Picosecond reversible reaction kinetics of excited p-dimethylaminoacetophenone and m-methyl-p-cyanodimethylaniline studied by oxygen quenching technique

Double fluorescence of p-dimethylacetophenone (DMAPh) in CH₃CN and m-methyl-p-cyanodimethylaniline (MCDMA) in CH₂Cl₂ has been observed and analyzed in terms of reversible excited state isomerisation of the primary excited form b^* to the strongly polar rotamer a^*. Using the oxygen quenching technique, the kinetics of the reactions have been solved and all rate constants separated. The “formal” lifetimes of the species b^*, τ_b ≡ (k_bf + k_bd + k_ba)^?1, are 1 ps and 2.2 ps for DMAPh and MCDMA, respectively. The first value fits well to the reorientation relaxation time of acetonitrile.     

Kinetic features of the adsorption of sodium oleate on talc

Kinetic features of the adsorption of sodium oleate on dispersed talc samples of Onotskoe deposit are studied. It is shown that adsorption proceeds in two steps with different rate constants of adsorption k _a and desorption k _d. It is found that at the first step, lasting for nearly one hour under experimental conditions, the process is characterized by high k _a and k _d values (1.23 × 10^?2 m²/(mol min) and 1.06 × 10^?5 m²/(L min), respectively). It is concluded that the final stage associated with the rearrangement of molecules on a talc surface lasts for a long time with k _a = 2.12 × 10^?3 m²/(mol min) and k _d = 1.82 × 10^?6 m²/(L min).     

A diode laser study of the Cl + CH3CO reaction

Real-time kinetic measurements are reported for the Cl + CH₃CO → CH₂CO + HCl reaction. The experiments utilize infrared spectroscopy to determine the time dependence of the ketene formed via this reaction and of the CO produced from the subsequent rapid reaction between chlorine atoms and ketene. The reaction is investigated over a pressure range of 10–200 torr and a temperature range of 215–353 K. Within experimental error the rate constant under these conditions is k_5a = (1.8 ± 0.5) × 10⁻¹⁰ cm³ s⁻¹. We have also examined the Cl + CH₂CO reaction and found it to have a rate constant of k₆ = (2.5 ± 0.5) × 10⁻¹⁰ cm³ s⁻¹ independent of temperature. © John Wiley & Sons, Inc. Int J Chem Kinet 29: 421–429, 1997.     

Validation of a thermal decomposition mechanism of formaldehyde by detection of CH2O and HCO behind shock waves

The thermal decomposition of formaldehyde was investigated behind shock waves at temperatures between 1675 and 2080 K. Quantitative concentration time profiles of formaldehyde and formyl radicals were measured by means of sensitive 174 nm VUV absorption (CH₂O) and 614 nm FM spectroscopy (HCO), respectively. The rate constant of the radical forming channel (1a), CH₂O + M → HCO + H + M, of the unimolecular decomposition of formaldehyde in argon was measured at temperatures from 1675 to 2080 K at an average total pressure of 1.2 bar, k_1a = 5.0 × 10¹⁵ exp(‐308 kJ mol^?1/RT) cm³ mol^?1 s^?1. The pressure dependence, the rate of the competing molecular channel (1b), CH₂O + M → H₂ + CO + M, and the branching fraction β = k_1a/(kA_1a + k_1b) was characterized by a two‐channel RRKM/master equation analysis. With channel (1b) being the main channel at low pressures, the branching fraction was found to switch from channel (1b) to channel (1a) at moderate pressures of 1–50 bar. Taking advantage of the results of two preceding publications, a decomposition mechanism with six reactions is recommended, which was validated by measured formyl radical profiles and numerous literature experimental observations. The mechanism is capable of a reliable prediction of almost all formaldehyde pyrolysis literature data, including CH₂O, CO, and H atom measurements at temperatures of 1200–3200 K, with mixtures of 7 ppm to 5% formaldehyde, and pressures up to 15 bar. Some evidence was found for a self‐reaction of two CH₂O molecules. At high initial CH₂O mole fractions the reverse of reaction (6), CH₂OH + HCO ? CH₂O + CH₂O becomes noticeable. The rate of the forward reaction was roughly measured to be k₆ = 1.5 × 10¹³ cm³ mol^?1 s^?1. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 157–169 2004     

Kinetics of the peroxy RFO2 radical formation in the RHF-O2-CO2 gaseous mixtures

The kinetic data on the molecular oxygen activity of CH₃CH^·, CH₃CF₂ ^· and CF₃CHF^· radicals are reported. In laboratory, these radicals were generated by pulsed (12 ns) electron beam interaction with the gaseous RHF-O₂-CO₂ mixtures containing large excess of carbon dioxide (RHF = CH₃CH₂F, CH₃CHF₂ or CH₂FCF₃). The transient product (O₃ or RFO₂ ^·) formation was monitored by the UV absorptions at 250 nm and the rate constants of Reactions (4) and (9) were obtained. The values of k ₉ diminished with increasing number of fluorine atoms in RHF molecule. For CH₃CH₂F and CH₃CHF₂ the k ₉’s were equal to (8.8–10.2)^·10⁻¹⁴cm^{3 ·}s⁻¹ and (7.3–8.4)^·10⁻¹⁴cm^{3 ·}s⁻¹, respectively, and seem to be determined for the first time. In the case of CH₂FCF₃ the obtained value of k _CF3CHF+O2 = 5.20±0.76^·10⁻¹⁴cm^{3 ·}s⁻¹ is much higher than the value published in the literature.⁴ The other determined rate constant data are comparable to the literature values.     

Chemisorbed Oxygen Species over the (110) Face of SnO2

The electronic states of chemisorbed oxygen species on the (110) face of SnO₂ and their variations caused by heat treatments and/or O₂ exposure have been investigated. The reactivities of the chemisorbed oxygen species for methane oxidations were also examined.Four different chemisorbed oxygen species (O₂ ^2-, O_2-, O^-, O_b) were observed, in addition to the lattice oxygen (O^2-), on the surface of the stabilized (110) surface of SnO₂ after O₂ exposure. The O_b species was assumed to be the bridging oxygen at the topmost layer of the SnO₂ (110) surface having no neighboring oxygen vacancies. The electronic state of O_b was converted to O^- upon CH₄ exposure at 473 K by coupling with newly produced vacancies at the bridging site of the SnO₂ (110) surface.     

Low Energy Electron Attachment by Some Chlorosilanes

In this paper, the rate coefficients (k) and activation energies (E_a) for SiCl₄, SiHCl₃, and Si(CH₃)₂(CH₂Cl)Cl molecules in the gas phase were measured using the pulsed Townsend technique. The experiment was performed in the temperature range of 298–378 K, and carbon dioxide was used as a buffer gas. The obtained k depended on temperature in accordance with the Arrhenius equation. From the fit to the experimental data points with function described by the Arrhenius equation, the activation energies (E_a) were determined. The obtained k values at 298 K are equal to (5.18 ± 0.22) × 10⁻¹⁰ cm³·s⁻¹, (3.98 ± 1.8) × 10⁻⁹ cm³·s⁻¹ and (8.46 ± 0.23) × 10⁻¹¹ cm³·s⁻¹ and E_a values were equal to 0.25 ± 0.01 eV, 0.20 ± 0.01 eV, and 0.27 ± 0.01 eV for SiHCl₃, SiCl₄, and Si(CH₃)₂(CH₂Cl)Cl, respectively. The linear relation between rate coefficients and activation energies for chlorosilanes was demonstrated. The DFT/B3LYP level coupled with the 6-31G(d) basis sets method was used for calculations of the geometry change associated with negative ion formation for simple chlorosilanes. The relationship between these changes and the polarizability of the attaching center (α_centre) was found. Additionally, the calculated adiabatic electron affinities (AEA) are related to the α_centre.     

Kinetics and quantification of the electrophilic reactivities of substituted thiophenes and structure‐reactivity relationships

Second‐order rate constants (k₁) have been measured spectrophotometrically for reactions of 2‐methoxy‐3‐X‐5‐nitrothiophene 1a‐c (X = NO₂, CN, and COCH₃) with secondary cyclic amines (pyrrolidine 2a , piperidine 2b , and morpholine 2 c ) in CH₃CN and 91:9 (v/v) CH₃OH/CH₃CN at 20°C. The experimental data show that the rate constants (k₁) values exhibit good correlation with the parameters of nucphilicity (N) of the amines 2a‐c and are consistent with the Mayr's relationship log k (20°C) = s(E + N). We have shown that the electrophilicity parameters E derived for 1a–c and those reported previously for the thiophenes 1d‐g (X = SO₂CH₃, CO₂CH₃, CONH₂, and H) are linearly related to the pK_a values for their gem‐dimethoxy complexes in methanol. Using this correlation, we successfully evaluated the electrophilicity E values of 12 structurally diverse electrophiles in methanol for the first time. In addition, a satisfactory linear correlation (r² = 0.9726) between the experimental (log k_exp) and the calculated (log k_calcd) values for the σ‐complexation reactions of these 12 electrophiles with methoxide ion in methanol has been observed and discussed.     

Methyl radical kinetics: relaxation versus H-atom abstraction in flash-photolyzed mixtures of methyl iodide and diluent gases

Time-resolved mass spectrometry has been used to deduce the de-excitation rate (k_d) of photolytically produced “hot” methyl radicals relative to the H-atom abstraction rate (K_a) from CH₃¹. The rate-constant ratios k_d(M)/k_a f Me = He, Ar, N₂, and CH₃l are 0.22 ± 0.02, 0.36 ± 0.03, 0.86 ± 0.13 and 9.6 ± 0.8, respectively.     

Bifunctional metal-free KAUST Catalysis Center 1 (KCC-1) as highly active catalyst for syngas production via methane partial oxidation

The metal-free KAUST Catalysis Center 1 (KCC-1) was synthesized through microemulsion method with microwave assistance and was assessed for methane partial oxidation (MPO) under various operating conditions. The electronic spin resonance spectroscopy, pyridine-probed infrared spectroscopy, and temperature-programmed desorption of oxygen measurement indicated that the concentration of BrØnsted acid sites and oxygen vacancies in KCC-1 were at least 2-fold higher than its counterparts, which benefited MPO activity via the promotion of adsorption and dissociation of gaseous reactants. The principal species detected by post-reaction X-ray photoelectron spectroscopy (XPS) was surface-adsorbed oxygen species; its relative percentages among all oxygen species reduced in the order spent KCC-1 (77.1%) > spent MCM-41 (Mobil Composition of Matter number 41; 41.4%) > spent SiO₂ (?). The catalytic performance followed the same trend, suggesting that the surface-adsorbed oxygen species was the key factor for MPO process. Additionally, the carbon deposition rate increased in the order SiO₂ (16.8 mol/g_cat/s) > MCM-41 (11.7 mol/g_cat/s) > KCC-1 (7.7 mol/g_cat/s), consistent with the results of post-reaction Raman measurements. By coupling the in situ Fourier-transform infrared and XPS results, it is suggested that the high concentration of oxygen vacancies in KCC-1 contributed to activate the CH₄ molecules on acid sites via different O1-assisted kinetically relevant C–H bond activation mechanism for combustion-reforming pathway; meanwhile it provided an excellent adsorption-desorption cycle of O^2? species to inhibit the carbon deposition, thus creating a bifunctional reaction mechanism in MPO reaction.     

Catalytic decomposition of azomethane and reactions of adsorbed methyl radicals on the molybdenum surface

The methods of temperature-programmed reaction/desorption (TPR/TPD) are used to study azomethane (CH³N=NCH³) decomposition and the reactions of the products of its pyrolysis (CH ₃ ^* radicals and N₂) on the polycrystalline molybdenum surface. A TPR spectrum of adsorbed azomethane decomposition shows mainly N₂, H₂, and unreacted azomethane. Upon preliminary adsorption of azomethane pyrolysis products on a catalyst sample, a TPR spectrum shows N₂, H₂, and CH₄ in comparable amounts. The difference in the composition of desorption products found for these two types of experiments shows that, in the decomposition of adsorbed azomethane, surface methyl moieties are not formed. The rate constants were calculated for the dissociation of adsorbed CH₃, CH₂, and CH, recombination of hydrogen atoms with each other and with CH₃ and CH₂, and the recombinative desorption of nitrogen atoms. Deceased.     

Effect of mixed H2O?CH3CN solvent on intramolecular general base-catalyzed cleavage of ionized phenyl salicylate in the presence of DL-proline

Nucleophilic second-order rate constant (K_n ^ms) for the reaction of DL-proline with ionized phenyl salicylate (PS⁻) shows a nonlinear decrease with the increase in the content of CH₃CN in mixed aqueous solvents at ≤50% v/v CH₃CN. The values of k_n ^ms show a mild increase with the increase in the content of CH₃CN at >50% v/v CH₃CN. The effect of solvent on k_n ^ms is explained in terms of solvent effect on the pK_a of the conjugate acids of leaving group (i.e. phenolate ion) and DL-proline.     

Association reactions of trimethylsilyl ions

Adduct ions, [M + (CH₃)₃Si]⁺, were produced by bimolecular association reactions of trimethylsilyl ions, (CH₃)₃Si⁺, with acetone, cydohexaoone, anisole, dimethyl ether, 2,5-dimethylfuran, 2-methylfuran and furan in ion cyclotron resonance experiments at 300 K and at pressures of ～10^?7 Torr (1 Torr = 133.3 Pa). The rate constants, k_a, for the association reactions varied from 100% to 2% of the collision rate constants, k_c. The rate constants were independent of pressure, except for furan. Measurements were also made of bond dissociation energies for these adduct ions, D[(CH₃)₃Si⁺–X], from equilibrium measurements. The association efficiency, k_a/k_c, increased with increasing bond dissociation energy and with increasing numbers of degrees of freedom, in qualitative agreement with theoretical predictions. Observations pertinent to the dependence of k_a on reactant temperature and relative kinetic energy are discussed. The possibility of determining ion-neutral complex binding energies from radiative association rate constants is considered.     

Porous nanographene formation on γ-alumina nanoparticles via transition-metal-free methane activation

γ-Al₂O₃ nanoparticles promote pyrolytic carbon deposition of CH₄ at temperatures higher than 800 °C to give single-walled nanoporous graphene (NPG) materials without the need for transition metals as reaction centers. To accelerate the development of efficient reactions for NPG synthesis, we have investigated early-stage CH₄ activation for NPG formation on γ-Al₂O₃ nanoparticles via reaction kinetics and surface analysis. The formation of NPG was promoted at oxygen vacancies on (100) surfaces of γ-Al₂O₃ nanoparticles following surface activation by CH₄. The kinetic analysis was well corroborated by a computational study using density functional theory. Surface defects generated as a result of surface activation by CH₄ make it kinetically feasible to obtain single-layered NPG, demonstrating the importance of precise control of oxygen vacancies for carbon growth.

Oxygen vacancies on the (100) surface of γ-Al₂O₃ nanoparticles catalyse CH₄-CVD for single-layered nanoporous graphenes with no transition metal reaction centre. The rate-limiting step is the proton transfer (PT) in the activation of CH₄ on them.     

Reactions of Adsorbed CH3 Radicals and the Products of Their Decomposition at Mo and Cu Surfaces According to Spatially Resolved TPR Data

Reactions in the layer of CH₃ radicals adsorbed on the surfaces of polycrystalline molybdenum and copper were studied using the method of temperature-programmed reaction (TPR). After N₂ and CH₃ ^· adsorption (the products of azomethane pyrolysis) on molybdenum, N₂, H₂, and CH₄ were observed in comparable amounts in the TPR spectrum. At the same time, only methane was detected in the TPR products on the copper surface. The spatial distributions of CH₄ desorption flows were measured, which were indicative of translational excitation of these molecules. The direct measurements of the rates of the CH₄ molecules desorbed from the copper surface showed that their translational energy was 10–15 times greater than the thermal one. The mechanisms of reactions on the Mo and Cu surfaces are proposed. The rate constants were calculated for some of the elementary steps.     

Absolute rate constants for the addition of hydroxymethyl radicals to alkenes in methanol solution

Absolute rate constants and their temperature dependencies were determined for the addition of hydroxymethyl radicals (CH₂OH) to 20 mono- or 1,1-disubstituted alkenes (CH₂ = CXY) in methanol by time-resolved electron spin resonance spectroscopy. With the alkene substituents the rate constants at 298 K (k₂₉₈) vary from 180 M^?1s^?1 (ethyl vinylether) to 2.1 middot; 10⁶ M^?1s^?1 (acrolein). The frequency factors obey log A/M^?1s^?1 = 8.1 ± 0.1, whereas the activation energies (E_a) range from 11.6 kJ/mol (methacrylonitrile) to 35.7 kJ/mol (ethyl vinylether). As shown by good correlations with the alkene electron affinities (EA), log k₂₉₈/M^?1s^?1 = 5.57 + 1.53 · EA/eV (R² = 0.820) and E_a = 15.86 ? 7.38 · EA/eV (R² = 0.773), hydroxymethyl is a nucleophilic radical, and its addition rates are strongly influenced by polar effects. No apparent correlation was found between E_a or log k₂₉₈ with the overall reaction enthalpy. © 1995 John Wiley & Sons, Inc.     

Effects of mixed CH3CN(SINGLEBOND)H2O solvents on rate of intramolecular general base-catalyzed cleavage of ionized phenyl salicylate in the presence of 3-aminopropan-1-ol, 1,2-diaminoethane,and glycine

The effects of mixed CH₃CN(SINGLEBOND)H₂O solvents on rates of aminolysis of ionized phenyl salicylate, PS⁻, reveal a nonlinear decrease in the nucleophilic second-order rate constants, k_n^ms, (for aminolysis) with increase in the content of CH₃CN until it becomes ∼50%, v/v. The values of k_n^ms remain almost unchanged with change in the CH₃CN content within 50 to 70 or 80%, v/v. The effects of mixed CH₃CN(SINGLEBOND)H₂O solvents on pK_a of leaving group, phenol, and protonated amine nucleophile have been concluded to be the major source for the observed mixed solvent effects on k_n^ms. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 301–307, 1998.     

Kinetics of the radical reactions in pulse radiolyzed RHF — CO2(SF6)-O2-NO gaseous mixtures; RHF = CH3CH2F, CH3CHF2, CH3CF3, CH2FCF3

The kinetics of the peroxy radicals RHFO₂ reactions with NO has been studied by using pulse radiolysis and UV absorption spectroscopy. The rate constants of interaction of oxygen atoms with NO − k ₂ = 2.2±0.2·10⁻¹² cm³·s⁻¹ and NO₂ − k ₃ = 2.1±0.2·10⁻¹¹ cm³·s⁻¹ were found in agreement with the literature values. The bath gases (SF₆ or CO₂) have got minor effect on the rate constants of RHFO₂+NO→NO₂+prod. reactions; RHFO₂ = CH₃CH₂O₂, CH₃CHFO₂, CH₃CF₂O₂, CF₃CH₂O₂, CF₃CHFO₂. The obtained rate coefficients are in the scope of the literature values, although they are lower than those recommended in NIST database. The reasons are discussed.     

Oxidation of dimethyl ether: Absolute rate constants for the self reaction of CH3OCH2 radicals,the reaction of CH3OCH2 radicals with O2, and the thermal decomposition of CH3OCH2 radicals

The rate constant for the reaction of CH₃OCH₂ radicals with O₂ (reaction (1)) and the self reaction of CH₃OCH₂ radicals (reaction (5)) were measured using pulse radiolysis coupled with time resolved UV absorption spectroscopy. k₁ was studied at 296K over the pressure range 0.025–1 bar and in the temperature range 296–473K at 18 bar total pressure. Reaction (1) is known to proceed through the following mechanism: CH₃OCH₂ + O₂ ↔ CH₃OCH₂O₂^# → CH₂OCH₂O₂H^# → 2HCHO + OH (k_prod) CH₃OCH₂ + O₂ ↔ CH₃OCH₂O₂^# + M → CH₃OCH₂O₂ + M (k_RO2) k = k_RO2 + k_prod, where k_RO2 is the rate constant for peroxy radical production and k_prod is the rate constant for formaldehyde production. The k₁ values obtained at 296K together with the available literature values for k₁ determined at low pressures were fitted using a modified Lindemann mechanism and the following parameters were obtained: k_RO2,0 = (9.4 ± 4.2) × 10⁻³⁰ cm⁶ molecule⁻² s⁻¹, k_RO2,∞ = (1.14 ± 0.04) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, and k_prod,0 = (6.0 ± 0.5) × 10⁻¹² cm³ molecule⁻¹ s⁻¹, where k_RO2,0 and k_RO2,∞ are the overall termolecular and bimolecular rate constants for formation of CH₃OCH₂O₂ radicals and k_prod,0 represents the bimolecular rate constant for the reaction of CH₃OCH₂ radicals with O₂ to yield formaldehyde in the limit of low pressure. k_RO2,∞ = (1.07 ± 0.08) × 10⁻¹¹ exp(−(46 ± 27)/T) cm³ molecule⁻¹ s⁻¹ was determined at 18 bar total pressure over the temperature range 296–473K. At 1 bar total pressure and 296K, k₅ = (4.1 ± 0.5) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ and at 18 bar total pressure over the temperature range 296–523K, k₅ = (4.7 ± 0.6) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. As a part of this study the decay rate of CH₃OCH₂ radicals was used to study the thermal decomposition of CH₃OCH₂ radicals in the temperature range 573–666K at 18 bar total pressure. The observed decay rates of CH₃OCH₂ radicals were consistent with the literature value of k₂ = 1.6 × 10¹³exp(−12800/T)s⁻¹. The results are discussed in the context of dimethyl ether as an alternative diesel fuel. © 1997 John Wiley & Sons, Inc.     

Atmospheric chemistry of acetone: Kinetic study of the CH3C(O)CH2O2+NO/NO2 reactions and decomposition of CH3C(O)CH2O2NO2

Pulse radiolysis was used to study the kinetics of the reactions of CH₃C(O)CH₂O₂ radicals with NO and NO₂ at 295 K. By monitoring the rate of formation and decay of NO₂ using its absorption at 400 and 450 nm the rate constants k(CH₃C(O)CH₂O₂+NO)=(8±2)×10⁻¹² and k(CH₃C(O)CH₂O₂+NO₂)=(6.4±0.6)×10⁻¹² cm³ molecule⁻¹ s⁻¹ were determined. Long path length Fourier transform infrared spectrometers were used to investigate the IR spectrum and thermal stability of the peroxynitrate, CH₃C(O)CH₂O₂NO₂. A value of k₋₆≈3 s⁻¹ was determined for the rate of thermal decomposition of CH₃C(O)CH₂O₂NO₂ in 700 torr total pressure of O₂ diluent at 295 K. When combined with lower temperature studies (250–275 K) a decomposition rate of k₋₆=1.9×10¹⁶ exp (−10830/T) s⁻¹ is determined. Density functional theory was used to calculate the IR spectrum of CH₃C(O)CH₂O₂NO₂. Finally, the rate constants for reactions of the CH₃C(O)CH₂ radical with NO and NO₂ were determined to be k(CH₃C(O)CH₂+NO)=(2.6±0.3)×10⁻¹¹ and k(CH₃C(O)CH₂+NO₂)=(1.6±0.4)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹. The results are discussed in the context of the atmospheric chemistry of acetone and the long range atmospheric transport of NO_x. © John Wiley & Sons, Inc. Int J Chem Kinet: 30: 475–489, 1998