Wendell Forst

Gas‐Phase Kinetics of Hydroxyl Radical Reactions with Alkenes: Experiment and Theory

Reactions of the hydroxyl radical with propene and 1-butene are studied experimentally in the gas phase in a continuous supersonic flow reactor over the range 50≤T/K≤224. OH radicals are produced by pulsed laser photolysis of H(2)O(2) at 266 nm in the supersonic flow and followed by laser-induced fluorescence in the (1, 0) A(2)Σ(+)←X(2)Π(3/2) band at about 282 nm. These reactions are found to exhibit negative temperature dependences over the entire temperature range investigated, varying between (3.1-19.2) and (4.2-28.6)×10(-11) cm(3) molecule(-1) s(-1) for the reactions of OH with propene and 1-butene, respectively. Quantum chemical calculations of the potential energy surfaces are used as the basis for energy- and rotationally resolved Rice-Ramsperger-Kassel-Marcus calculations to determine the rate constants over a range of temperatures and pressures. The negative temperature dependences of the rate constants are explained by competition between complex redissociation and passage to the adducts by using a model with two transition states. The results are compared and contrasted with earlier studies and discussed in terms of their potential relevance to the atmosphere of Saturn.

Tunneling in the reaction of acetone with OH

Based on recent detailed quantum mechanical computations of the mechanism of the title reaction and, this paper presents kinetics analysis of the overall rate constant and its temperature dependence, for which ample experimental data are available for comparison. The analysis confirms that the principal channel is the formation of acetonyl radical + H(2)O, while the channel leading to acetic acid is of negligible importance. It is shown that the unusual temperature dependence of the overall rate constant, as observed experimentally, is well accounted for by standard RRKM treatment that includes tunneling. This treatment is applied at the microcanonical level, with chemically activated distribution of entrance species, i.e. using a stationary rather than a thermal distribution that incorporates collisional energy transfer and competition between the redissociation and exit channel. A similar procedure is applied to the isotopic reaction acetone-d6 + OH with equally satisfying results, so that the experimental temperature dependence of the KIE (kinetic isotope effect) is perfectly reproduced. This very good agreement between calculation and experiment is obtained without any fitting to experimental values and without any adjustment of the parameters of calculation.

Microcanonical variational theory of radical recombination by inversion of interpolated partition function. Part 2.—CX3+ O2(X = H, F, Cl)

The title theory (MVIPF) has been applied to the recombinations of O2 with the radicals CH3, CF3, CF2Cl, CFCl2 and CCl3 and compared, with generally good results, to experimental data which are available for all radicals but one (CF2Cl). In particular, MVIPF, which is based on minimum information and uses an adjustable parameter (c) in a Gaussian-type switching function, correctly reproduces the fact that the recombination CH3+ O2 has a positive temperature coefficient, while all the other radical recombinations have a negative temperature coefficient. Consolidated results by MVIPF for the nine recombination reactions for which experimental data are available (the title reactions, plus CH3+ H, CH3+ CH3, CH2Cl + CH2Cl, CHCl2+ CHCl2, CCl3+ CCl3), yield a linear relation for a function of the logarithm of c, which in principle allows one to predict a recombination rate constant in a series of similar reactions within a factor of two.

Ultraviolet absorption spectra of the CH2Cl and CHCl2 radicals and the kinetics of their self-recombination reactions from 273 to 686 K

The UV absorption spectra of the chloromethyl (CH2Cl) and dichloromethyl (CHCl2) radicals have been determined between 197.5 and 230 nm, together with the absolute rate constants for their association reactions: CH2Cl + CH2Cl → Products (1a) CHCl2+ CHCl2→ Products (1b) as a function of temperature from 273 to 686 K and between 29 and 760 Torr N2 total pressure. The transient decays of the radicals were monitored by time-resolved UV absorption following the flash photolysis of Cl2 mixed with the parent molecules (CH3Cl or CH2Cl2). At a resolution of 2 nm, no vibrational structure was detected in either spectrum which both appear as strong broad electronic bands with maxima around 200 nm (CH2Cl) and 215 nm (CHCl2). At these wavelengths the absolute absorption cross-sections were measured as (1.45 ± 0.16)× 10–17 and (1.37 ± 0.24)× 10–17 cm2 molecule–1, respectively, relative to that of CH3O2 at 240 nm (4.55 × 10–18 cm2 molecule–1). The experimental results, supported by RRKM calculations, demonstrate that the measured rate constants for removal of the radicals correspond to the high-pressure limiting rate constants for recombination, both of which exhibit a negative temperature dependence, represented by the following expressions: CH2Cl: k1a=(2.8 ± 0.3)× 10–11(T/298)–(0.85 ± 0.14) cm3 molecule–1 s–1. CHCl2: k1b=(9.3 ± 1.7)× 10–12(T/298)–(0.74 ± 0.10) cm3 molecule–1 s–1. Errors are 1 σ. The present results are compared to existing data on the self-recombination reactions of the CH3 and CCl3 radicals; the negative temperature dependences of the self-association rate constants for the series CH3, CH2Cl, CHCl2 and CCl3 are shown to be consistent within the framework of a recently developed variational transition-state theory method.

Kinetics of the combination reactions of chlorofluoromethylperoxy radicals with NO2 in the temperature range 233–373 K

The rate parameters for the combination reactions of CCl3O2, CCl2FO2 and CF3O2 radicals with NO2 were measured in the pressure range 1–10 Torr and from 233 to 373 K. Experiments were performed by pulsed laser photolysis and time-resolved mass spectrometry. Al reactions are in the fall-off region and are significantly faster than the equivalent reaction of CH3O2. They exhibit strong negative temperature coefficients and the rate constants increase in the series from CF3O2 to CCl3O2. The experimental equilibrium constant was obtained for the reaction of CCl2FO2 at 273 K, by using previously determined rate constants for the reverse reaction. This determination allowed us to obtain ΔH° of the reaction by calculating the structural and spectroscopic parameters by the semi-empirical MNDO method. Assuming that the value of ΔH° is the same for all reactions of the series it was possible to calculate the temperature dependence of all equilibrium constants and the bond dissociation energies D°298(CX3O2—NO2)= 105 ± 5 kJ mol–1(X = For Cl). ARRKM model was set up for these reactions and calibrated for the reaction of CCl2FO2. The model is shown to reproduce quite adequately the fall-off curves and with the substitution of chlorine for fluorine in the series of radicals investigated. This model was used to extrapolate the low-pressure data to high pressure, where no experimental data were available, and to calculate the kinetic parameters for the CClF2O2 reaction, which could not be investigated experimentally in the present study. Kinetic parameters are reported for the association reaction of the entire series of chlorofluoromethylperoxy radicals with NO2.

Kinetic and mechanistic study of the pressure and temperature dependence of the reaction CH3O + NO

New kinetic measurements for the CH3O + NO reaction have been performed using two different techniques. The discharge flow (DF) technique has been used to investigate the 0.5–5 Torr and 248–473 K pressure and temperature ranges and pulsed laser photolysis (PLP) has been used for the 30–500 Torr and 284–364 K ranges. These new results represent an extension of the pressure and temperature ranges investigated previously. This reaction is known to present two reaction pathways, the association pathway yielding CH3ONO and the disproportionation pathway yielding CH2O+HNO. Based on literature and present experimental data, using the results of ab initio calculations, a multichannel RRKM analysis was developed to interpret the experimental results. This analysis has shown that the disproportionation reaction occurs simultaneously by both a direct hydrogen abstraction reaction, and via the formation of energized CH3ONO* complex in competition with the association reaction. The RRKM analysis, fitted to present and previous data, has yielded a second-order limiting low-pressure value of 2.5 × 10−12 cm3 molecule−1 s−1 at 298 K, with a complex temperature dependence. The limiting high-pressure rate constant derived in the same way is k∞ = (3.4 ± 0.4) × 10−11(T/298)−0.75. The model allows the prediction of CH3O loss rate constants and of the branching ratios in the 1–760 Torr and 220–600 K ranges. For a convenient presentation of the overall rate constant, an analytical expression using the conventional Troe expression with a temperature-dependent addition constant, has been fitted to the results of the RRKM analysis.

Chemical activation in OH radical-oxidation of 1-n-alkenes

Statistical kinetic calculations, using RRKM-master equation analysis and phase space theory, were performed to determine the importance of chemical activation in the subsequent evolution of the β-hydroxy-1-alkoxy radicals produced in the troposphere OH-oxidation of 1-n-alkenes via the exothermic reaction R–CH(OH)–CH2O2 + NO → R–CH(OH)–CH2O2NO* → R–CH(OH)–CH2O + NO2; the series of radicals from β-hydroxy-ethoxy to -1-hexoxy was considered. At 298 K and 1 atm the fraction of promptly reacting alkoxy radical (on a subnanosecond time-scale) decreases from 75% for the C3-radical to 20% for the C6-radical, the C2-radical being a particular case (higher dissociation barrier height) with only 30% of prompt dissociation. The thermal unimolecular rate constants of these alkoxy radicals were determined by RRKM calculations of the fall-off curves. From the totality of these results it is deduced: (i) the troposphere impact of chemical activation is important for C2-hydroxy-alkoxy radical and specially in the upper troposphere (ii) for C3 chain length radicals, a chemical activation, by favouring the unimolecular reactions, decreases the fraction of O2 reaction, the major effect being predicted for C4 radical. For all radicals except C2 radical the unimolecular reactions are predicted to be preponderant, and at low temperature the isomerisation, when possible from a –CH2– group, is the major way of reaction.

Approximations for angular momentum-conserved polyatomic vibrational-rotational sum and density of states under a central potential

Abstract An approximation is suggested for the calculation of the angular momentum-conserved rotational sum of states for polyatomic molecules under a central potential, based on an interpolation between high-J and low-J forms (J = total angular momentum). A similar procedure is suggested for an approximation to the angular momentum-conserved polyatomic vibrational-rotational sum of states, which obviates a computationally intensive convolution integral. The procedure is checked against rigorous results with satisfactory results. It can be applied to various fragment symmetries in both the phase space limit and at arbitrary interfragment distance, which is of some interest in variational routines.

Gas-phase kinetics of the hydroxyl radical reaction with allene: absolute rate measurements at low temperature, product determinations, and calculations.

The gas phase reaction of the hydroxyl radical with allene has been studied theoretically and experimentally in a continuous supersonic flow reactor over the range 50 ≤ T/K ≤ 224. This reaction has been found to exhibit a negative temperature dependence over the entire temperature range investigated, varying between (0.75 and 5.0) × 10(-11) cm(3) molecule(-1) s(-1). Product formation from the reaction of OH and OD radicals with allene (C(3)H(4)) has been investigated in a fast flow reactor through time-of-flight mass spectrometry, at pressures between 0.8 and 2.4 Torr. The branching ratios for adduct formation (C(3)H(4)OH) in this pressure range are found to be equal to 34 ± 16% and 48 ± 16% for the OH and OD + allene reactions, respectively, the only other channel being the formation of CH(3) or CH(2)D + H(2)CCO (ketene). Moreover, the rate constant for the OD + C(3)H(4) reaction is also found to be 1.4 times faster than the rate constant for the OH + C(3)H(4) reaction at 1.5 Torr and at 298 K. The experimental results and implications for atmospheric chemistry have been rationalized by quantum chemical and RRKM calculations.

SUM AND DENSITY OF STATES OF POLYATOMIC SYSTEMS WITH HINDERED ROTORS

The density or sum of states for a collection of independent oscillators, free rotors, and one‐dimensional hindered rotors is obtained with good accuracy by numerical inversion of the corresponding total partition function by the method of steepest descents. The hindered‐rotor partition functions are used in both classical and quantum forms, the latter in the approximation proposed by Truhlar [J. Comput. Chem., 12, 266 (1991)]. The numerical inversion compares well with analytical results obtained in a simple artificial case and also with an exact count of states in a large ethane‐like system. Inversion of the hindered‐rotor classical partition function is shown to lead to a somewhat different energy dependence of the sum or density of states, relative to the quantum counterpart, which is considered to be a more realistic representation. The routines presented are simple and fast enough to be of use in microcanonical rate calculations.