On the mechanism of the reaction of organosilicon hydrides with dichlorocarbene generated from sodium trichloroacetate

The reaction of sodium trichloroacetate with various organosilicon hydrides in 1,2-dimethoxyethane was investigated and the products, α-triorganosilyldichloromethanes, were formed in yields of 20–50%. The relative rate constants of these hydrosilanes in such insertion reactions of dichlorocarbene were determined by means of competition reactions. The relative reactivities of a series of alkyl substituted hydrosilanes correlate well with the Taft σ* constants for the substituents on silicon, with a ?* value of ?1.07, and a series of aryl substituted hydrosilanes also shows good linear correlation of the log k_rel values with Taft σ* constants, giving ?* value of ?1.18. The hydrogen isotope effect in the reaction, k_H/k_D 1.26 ± 0.02. Based on the observed results, it was concluded that the insertion of CCl₂ into the SiH bond proceeds by a three-center concerted process in which charge separation in the transition state is not large, as suggested by Seyferth for the related PhHgCCl₂Br/XC₆H₄SiMe₂H reactions.     

Analysis of nitromethane thermal decomposition at low temperatures

The thermal decomposition of nitromethane (NM) over the temperature range from 580 to 700 K at pressures of 4 Torr to 40 atm was analyzed. On the basis of literature data, with the use of theoretical transitional curves of the modified Kassel integral, the rate constants k of NM decomposition at the upper pressure limit were determined. The values thus obtained are in good agreement with the results of extrapolation of the high-temperature (1000–1400 K) k _{1, ∞} values to lower temperatures. The reasons for which the NM decomposition rate constants differ by two orders of magnitude at low temperatures are considered. A general expression for the NM decomposition rate constant at the upper pressure limit over the 580–1400 K temperature range was determined: k _{1, ∞} = (1.8 ± 0.7) × 10¹⁶ exp((?58.5 ± 2)/R _T ) s^?1. These data disprove the hypothesis that a nitro-nitrite rearrangement takes place during the NM decomposition at low temperatures.     

Non-and near-resonant (vv) energy transfer between the isotopes of N2 and of CO in liquid Ar and in the gas phase at 85 k

Rate constants for vibrational energy transfer have been measured in the system N₂(v= 1) +CO(v=0) as a function of energy mismatch by using isotopic derivatives of N₂ and of CO. These rate constants have been determined both in the gas phase and in liquid argon solution at 85 K. For non-resonant processes we find k_L=k_G for the nearest to resonant system we find k_L <k_G. This result is considered to arise because long range forces important in near-resonant energy transfer operate to a lesser extent in the liquid phase.     

Electrochemical and quantum-chemical studies of chromium(III,II) fluoride complexes in alkali chloride melts

The standard rate constants (k _s) of charge transfer on a glass carbon electrode were determined for the Cr(III)/Cr(II) redox pair in the NaCl-KCl-K₃CrF₆, KCl-K₃CrF₆, and CsCl-K₃CrF₆ systems at 973–1173 K by cyclic voltammetry. The k _s constant was found to increase at elevated temperatures and the following nonmonotonic dependence of k _s on the nature of the outer-spheric cation was found: k _s (CsCl) > k _s (NaCl-KCl) > k _s (KCl). On the basis of quantum-chemical data for the M₃CrF₆ + 18MCl (M = Na, K) model systems, it was shown that the complex chromium particles with four or five outer-spheric sodium or potassium cations had maximum thermodynamic stability. Quantum-chemical calculations were performed to interpret the experimental data on the effect of the second coordination sphere of the complexes on the standard charge transfer rate constants.     

Thermal stability of amino acids

Thermal decomposition of L-α-amino acids RCH₂(NH₂)COOH where R = Me₂CH, Me₂CHCH₂, MeEtCH, and C₆H₅CH₂ was studied at temperatures below the melting points of their crystals. From the effective rate constants of the first order reactions energy parameters in the Arrhenius equation were calculated. Correlations between the reaction rate constants k _R and the inductive constants σ* of substituents R and also between the rate constants of the reactions and the dipole moments of amino acids was established. Value of ρ* parameter +8.8 in the Taft equation indicates the heterolytic mechanism of transformation of the amino acids. Chromato-mass spectrometric analysis of decomposition products shows that condensation, decarboxylation, and deamination of the amino acids take place.     

Rate constants for the reaction of OH radicals with 2-methyl-2-butene over the temperature range 297–425 K

Absolute rate constants, k₂, for the reaction of OH radicals with 2-methyl-2-butene have been determined over the temperature range 297–425 K using a flash photolysis-resonance fluorescence technique. The Arrhenius expression obtained was k₂ = 3.6 × 10^?11 exp [(450 ± 400)/RT] cm³ molecule^?1 s^?1.     

The question of a pressure effect in the reaction OH + HNO3. A laser flash-photolysis resonance-absorption study

Absolute rate constants, k, of the reaction OH + HNO₃ were determined using a pulsed laser photolysis-resonance absorption technique. The measured values, in cm³ mol^-1 s^-1 at ±3σ, 10^-10k(1–16 Torr HNO₃) = 7.57 ± 0.64, k(500 Torr N₂) = 7.20 ± 0.66 and k(600 Torr SF₆) = 8.37 ± 0.45, indicate that any pressure effect on k at 297 K is less than the experimental uncertainty of 10%.     

Direct measurement of proton dissociation in thhe excited state of protonated 1-aminopyrene with picosecond pulses

Direct measurement of proton dissociation in the excited singlet state of protonated I-aminopyrene in a mixture of H₂O (or D₂O) and acetonitrile (1:1) has been carried out by means of picosecond time-resolved spectroscopy. The proton dissociation retes k¹₁ and k^D+₁ at about 300 K were determined tobe 3.7 (±0.6) * 10⁹ s^-1, respectively. These rates are in very good agreement with those obtained by nanosecond time-resolved spectroscopy with fluorometry. The excited singlet state ;K^*₂ value of l-aminopyrene was also determined by dynamic analyses.     

Absolute rate constants for the reaction of O(3P) aroms with n-butane and NO(M  Ar) over the temperature range 298–439 K

Absolute rate constants for the reaction of O(³P) atoms with n-butane (k₂) and NO(M  Ar)(k₃) have been determined over the temperature range 298–439 K using a flash photolysis-NO₂ chemiluminescence technique. The Arrhenius expressions obtained were k₂ = 2.5 × 10^?11exp[-(4170 ± 300)/RT] cm³ molecule^?1 s^?1, k₃ = 1.46 × 10^?32 exp[940 ± 200)/ RT] cm⁶ molecule^?2 s^?1, with rate constants at room temperature of k₂ = (2.2 ± 0.4) × 10^?14 cm³ molecule^?1 s^?1 and k₃ = (7.04 ± 0.70)×10^?32 cm⁶ molecule^?2 s^?1. These rate constants are compared and discussed with literature values.     

Adiabatic explosion in the flash photolysis of CIFCl2H2 mixtures

Incubation period and explosive limits in the flash photolysis of a CIFCl₂H₂ mixture has been measured. The ratio of rate constants k₃₁/k₁₃ for the reversible reaction Cl+CIF α Cl₂ + F (k₁₃ is the rate constant for the forward reaction, k₃₁ for the back reaction) was measured: k₃₁/k₁₃ = 270 ? 65 (T = 295 K). Photothermal explosion theory has been generalized for the case of a chain process with the three active centers.     

Measurement of reaction rate of HCl(υ = 1) with oxygen atoms

Relative rate constants k₂/k₁ for the reaction of vibrationally excited HCl(υ = 1) with oxygen atoms HCl(1)+O → OH+Cl, (k₂) to the reaction HCl(0)+O → OH+Cl, (k₁) was measured in a flow system (1–2 torr) at room temperature. Vacuum UV resonance fluorescence was used for the detection of the product chlorine atoms, combined with infrared fluorescence to follow the HCl (υ = 1) decay. In this way vibrational relaxation by oxygen atoms could be excluded from the pure chemical reaction rate. The measured ratio k₂/k₁ is 300 ± 100.     

Temperature dependence of vibrational energy transfer in liquids: VV relaxation of CO(ν = 1) by O2 in liquid Ar from 86 to 145 K

The rate constant for the energy transfer process

has been measured for CO and O₂ dilute in liquid Ar along the coexistence curve between 86 and 145 K. The results are compared to gas phase measurements of this rate constant over the same temperature range. The ratio of the rate constants in the two phases is compared to that predicted by an application of the isolated binary collision (IBC) model for energy trasnfer, which scales the energy transfer rate constants by the rate constants for binary collisions in the two phases. The theoretical predictions of the IBC model are that the liquid state energy transfer rate constant, k^ℓ, should be 30–60% greater than the gas phase energy transfer rate constant, k^g. The measured value of k^ℓ is, within the experimental error, equal to that for k^g. Possible reasons for the discrepancy are discussed.     

Kinetics and equilibria of dialkylthallium(III) complexes of some thiacrown-ethers in acetonitrile

The rate constants k₁ and activation enthalpies H of complex formation of dialkylthallium(III) perchlorates with some thia-18-crown-6 ethers were determined in acetonitrile. The equilibrium constants K were also obtained. More symmertrical thiacrown-ethers give smaller k₁ and K values, which are mainly regulated by the entropy term rather than the enthalpy term. Diethylthallium(III) ion gives smaller rate constants than those of dimethylthallium(III) ion. The K values of these complexes decrease as oxygen of the 18-crown-6 was replaced by sulfur.     

Kinetics of the liquid-phase oxidation of 2-hydroxycyclohexanone

The kinetics of oxygen uptake in the azobisisobutyronitrile-initiated oxidation of 2-hydroxycy-clohexanone (RCH(OH)C(O)R) at 323 K has been investigated (chlorobenzene solvent, [RCH(OH)C(O)R] = 0.15-1.30 mol/L, initiation rate of w _i = 0.016–0.473 mol L^?1 s^?1). The effective oxidizability parameter k _{p, eff}(2k _t,eff)^?0.5 has been determined by solving the inverse kinetic problem using the entire data array obtained while varying [RCH(OH)C(O)R] and w _i. The rate constants of chain propagation and termination and the equilibrium constant of the addition of the hydroxyperoxyl radical to the ketoalcohol have been derived from the dependence of the oxidizability parameter on the substrate concentration. The rate constant of bimolecular chain initiation has been calculated to be k ₀ = 9.7 × 10^?7 L mol^?1 s^?1. The ratio of the rate constants of quadratic-law peroxyl radical recombination reactions without and with chain termination (k _eff′/k _t,eff′) increases with increasing [RCH(OH)C(O)R], passes through a maximum at a substrate concentration of 0.8 mol/L, and then falls off. It is demonstrated that this effect is due to the variation of the proportions of the hydroperoxyl and organic (1-hydroxy-2-oxocyclohexylperoxyl or 1,2-dihydroxy-1-cyclo-hexylperoxyl) radicals in the reaction medium.     

Absolute Rate constants for O + No + N2 → NO2 + N2 from 217–500 K

Flash photolysis of NO coupled with time resolved detection of O via resonance fluorescence has been used to obtain rate constants for the reaction O + NO + N₂ → NO₂ + N₂ at temperatures from 217 to 500 K. The measured rate constants obey the Arrhenius equation k = (15.5 ± 2.0) × 10^?33 exp(1160 ± 70)/1.987 T] cm⁶ molecule^?2 s^?1. An equally acceptable equation describing the temperature dependence of k is k = 3.80 × 10^?27/T^1.82 cm⁶ molecule^?2 s^?1. These results are discussed and compared with previous work.     

The reaction of fluorine atoms with formyl fluoride and the CFO self-reaction at 293 K

A fast-flow apparatus with mass spectrometric detection was used to study the system F + CHFO between 2 and 3.5 mbar total pressure. The rate constant of the primary reaction was evaluated directly to yield at 298 K k₍₁₎ = (8.8 ± 1.4) * 10^?13 cm³ * molecule^?1 * s^?1. Numerical modelling was used to determine the rate constant at 298 K of the subsequent reaction CFO + CFO → CF₂O + CO: k₍₂₎ = (4.9 ± 2.0) * 10^?11 cm³ * molecule^?1 * s^?1. The possible occurrences of secondary reactions, CFO + F + M → CF₂O + M, and CFO + F₂ → CF₂O + F, can be excluded under the present conditions. © 1993 John Wiley & Sons, Inc.     

Energy distribution among reaction products. XIV. F + CH4, F + CH3X(X  Cl,Br, I), F + CHnCl4−n(n = 1−3)

The infrared chemiluminescence technique has been used to obtain relative rate constants k(ν′) for HF(ν′) formed in the following reaction:

For reaction (1) the detailed rate constants [k(ν′ = 1) = 0.30;k(ν′ = 2) = 1.00; k(ν′ = 3) = 0.15; mean fraction of the available energy entering vibration <?^′_ν> = 0.56] confirmed, at much lower reagent pressures, results obtained by previous workers. In series I there was a slight increase in fraction of the energy entering vibration as the molecular reagent altered from CH₃Cl to CH₃Br to CH₃I, viz <?^′_ν> = 0.50 (1a), <?^′_ν> = 0.58 (1b), <?^′_ν> = 0.60 (1c). In series 2, by contrast, there was a marked decrease in fractional conversion of the available energy into vibration with increasing chlorination of the molecular reagent; <?^′_ν> = 0.50 (1a), <?^′_ν> = 0.23 (2a), <?^′_ν> = 0.13 (2b). The rate constants into ν′ = 0, k(ν′ = 0), were obtained by extrapolation of surprisal plots; the trends for both series were, however, also evident from k(ν′ > 0). No separate initial rotational distribution was observed for any of these reactions, indicating that the peak of the initial distribution is not far removed from a 300 K thermal distribution. The decrease in <?^′_ν> for the HF products along series 2 was tentatively ascribed to increasing internal excitation in the ejected radicals CH₂Cl, CHCl₂, CCl₃, due to increase in the number of secondary encounters between HF and the departing radical.     

Studies on the influence of vibrational excitation of CF2Cl* radicals on the rate of their reaction with HCl

A method is proposed for studying the influence of vibrational excitation of radicals on their reactivity in bimolecular reactions. Investigations of the reaction CF₂Cl + HCl → CF₂ HCl + Cl by this method show for the first time that this reaction is accelerated by vibrational excitation of CF₂Cl* radicals. Under the experimental conditions, it was found that k^*₁/k₁ ? 6.0.     

Temperature dependence and branching ratio of the C2H5OH+OH reaction

Rate constants of hydrogen abstraction from C₂H₅OH by hydroxyl radicals have been measured in the temperature range 300–1000 K by laser-induced fluorescence detection of OH. An Arrhenius expression k(T) = (4.4 ± 1.0) × 10⁻¹² × exp[(–274 ± 90) K/T] cm³/s was derived. Mass spectrometric investigation of the reaction products resulted in a yield of (75 ± 15)% for the CH₃CHOH product channel at 300 K.