Ferenc Márta

Direct kinetic study of reactions of hydroxyl radicals with alkyl formates

Abstract Rate coefficients were determined for the gas phase reactions of hydroxyl radicals with a series of alkyl formats. Experiments were carried out using the isothermal fast flow method coupled with resonance fluorescence detection. The obtained room temperature rate coefficients are (in 10−13cm3 molecule−1s−1 units): 1.83 ± 0.33 (methyl formate), 9.65 ± 0.43 (ethyl formate), 18.73 ± 0.83 (isopropyl formate), 6.77 ± 0.38 (tert-butyl formate) and 1.62 ± 0.13 (methyl chloroformate). These results are compared with the literature data. In addition estimations are made for the partial reactivities of the formate group and for the hydrocarbon groups adjacent to HC(O)O. Moreover, it has been found that the partial reactivity of the HC(O)O group (in reactions of OH with formates) is two orders of magnitude smaller than that of the HC(O) group (in reactions of OH with aldehydes). This has been explained using the results of ab initio calculations at the G3MP2//MP2(full)/6-31G(d) level of theory.

Laser spectrometry and kinetics of selected elementary reactions of the acetonyl radical

The laser induced fluorescence excitation spectrum (LIF) and the ultraviolet absorption spectrum (TA) of the acetonyl radical (CH3C(O)CH2) were remeasured by using the time-resolved fast discharge flow (DF) and laser flash photolysis (LP) experimental techniques (T = 298 K). The absorption cross section of σ(acetonyl, 207 nm) = (3.16 ± 0.61) × 10−18 cm2 molecule−1 was determined calibrated against the acetyl-peroxyl radical (CH3C(O)O2) in LP/TA measurements. The kinetics of the reactions of CH3C(O)CH2 with the open shell reaction partners O2 (1), NO (2), NO2 (3) and H (4) were studied by using the DF method with LIF detection of the acetonyl radical at 298 ± 1 K and 2.85 ± 0.05 mbar He pressure. The rate constants for the overall reactions were determined in units of cm3 molecule−1 s−1 to be k1 = (3.49 ± 0.51) × 10−13, k2 = (1.04 ± 0.19) × 10−11, k3 = (3.25 ± 0.65) × 10−11 and k4 ≥ 3 × 10−10 with 2σ accuracy given. The acetonyl radical was found to react similarly to alkyl radicals by comparison with literature results. A reduced reactivity was observed toward O2 and NO that might be attributed to the resonance stabilisation of the acetonyl radical. No such effect was observed for the NO2 and H atom reactions.

Structural effects in the decay kinetics of 1-naphthyl derivative-triethylamine exciplexes

Abstract The formation and decay kinetics of fluorescence spectra of triethylamine (TEA) exciplexes with naphthalene and 1-naphthalyl derivatives were investigated in cyclohexane. The stability of the singlet exciplexes was shown to determine the kinetics. The loosely bound exciplexes between TEA and naphthalene or 1-methylnaphthalene were formed in reversible processes, while the tightly bound complexes of TEA with 1-naphthyl- N -methyl-carbamate or with 1-naphthyl-acetate were the product of irreversible reactions of the singlet excited aromatic acceptor with the ground state aliphatic amine molecules. The increases electron affinity of the carbamate and acetate derivatives promoted the formation of ground state complexes and made possible a second route of exciplex formation. Evaluation of the time-resolved fluorescence results supplied the room temperature rate coefficients for the elementary steps.

Kinetics and energy transfer in the thermal decomposition of 2-methyloxetane and 3-methyloxetane

The thermal decomposition of 2-methyloxetane and 3-methyloxetane has been studied between 660 and 760 K in the pressure range 0.01–3 kPa. In the pressure-independent range, rate coefficient expressions log(k∞1/s–1)=(14.53 ± 0.12)–(249.2 ± 2.2 kJ mol–1)/2.303RT and log(k∞2/s–1)=(15.67 ± 0.17)–(269.8 ± 3.3 kJ mol–1)/2.303RT were determined for 2-methyloxetane decomposition into C3H6+ HCHO (k1) and C2H4+ CH3CHO (k2), respectively, while for 3-methyloxetane decomposition into C3H6+ HCHO (k3) the following kinetic parameters were obtained: log(k∞3/s–1)=(15.38 ± 0.27)–(258.7 ± 3.7 kJ mol–1)/2.303RT. The pressure dependence of the homogeneous decomposition rate and the efficiency of the gas-phase collisional energy transfer have been studied at 743 K. A value of 〈ΔE〉d= 1500 ± 300 cm–1 was extracted from the investigation of the pressure dependence of the two-channel decomposition of 2-methyloxetane. Finally, the efficiency of the surface–gas energy transfer has been studied by the ‘variable encounter method’ in the range 750–1100 K. At 750 K the average energy transferred per collision with the wall was determined to be 2600 cm–1 for both methyloxetanes; however, 〈ΔE′〉 decreased considerably with increasing temperature. The results on the collision efficiencies were discussed and compared with literature data for related molecules.

Reactions of CF3 radicals with benzotrifluoride and the C—H bond strength in C6H5CF3 and C6H6

The reactions of CF3 with benzotrifluoride (BTF) were studied at 1029, 1070 and 1132 K. The RCF3H/R½C2F6[BTF] ratio was found to decrease with increasing benzotrifluoride pressure, which was interpreted as simultaneous formation of C2F6 by CF3 recombination and an inversion reaction between CF3 and C6H5CF3.From experiments in the absence and presence of hydrogen, we obtained log(k2/l. mol–1 s–1)=(9.1 ± 0.3)–(9.4 ± 1.2)/θ and log(k7/l. mol–1 s–1)=(8.7 ± 0.1)–(9.3 ± 0.1)/θ, (θ= 2.303 RT/kcal mol–1) for the rate coefficients of H-abstraction by CF3 from C6H5CF3 and H2, respectively. Hence we obtained, with an assumed value for the activation energy of step (–2), D°(CF3C6H4—H)= 109.0 ± 2.5 kcal mol–1. CF3H + C6H4CF3→ CF3+ C6H5CF3(–2)

Surface-gas energy transfer in the oxetane decomposition system

The method called the variable encounter method was applied in the oxetane decomposition system for the study of vibrational energy transfer between gas molecules and the surface. The average probability of reaction per collision was derived from the experimental data and compared with theoretical calculations based on various energy transfer probability models. The Gaussian model fits the data well. The average down stepsize was found to be 3100 cm−1 at 750 K and it decreased to 2200 cm−1 at 1100 K.AbstractТак называемый метод изменяющихся столкновений применяли для исследования передачи колебательной энергии между молекулами газа и поверхностью в разложении оксетана. Среднюю вероятность реакции на одно столкновение получали из экспериментальных данных и сравнивали с теоретическими расчетами, исходящими из моделей вероятностей передачи различных энергий. Гауссовское распределение хорошо согласуется с экспериментапьными данными. Средняя нижняя величина ступени равна 3100 cm−1 при 750 К и уменьшается до 2200 cm−1 при 1100 К.

Collisional energy transfer in the thermal decomposition of oxetane

The first order rate coefficient for the thermal decomposition of oxetane and oxetane-d2 has been investigated at two temperatures as a function of pressure. Gas phase collisional relaxation results are obtained by using RRKM theory and various energy tranfer probability models.AbstractКонстанту скорости первого порядка для термического разложения оксетана и оксетана-d2 измеряли при двух температурах в зависимости от давления. Данные релаксации за счет столкновений в газовой фазе получены, исходя из теории RRKM и на основе различнх моделей вероятностей передачи энергии.

Excimer formation in the photochemistry of aliphatic ketones I: concentration dependence of quantum yields

The photochemistry of 2-pentanone was studied in iso-octane at 313 nm. The quantum yields of the Norrish type I and II primary processes were found to increase with increasing ketone concentration and to attain limiting values at high concentrations (above about 0.3 mol dm−3. The triplet yield, studied by means of the sensitized biacetyl phosphorescence technique, showed a similar concentration dependence. The enhancement of singlet → triplet intersystem crossing by ground state ketone molecules was interpreted by assuming singlet excimer formation. The excimer may redissociate into singlet state and ground state ketone molecules or may enter intersystem crossing followed by dissociation of the excimer triplet state. Evaluation of the experimental results yielded rate coefficients for the first-order intersystem crossing step and for the type II decomposition from the triplet state.

Competitive bromination kinetics of CH3Br and CH2ClBr

The relative-rate method with gas-chromatographic product analysis was applied to study the kinetics of the reactions Br + CH3Br → CH2Br + HBr (1) and Br + CH2ClBr → CHClBr + HBr (2) The rate coefficient ratio of k1/ k2 = (1.6 ± 0.2) exp[(-15.2 ± 0.3) kJ mol-1/ RT] was determined in the temperature range of 353 - 410 K.

The kinetics of hydroxyl radical reactions with cyclopropane and cyclobutane

A laser flash photolysis/resonance fluorescence investigation has been carried out to study the kinetics of the overall reactions OH + cyclopropane (1) and OH + cyclobutane (2) in the temperature range 298–490 K and at 298 K, respectively. The following kinetic parameters have been determined: k1 =(3.9 ±0.6) 10−12exp- (2.2 ± 0.1)kcal mol−1/RT molecule−1cm3s−1, k2(298 K) = (17.5 ± 1.5)10−13molecule−1 cm3s−1.