). The two sets of calculations agree qualitatively with each other and with experimental observations.     

Kinetics of the reaction between methane and iodine from 830 to 1150 K in the presence and absence of oxygen

The formation of methyl iodide was determined by radiochemical methods and by massspectroscopic analyses in mixtures of Ar–CH₄–I₂ and Ar? CH₄? I₂? O₂, heated by a reflected shock wave to temperatures of 830–1150 K. The rate of formation of CH₃I was consistent with the chain mechanism where the indicated rate constant for reaction between I and CH₄ is given by k₂(cm³/mol · s) = 10^14.17 exp(?32.9 ± 0.8 kcal/mol/RT). No effect on the reaction rate by the presence of O₂ was detected. However, in one experiment at 1097 K with 3.86 mol % O₂ the formation of CH₂O was indicated by the mass-spectroscopic analysis, presumably from the reaction of O₂ with CH₃.