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The thermal dissociation reactions of C2F4 and C2F6 were studied in shock waves over the temperature range 1000-4000 K using UV absorption spectroscopy. Absorption cross sections of C2F4, CF2, CF, and C2 were derived and related to quantum-chemically modeled oscillator strengths. After confirming earlier results for the dissociation rates of C2F4, CF3, and CF2, the kinetics of secondary reactions were investigated. For example, the reaction CF2 + CF2 → CF + CF3 was identified. Its rate constant of 1010 cm3 mol-1 s-1 near 2400 K is markedly larger than the limiting high-pressure rate constant of the dimerization CF2 + CF2 → C2F4, suggesting that the reaction follows a different path. When the measurements of the thermal dissociation CF2 (+Ar) → CF + F (+Ar) are extended to temperatures above 2500 K, the formation of C2 radicals was shown to involve the reaction CF + CF → C2F + F (modeled rate constant 8.0 × 1012 (T/3500 K)1.0 exp(-4400 K/T) cm3 mol-1 s-1) and the subsequent dissociation C2F (+Ar) → C2 + F + (Ar) (modeled limiting low-pressure rate constant 3.0 × 1016 (T/3500 K)-4.0 exp(-56880 K/T) cm3 mol-1 s-1). This mechanism was validated by monitoring the dissociation of C2 at temperatures close to 4000 K. Temperature- and pressure-dependences of rate constants of reactions involved in the system were modeled by quantum-chemistry based rate theory.

The reactions of anionic metal clusters Mn– with O2 (M = V (n = 1–15), Cr (n = 1–15), Co (n = 1–12), and Ni (n = 1–14)) are investigated from 300 to 600 K using a selected-ion flow tube. All rate c...

The thermal dissociation of C2F5I was studied in shock waves monitoring UV absorption signals from the reactant C2F5I and later formed reaction products such as CF, CF2, and C2F4. Temperatures of 950–1500 K, bath gas concentrations of [Ar] = 3 × 10−5–2 × 10−4 mol cm−3, and reactant concentrations of 100–500 ppm C2F5I in Ar were employed. Absorption-time profiles were recorded at selected wavelengths in the range 200–280 nm. It was found that the dissociation of C2F5I → C2F5 + I was followed by the dissociation C2F5 → CF2 + CF3, before the dimerization reactions 2CF2 → C2F4 and 2CF3 → C2F6 and a reaction CF2 + CF3 → CF + CF4 set in. The combination of iodine atoms with C2F5 and CF3 had also to be considered. The rate constant of the primary dissociation of C2F5I was analyzed in the framework of statistical unimolecular rate theory accompanied by a quantum-chemical characterization of molecular parameters. Rates of secondary reactions were modelled as well. Experimental rate constants for the dissociations of C2F5I and C2F5 agreed well with the modelling results. The comparably slow dimerization 2CF2 → C2F4 could be followed both by monitoring reactant CF2 and product C2F4 absorption signals, while CF3 dimerization was too fast to be detected. A competition between the dimerization reactions of CF2 and CF3, the recombination of CF2 and CF3 forming C2F5, and CF-forming processes like CF2 + CF3 → CF + CF4 finally was discussed.

This article, the seventh in the series, presents kinetic and photochemical data sheets evaluated by the IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation. It covers an extension of the gas-phase and photochemical reactions related to Criegee intermediates previously published in Atmospheric Chemistry and Physics (ACP) in 2006 and implemented on the IUPAC website up to 2020. The article consists of an introduction, description of laboratory measurements, a discussion of rate coefficients for reactions of O3 with alkenes producing Criegee intermediates, rate coefficients of unimolecular and bimolecular reactions and photochemical data for reactions of Criegee intermediates, and an overview of the atmospheric chemistry of Criegee intermediates. Summary tables of the recommended kinetic and mechanistic parameters for the evaluated reactions are provided. Data sheets summarizing information upon which the recommendations are based are given in two files, provided as a Supplement to this article.

The reaction of CF2 with H2 was studied in shock waves by monitoring UV absorption signals. CF2 was prepared by thermal dissociation of C2F4 (or of c-C3F6). The rate constant of the reaction CF2 + H2 → CHF + HF near 2000 K was found to be close to 1011 cm3 mol-1 s-1, consistent with earlier information on the reverse reaction CHF + HF → CF2 + H2 and a modeled equilibrium constant. The kinetic studies were accompanied by spectroscopic investigations. Absorption cross sections of C2F4 between 190 and 220 nm were measured near 1000 K and compared with room temperature values from the literature. Likewise, absorption cross sections of CF2 near 2000 K were measured between 210 and 300 nm and compared with room temperature data. Additional, superimposed, absorption signals were recorded during the reaction and identified by their time-dependence and by quantum-chemical calculations employing time-dependent density functional theory. A previously unknown absorption spectrum of CHF radicals near 200 nm was identi...

The unimolecular dissociation of CH2F2 leading to CF2 + H2, CHF + HF, or CHF2 + H is investigated by quantum-chemical calculations and unimolecular rate theory. Modeling of the rate constants is accompanied by shock wave experiments over the range of 1400–1800 K, monitoring the formation of CF2. It is shown that the energetically most favorable dissociation channel leading to CF2 + H2 has a higher threshold energy than the energetically less favorable one leading to CHF + HF. Falloff curves of the dissociations are modeled. Under the conditions of the described experiments, the primary dissociation CH2F2 → CHF + HF is followed by the reaction CHF + HF → CF2 + H2. The experimental value of the rate constant for the latter reaction indicates that it does not proceed by an addition–elimination process involving CH2F2* intermediates, as assumed before.

The thermal dissociation reaction CF3 (+ Ar) → CF2 + F (+ Ar) was studied in incident and reflected shock waves by monitoring UV absorption signals of the primary dissociation product CF2. CF3 radicals were produced by thermal decomposition of CF3I. Accounting for secondary reactions of F atoms, rate constants for the unimolecular dissociation were derived. Experimental parts of the falloff curves were obtained over the ranges 1544 - 2106 K and 1.0×〖10〗^(-5)≤["Ar" ]≤9.3×〖10〗^(-5 ) "mol " 〖"cm" 〗^"-3" . Theoretical modelling allowed for a construction of the full falloff curves connecting the limiting low pressure rate constants k_0=["Ar" ] 2.5×〖10〗^18 (T⁄(2000 "K" ))^(-5.1) " exp" ((-42450 "K" )⁄T) 〖"cm" 〗^"3" 〖"mol" 〗^(-1) "s" ^(-1) with the limiting high pressure rate constants k_∞= 1.6×〖10〗^16 (T⁄(2000 "K" ))^(-1.3) "exp" ((-43250 "K" )⁄T) "s" ^(-1) (center broadening factors of F_cent= 0.25, 0.22, and 0.20 at 1500, 2000, and 2500 K, respectively, were used). The influence of simplifications of falloff expressions and of limiting rate constants on the representation of experimental data is discussed.

The falloff curves of the unimolecular dissociation CF3I (+Ar) - CF3 + I (+Ar) are modelled by combining quantum-chemical characterizations of the potential energy surface for the reaction, standard unimolecular rate theory, and experimental information on the average energy transferred per collision between excited CF3I and Ar. The (essentially) parameter-free theoretical modelling gives results in satisfactory agreement with data deduced from earlier shock wave experiments employing a variety of reactant concentrations (between a few ppm and a few percent in the bath gas Ar). New experiments recording absorption–time signals of CF3I, I2, CF2 and (possibly) IF at 450–500 and 200–300 nm are reported. By analysing the decomposition mechanism, besides the unimolecular dissociation of CF3I, these provide insight into the influence of secondary reactions on the experimental observations. Fil: Cobos, Carlos Jorge. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina Fil: Sölter, L.. Universität Göttingen; Alemania Fil: Tellbach, E.. Universität Göttingen; Alemania Fil: Troe, J.. Universität Göttingen; Alemania. Institut Max Planck fuer Bioanorganische Chemie; Alemania

The thermal decomposition of perfluorotriethylamine, (C2F5)3N, was investigated in shock waves by monitoring the formation of CF2. Experiments were performed over the temperature range of 1120-1450 K with reactant concentrations between 100 and 1000 ppm of (C2F5)3N in the bath gas Ar and with [Ar] in the range of (0.7-5.5) × 10-5 mol cm-3. The experiments were accompanied by quantum-chemical calculations of the energies of various dissociation paths and by rate calculations, in particular for the dissociation of C2F5via C2F5 → CF3 + CF2. The overall reaction can proceed in different ways, either by a sequence of successive C-N bond ruptures followed by fast C2F5 decompositions, or by a sequence of alternating C-C and C-N bond ruptures. A cross-over between the two pathways can also take place. At temperatures below about 1300 K, yields of less than one CF2 per (C2F5)3N decomposed were observed. On the other hand, at temperatures around 2000 K, when besides the parent molecule, CF3 also dissociates, yields of six CF2 per (C2F5)3N decomposed were measured. The rate-delaying steps of the dissociation mechanism at intermediate temperatures were suggested to be the processes (C2F5)NCF2 → (C2F5)N + CF2 and (CF2)N → N + CF2. The reduction of the CF2 yields at low temperatures was tentatively attributed to a branching of the mechanism at the level of (C2F5)2NCF2, from where the cyclic final product perfluoro-N-methylpyrrolidine, (C4F8)NCF3, is formed which was identified in earlier work from the literature., Universidad Nacional de La Plata

The role of collisional energy transfer in ion-molecule association reactions is analyzed. Rate constants for the formation of adducts between HCl and NO3−(HNO3)x (H2SO4)y or HSO4−(HNO3)x (H2SO4)y (with x = 0–2 and y = 0–2) in the buffer gases H2, He, and N2 and at temperatures between 150 and 300 K are considered. Quantum-chemical calculations of molecular parameters and statistical unimolecular rate theory are combined to model low-pressure rate constants whereas ion-molecule capture theory provides high-pressure rate constants. The comparison with experimental results indicates that energy transfer is dominated by overall collision numbers while weak-collision effects are only of minor importance. On the other hand, often neglected falloff effects between termolecular and bimolecular reaction behavior have to be taken into account.

The thermal decomposition of hexafluoropropylene oxide, C3F6O, to perfluoroacetyl fluoride, CF3COF, and CF2 has been studied in shock waves highly diluted in Ar between 630 and 1000 K. The measured rate constant k1 = 1.1 × 1014exp(-162(±4) kJ mol-1/RT) s-1 agrees well with literature data and modelling results. Using the reaction as a precursor, equimolar mixtures of CF3COF and CF2 were further heated. Combining experimental observations with theoretical modelling (on the CBS-QB3 and G4MP2 ab initio composite levels), CF3COF is shown to dissociate on two channels, either leading to CF2 + COF2 or to CF3 + FCO. By monitoring the CF2 signals, the branching ratio was determined between 1400 and 1900 K. The high pressure rate constants for the two channels were obtained from theoretical modelling as k5,∞(CF3COF → CF2 + COF2) = 7.1 × 1014exp(-320 kJ mol-1/RT) s-1 and k6,∞(CF3COF → CF3 + FCO) = 3.9 × 1015exp(-355 kJ mol-1/RT) s-1. The experimental results obtained at [Ar] ≈ 5 × 10-6 mol cm-3 were consistent with modelling results, showing that the reaction is in the falloff range of the unimolecular dissociation. The mechanism of secondary reactions following CF3COF dissociation has been analysed as well. Fil: Cobos, Carlos Jorge. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina Fil: Hintzer, K.. Dyneon GmbH; Alemania Fil: Sölter, L.. Universität Göttingen; Alemania Fil: Tellbach, E.. Universität Göttingen; Alemania Fil: Thaler, A.. Dyneon GmbH; Alemania Fil: Troe, J.. Universität Göttingen; Alemania. Max-Planck-Institut für Biophysikalische Chemie; Alemania

Limiting high pressure rate constants for the association of H with C6H5+ and of CH3, C2H5, and n-C3H7 radicals with C7H7+ molecular ions are analyzed in terms of “rigidity factors” arising from the anisotropy of the interaction potentials and “collision rate constants” in the absence of anisotropy (the latter corresponding to phase space theory, PST). Model calculations based on ab initio potential energy surfaces show that the PST rate constants kPST(T) exceed collision rate constants from conventional ion-molecule capture theory (in this case given by Langevin rate constants kL). They can be represented by kPST(T) = kL + kh.sph. where kh.sph. denotes hard-sphere collision numbers with collision radii r0. The r0 derived from the modelled kPST(T) are related to properties of the interaction potentials. Applications to other ion-molecule reactions are proposed.

In this paper, the electronically nonadiabatic Landau-Zener (LZ) mechanism for the vibrational relaxation v = 1 → v = 0 of NO(X2Π) in collisions with Ar(S01) is discussed. It corresponds to nonadiabatic transitions between two crossing vibronic potential energy surfaces (PESs) originating from vibrational states of the collision complex and supported by two coupled electronic PESs. The LZ rate coefficients k10LZ are calculated within the uniform Airy approach in the reaction coordinate approximation with parameters derived from ab initio PESs and an asymptotic estimation of the Franck-Condon factor in the nonadiabatic coupling region. The rate coefficients are close to the experimental rate coefficients available over the range of 900-2500 K, where the electronically adiabatic Landau-Teller (LT) mechanism with the rate coefficients k10LT does not make a noticeable contribution to the total relaxation rate. The ratio k10LZ/k10LT increases with temperature and the LZ and LT mechanisms have comparable rates at about 4000 K.

The thermal unimolecular dissociation of CH3F was studied in shock waves by monitoring the UV absorption of a dissociation product identified as CH2F. It is concluded that, under conditions applied, the formation of this species corresponds to a minor, spin-allowed, dissociation channel of about 3% yield. Near to the low-pressure limit of the reaction, on the other hand, the energetically more favourable dissociation leads to 3CH2 + HF on a dominant, spin-forbidden, pathway. By considering the multichannel character of the reaction, it is shown that, in contrast to the low-pressure range, the high-pressure range of the reaction should be dominated by CH2F formation. The channel-switching probably takes place at pressures higher than those applied in the present work. In addition to the two dissociation channels of CH3F producing 3CH2 + HF and CH2F + H, a third, spin-allowed, dissociation channel leading to 1CHF + H2 was also considered and estimated to proceed with a yield smaller than 0.5%. Besides the dissociation of CH3F, the dissociation of CH2F was studied by monitoring the UV spectrum of CH2F. Details of this spectrum were investigated. Similar to CH3F, the dissociation of CH2F can proceed on several dissociation channels, under the present conditions either to CHF + H or to CF + H2. After modelling single-channel falloff curves for all reaction pathways, coupling effects of multichannel dissociations were interpreted in the framework of multichannel unimolecular rate theory.

The thermal dissociation of CH2F2 and the reaction of CF2 with H2 was studied in shock waves over the temperature range 1800–2200 K, monitoring the absorption–time profiles at 248 nm. Besides contributions from CF2, the signals showed strong absorptions from secondary reaction products, probably mostly CH2F formed by the reaction CHF + H2 → CH2F + H. Rate constants of a series of possible secondary reactions were modeled, including falloff curves for the thermal dissociations of CHF, CHF2, and CH2F and rate constants of the reactions CHF + CH2F2 → CHF2 + CH2F, CHF + H2 → CH2F + H, H + CH2F2 → CHF2 + H2, H + CF2 → CF + HF, and H + CF → C + HF. On this basis, concentration–time profiles were simulated and compared with experimental absorption–time profiles.

The thermal decomposition of octafluorooxalane, C4F8O, to C2F4 + CF2 + COF2 has been studied in shock waves highly diluted in Ar between 1300 and 2200 K. The primary dissociation was shown to be followed by secondary dissociation of C2F4 and dimerization of CF2. The primary dissociation was found to be in its falloff range and falloff curves were constructed. The limiting low and high pressure rate constants were estimated and compared with modelling results. Quantum-chemical calculations identified possible reaction pathways, either leading directly to the final products of the reaction or passing through an open-chain CF2CF2CF2 intermediate which dissociates in a second step. Fil: Cobos, Carlos Jorge. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina Fil: Hintzer, K.. Dyneon GmbH; Alemania Fil: Sölter, L.. Universität Göttingen; Alemania Fil: Tellbach, E.. Universität Göttingen; Alemania Fil: Thaler, A.. Dyneon GmbH; Alemania Fil: Troe, J.. Universität Göttingen; Alemania. Max-Planck-Institut für Biophysikalische Chemie; Alemania

Anharmonic rovibrational numbers and densities of resonance states of HO 2 are determined for a number of potential energy surfaces and employing a variety of methods. It is shown that the results present smooth analytical continuations of the expressions derived for the bound states [Part II of this series, J. Troe, V.G. Ushakov, Chem. Phys. 346 (2008) 186] and that no major changes of these expressions arise at the energies where the dissociations to H + O 2 and HO + O open up. The results are also used to test the empirical local mode model of part I of this series [J. Troe, Chem. Phys. 190 (1995) 381] which allows one to approximately predict anharmonic densities of states without using explicit potential energy surfaces.

This article, the third in the series, presents kinetic and photochemical data evaluated by the IUPAC Subcommittee on Gas Kinetic Data Evaluation for Atmospheric Chemistry. It covers the gas phase and photochemical reactions of inorganic halogen species, which were last published in J. Phys. Chem. Ref. Data, in 2000 (Atkinson et al., 2000), were updated on the IUPAC website in 2003 and are updated again in the present evaluation. The article consists of a summary sheet, containing the recommended kinetic parameters for the evaluated reactions, and five appendices containing the data sheets, which provide information upon which the recommendations were made.

There has been considerable speculation about the role of water and water complexes in chemical gas-phase reactions, including the conjecture that water may act as a molecular catalyst through its ability to form hydrogen bonds. Here, we present kinetic studies in which the effect of water on the rate of the reaction between hydroxyl radicals and acetaldehyde has been measured directly in Laval nozzle expansions at low temperatures. An increasing enhancement of the reaction rate by added water was found with decreasing temperatures between 300 and 60 kelvin. Quantum chemical calculations and statistical rate theory support our conclusions that this observation is due to the reduction of an intrinsic reaction barrier caused by specific water aggregation. The results suggest that even single water molecules can act as catalysts in radical-molecule reactions.

This article, the second in the series, presents kinetic and photochemical data evaluated by the IUPAC Subcommittee on Gas Kinetic Data Evaluation for Atmospheric Chemistry. It covers the gas phase and photochemical reactions of Organic species, which were last published in 1999, and were updated on the IUPAC website in late 2002, and subsequently during the preparation of this article. The article consists of a summary table of the recommended rate coefficients, containing the recommended kinetic parameters for the evaluated reactions, and eight appendices containing the data sheets, which provide information upon which the recommendations are made.

Branching ratios for the reactions of O 2 + with all three xylene isomers have been measured as a function of temperature over a wide range (300–900 K) at a fixed number density (1.45 × 10 16 molecule cm −3 of helium) and for m -xylene over an extended buffer gas pressure (50–250 Torr of nitrogen) and temperature (473–623 K) range. Rate constants measured under selected conditions indicate that the reactions proceed at the collisional rate. Two main products were observed in each reaction: the stabilized parent ion, C 8 H 10 + (S) and a dissociative charge transfer product, C 7 H 7 + (D). The ratio of S/D was found to vary significantly with both temperature and pressure. At high pressure very little dissociation occurred. Results of statistical modeling similar to that used in our studies of n -alkylbenzenes represent the data well.

The distribution of translational, rotational, and vibrational energies in the fragments (benzylium ions and ethyl radicals) of the dissociation of n-propylbenzene cations has been determined by statistical adiabatic channel model/classical trajectory (SACM/CT) calculations. The reaction was treated by CT calculations of capture processes for transitional modes, starting with specified fragment energies. A short-range valence/long-range ion-induced dipole potential model for the transitional modes was employed. The derived distributions approach the results from phase space theory (PST) at small energies and angular momenta. At larger energies and angular momenta, the shapes of the distribution functions remain similar to those from PST; however, the average translational, rotational, and vibrational energies of the fragments increasingly differ from PST predictions. The present results are consistent with separate SACM/CT calculations on the same potential of specific rate constants k(E,J) and thermally averaged rate constants k ∞(T) of the dissociation/recombination reaction.

The kinetics of the pyrolysis of the ethylbenzene cation, C 8 H 10 + , has been studied in a turbulent ion flow tube (TIFT) from 623 to 673 K and at pressures from 30 to 250 Torr. The ions were prepared by the charge transfer reaction O 2 + + C 8 H 10 → O 2 + C 8 H 10 +* followed by collisional stabilization and then by thermal decomposition in a nitrogen buffer gas. The thermal decomposition rate constants increased with temperature from about 15 to150 s −1 , but did not vary with pressure, indicating the results refer to the high pressure limit. The experimental activation energy, 157.8 kJ mol −1 , is similar to the bond energy value, 168.3 (±1.2) kJ mol −1 , needed to model the data. Modeling of the system using a statistical adiabatic channel model/classical trajectory (SACM/CT) approach provided complete falloff curves for the dissociation and recombination of ethylbenzene. Similar modeling is also presented for the previously published data on n -propylbenzene cations. The temperature and pressure dependences of the rate coefficients for dissociation and recombination in the falloff range are represented in analytical form. The chosen format corresponds to that employed in data compilations for the corresponding neutral reaction systems.

Benzylium versus tropylium ion yields from the fragmentation of ethylbenzene cations at various excitation energies are studied by forming excited ethylbenzene cations by charge transfer from a series of chargetransfer agents and by identifying the benzylium ion by its secondary reaction with neutral ethylbenzene. At lower excitation energies, the tropylium ion yield decreases with increasing energy from values near 16% (at an energy of 230 kJ mol -1 ) to 5% (at an energy of 500 kJ mol -1 ). At higher excitation energies, the tropylium ion yield increases again, which is attributed to secondary isomerization of the vibrationally highly excited benzylium ion arising from the primary fragmentation. It is suggested that this isomerization competes with radiative cooling of the excited benzylium ion. The experimental observations are rationalized in the framework of statistical unimolecular rate theory and electronic structure calculations.

The thermal dissociation of octafluorocyclobutane, c-C4F8, was studied in shock waves over the range 1150-2300 K by recording UV absorption signals of CF2. It was found that the primary reaction nearly exclusively produces 2 C2F4 which afterwards decomposes to 4 CF2. A primary reaction leading to CF2 + C3F6 is not detected (an upper limit to the yield of the latter channel was found to be about 10 percent). The temperature range of earlier single pulse shock wave experiments was extended. The reaction was shown to be close to its high pressure limit. Combining high and low temperature results leads to a rate constant for the primary dissociation of k1 = 1015.97 exp(-310.5 kJ mol-1/RT) s-1 in the range 630-1330 K, over which k1 varies over nearly 14 orders of magnitude. Calculations of the energetics of the reaction pathway and the rate constants support the conclusions from the experiments. Also they shed light on the role of the 1,4-biradical CF2CF2CF2CF2 as an intermediate of the reaction., Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas

The thermal decomposition of H2O2 was studied behind reflected shock waves using absorption spectroscopy at 215 nm with light from a laser source and at 215, 230, and 290 nm with light from a UV lamp. Due to the improved detection sensitivity for H2O2 and HO2, rate constants for the reactions H2O2 (+Ar) → 2HO (+Ar) (1), HO2 + HO2 → H2O2 + O2 (3) and HO + HO2 → H2O + O2 (4) could be derived with much better precision than possible in earlier work. Rate constant minima for reactions (3) and (4) near 800 and 1100 K, respectively, were reconfirmed. Falloff curves of reaction (1) near to the low pressure limit were measured in experiments at about 1, 4, and 15 bar. Limiting low pressure rate constants for reaction (1) of about k1,0 = [Ar]1016.36±0.23exp[−21 962(±608) K/T] cm3 mol−1 s−1 were obtained over the temperature range 950–1250 K. The deviations from the low pressure limit at relatively low pressures may provide experimental evidence for non-exponential lifetime distributions of dissociative H2O2 such as recently observed in classical trajectory calculations on the ab initio potential of H2O2.

Specific rate consts. k(E) of the photoisomerization of four isotopomers of trans-stilbene, of 4-chloro-(4CS) and 4-methyl-trans-stilbene (4MS), of all-trans-1,4-diphenyl-1,3-butadiene (DPB), and of 4-(dimethylamino)-4'-cyanostilbene (DCS) are represented by conventional RRKM theory, using new ab initio calcns. of excited state frequencies. Data from supersonic beam expts. are reproduced quant. when the barrier heights are fitted to the expts. and activated complex frequencies are taken unchanged from the parent mols. The main difference to earlier work lies in the smaller frequencies now calcd. for the reaction coordinates. The results of this work clearly support an adiabatic mechanism of the reaction which can very well be quantified by statistical unimol. rate theory. [on SciFinder(R)]

The potential energy surface of the reaction H+O2→HO2 is characterized by ab initio calculations. Based on these results, classical trajectory calculations of the capture of T, D, H, and muonium by O2 are made. Thermal rate constants as well as energy- and angular momentum-specific rate constants are determined. The calculated high pressure recombination rate constants at 300 K agree well with the limited experimental information available so far which gives confidence in the computed temperature dependence (calculations over the temperature range 30 to 5000 K). The calculated rate coefficients are represented in comparison to results from phase space theory which leads to the rigidity factors of the reaction. Markedly nonadiabatic dynamics is observed in the case of muonium+O2 while the reaction T+O2 approaches adiabatic dynamics. The observed dynamic isotope effect is most pronounced at low temperatures whereas it nearly disappears at high temperatures.

The thermal decomposition reactions of CF3CF2H and CF3CFHCF3 have been studied in shock waves by monitoring the appearance of CF2 radicals. Temperatures in the range 1400–2000 K and Ar bath gas concentrations in the range (2–10) × 10−5 mol cm−3 were employed. It is shown that the reactions are initiated by C–C bond fission and not by HF elimination. Differing conclusions in the literature about the primary decomposition products, such as deduced from experiments at very low pressures, are attributed to unimolecular falloff effects. By increasing the initial reactant concentrations in Ar from 60 to 1000 ppm, a retardation of CF2 formation was observed while the final CF2 yields remained close to two CF2 per C2F5H or three CF2 per C3F7H decomposed. This is explained by secondary bimolecular reactions which lead to comparably stable transient species like CF3H, releasing CF2 at a slower rate. Quantum-chemical calculations and kinetic modeling help to identify the reaction pathways and provide estimates of rate constants for a series of primary and secondary reactions in the decomposition mechanism., Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas

Systematic fluorescence lifetime measurements of trans-stilbene in bath gases nitrogen, methane, ethane and propane at pressures between 1 mbar and 10 bar were analysed by comparison with master equation simulations. The results show that the low-pressure regime of the unimolecular photoisomerization reaction is located well below 1 bar. The increase in the isomerization rate coefficient observed with further increasing pressure contradicts the basic assumption of the Lindemann model and is discussed in terms of the solvent-induced changes of the potential energy surface. The role of restricted intramolecular vibrational energy redistribution (IVR) in determining the observed photoisomerization dynamics is clearly of minor importance.

A uniform and systematic approach to the calculation of the capture rate constant is presented in the case of most typical long-range intermolecular interactions. We consider ion-linear dipole rotor, ion-axially symmetric quadrupole rotor and two dipole linear rotors with additional isotropic ion-induced dipole and induced dipole-induced dipole interactions. The results for the capture rate constants are given in analytical form for nonadiabatic and adiabatic capture of classical as well as quantum rotors in very wide range of dynamic and kinematic parameters of the capture events.

The thermal dissociation reaction C2F4(+ M) → 2CF2(+ M) was studied in shock waves monitoring CF2 radicals by their UV absorption. The absorption coefficients as functions of wavelength and temperature were redetermined and are represented in analytical form. Dissociation rate constants as functions of bath gas concentration [M] and temperature, from previous and the present work, are presented analytically employing falloff expressions from unimolecular rate theory. Equilibrium constants are determined between 1200 and 1500 K. The data are shown to be consistent, with a C–C bond energy of 67.5 (±0.5) kcal mol–1. High-pressure limiting rate constants for dissociation and recombination are found to be unusually small. This phenomenon can be attributed to an unusually pronounced anisotropy of the potential energy surface, such as demonstrated by quantum-chemical calculations of the potential energy surface. Fil: Cobos, Carlos Jorge. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; Argentina Fil: Croce, Adela Ester. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; Argentina Fil: Luther, K.. Universität Göttingen. Institut für Physikalische Chemie; Alemania Fil: Sölter, L.. Universität Göttingen. Institut für Physikalische Chemie; Alemania Fil: Tellbach, E.. Universität Göttingen. Institut für Physikalische Chemie; Alemania Fil: Troe, J.. Universität Göttingen. Institut für Physikalische Chemie; Alemania. Max-Planck-Institut für Biophysikalische Chemie; Alemania

Within the adiabatic channel treatment of ionmolecule capture we have calculated the low temperature capture rate constants of N2(1Σg+) and O2(3Σg−)in collisions with positive and negative ions. In both cases, the charge-quadrupole and the anisotropic charge-induced dipole interactions produce noticeable deviations from the Langevin rate constant. The rate constants calculated with account for the anisotropic interaction, in addition, are substantially affected by nuclear symmetry; in the case of O2(3Σg−), the fine-structure spin-spin interaction strongly manifests itself in the rate constants.

The reaction H+O2 (+Ar)→HO2 (+Ar) was studied in a high pressure flow cell in the bath gas argon at pressures between 1 and 900 bar and temperatures between 300 and 700 K. H atoms were generated by laser flash photolysis of NH3 at 193.3 nm, HO2 radicals were monitored by light absorption at 230 nm. The results are consistent with experimental low pressure rate constants k0=[Ar] 2.5×10−32(T/300 K)−1.3 cm6 molecule−2 s−1 and theoretical high pressure rate constants k∞=9.5×10−11(T/300 K)+0.44 cm3 molecule−1 s−1 from the literature. The intermediate falloff curve was found to be best represented by k/k∞=[x/(1+x)]Fcent1/(1+(a+logx)2/(N±ΔN)2) with x=k0/k∞, a≈0.3, N≈1.05, ΔN≈0.1 (+ΔN for (a+logx) 0), and Fcent(Ar)≈0.5 independent of the temperature. A comparison with literature data between 300 and 1200 K does not confirm major deviations from third order kinetics in earlier medium pressure experiments.

This article, the sixth in the ACP journal series, presents data evaluated by the IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation. It covers the heterogeneous processes involving liquid particles present in the atmosphere with an emphasis on those relevant for the upper troposphere/lower stratosphere and the marine boundary layer, for which uptake coefficients and adsorption parameters have been presented on the IUPAC website since 2009. The article consists of an introduction and guide to the evaluation, giving a unifying framework for parameterisation of atmospheric heterogeneous processes. We provide summary sheets containing the recommended uptake parameters for the evaluated processes. The experimental data on which the recommendations are based are provided in data sheets in separate appendices for the four surfaces considered: liquid water, deliquesced halide salts, other aqueous electrolytes and sulfuric acid.

Fine structure and hyperfine structure effects can influence the kinetics of unimolecular and complex-forming bimolecular reactions. The corresponding energetic effects are analyzed in the framework of the statistical adiabatic channel model (SACM). The reactions O+OH O2 + H, OH + CO H + CO2, C+ + OH products, C+ + NO products, and NO2 O + NO serve as examples.

Numbers and densities of vibrational states of non-rotating polyatomic molecules are calculated by a simplified model which accounts for Morse anharmonicities of stretching and for generalized empirical stretch-bend couplings of bending modes. At energies above the dissociation limit, adiabatic channel maxima restrict the molecular phase space. The model is applied to the NO 2 (X 2A 1 ), NO 2 (A 2 B 2 ), and H 3 + systems for which satisfactory agreement with the available experimental or calculational results is obtained.

Compact approximate equations for rovibrational densities of states ρ(E,J) for harmonic oscillator-rigid rotor molecular systems are derived. Linear molecules, spherical tops, prolate and oblate symmetric tops are considered. Vibrational densities of states are represented by the Whitten-Rabinovitch approximation which then is convoluted with the contribution from the K-rotor (for a symmetric top). Likewise, numbers of states W(E,J) are approximated for prolate symmetric top molecules. The derived expressions allow for quick calculations of specific rate constants k(E,J) of unimolecular reactions.

The thermal unimolecular decay of the benzyl radical has been investigated extensively by several groups. However, the reaction products could not be determined unambiguously. In this work the unimolecular bond fission of the benzyl radical is studied in a molecular beam experiment. The precursor molecules toluene and cycloheptatriene are expanded in a molecular beam and photodissociated with two photons at 248 or 193 nm, yielding in each case hot benzyl radicals. Since the internal energies lie above the dissociation limit, the benzyl radicals decay in a subsequent step. The reaction products are detected in a time-resolved manner with a quadrupole mass spectrometer on the molecular beam axis at low electron energies. The measured time-of-flight spectra provide information on the translational energy distribution of the products. In each case it is found that the hot benzyl radicals C 7 H 7 fragment under hydrogen loss to C 7 H 6 .

We report the first observation of a reaction which, in a single solvent, starts as a barrier crossing process at low pressure and turns into a relaxation on a barrierless potential at high pressure. The pressure and temperature dependences of trans-stilbene photoisomerization in n-alkanols were investigated by picosecond transient absorption spectroscopy. As in n-alkane solvents, at constant temperature, the rate coefficients k for rotation about the central double bond in each solvent exhibit a fractional power dependence on solvent viscosity η, k≈η−α, with 0 < α ⩽ 1. α varies little with solvent, but increases with temperature. This observation is discussed in terms of a solvent shift effect which causes with increasing solvent density a lowering of the barrier height. In n-propanol at high pressure the barrier height approaches zero. Even under these conditions, however, the observed decay dynamics remain monoexponential.

Anharmonic vibrational numbers and densities of states are calculated by quantum-corrected Monte Carlo integration of phase space volumes on ab initio potential energy surfaces for HO2, H2CO, and H2O2. Rotational dependences of the anharmonicity are also determined for H2CO. The total numbers of states W(E,J = 0) are approximated in analytical form. Using Whitten-Rabinovitch expressions as a reference, anharmonicity factors Fanh,rho(E,J = 0) for the densities of states are also given analytically. The results are used as a benchmark for comparisons with an empirical anharmonicity model (Troe, J. Chem. Phys. 1995, 190, 381). It was found that some modifications of this model are necessary for applications at high energies. The improved empirical model is intended to serve for estimates of anharmonic densities of states of larger molecular systems at high energies.

The rotational relaxation of t-stilbene and t-diphenylbutadiene in the first electronically excited singlet state was measured by picosecond anisotropic absorption spectroscopy. The viscosity η in n-alkane solvents was changed by increasing the pressure up to several kilobars. In all cases the viscosity dependence of the rotational relaxation time τor could be described by a modified form of the Stokes-Einstein-Debye equation. No solvent dependence was found for η ≤ 0.8 cP. Therefore we attribute the observed solvent dependence of photoisomerisation rates of t-stilbene and t-diphenylbutadiene measured under the same conditions to specific solvent-solute interactions that modify the path on the potential energy surface.

Recent experimental results on the thermal decomposition and the reverse recombination of alkyl and haloalkyl peroxynitrates are modelled with an SACM formalism. The molecules RO 2 NO 2 with R=CH 3 , CF 3 , CF 2 Cl, CFCl 2 , CCl 3 , and C 2 H 5 are considered. Detailed and simplified reduced falloff expressions are compared. Limiting low (k 0 ) and high pressure (k x ) rate constants, such as derived from a fit of these fallooff expressions to the experiments, are compared with absolute predictions, based on general knowledge about energy transfer ((△E) in k 0 ) on a recently proposed simple SACM estimate for k ∞ . The analysis also allows for a derivation of the bond energies of the mentioned molecules

This article, the fourth in the series, presents kinetic and photochemical data sheets evaluated by the IUPAC Subcommittee on Gas Kinetic Data Evaluation for Atmospheric Chemistry. It covers the gas phase and photochemical reactions of organic halogen species, which were last published in 1997, and were updated on the IUPAC website in 2006/07. The article consists of a summary sheet, containing the recommended kinetic parameters for the evaluated reactions, and four appendices containing the data sheets, which provide information upon which the recommendations are made.

The thermal dissociation of formaldehyde proceeds on three channels, the molecular-elimination channel H2CO --> H2 + CO (1), the radical-forming bond-fission channel H2CO --> H + HCO (2), and the bond-fission-initiated, intramolecular-hydrogen-abstraction channel H2CO --> H...HCO --> H2 + CO (3) which also forms molecular products. The kinetics of this system in the low-pressure range of the unimolecular reaction is shown to be governed by a subtle superposition of collisional channel coupling to be treated by solving a master equation, of rotational channel switching accessible through ab initio calculations of the potential as well as spectroscopic and photophysical determinations of the threshold energies and channel branching above the threshold energy for radical formation which can be characterized through formaldehyde photolysis quantum yields as well as classical trajectory calculations. On the basis of the available information, the rate coefficients for the formation of molecular and radical fragments are analyzed and extrapolated over wide ranges of conditions. The modeled rate coefficients in the low-pressure range of the reaction (neglecting tunneling) over the range 1400-3200 K in the bath-gas Ar in this way are represented by k0,Mol/[Ar] approximately 9.4 x 10(-9) exp(-33,140 K/T) cm3 molecule(-1) s(-1) and k0,Rad/[Ar] approximately 6.2 x 10(-9) exp(-36,980 K/T) cm3 molecule(-1) s(-1). The corresponding values for the bath-gas Kr, on which the analysis relies in particular, are k0,Mol/[Kr] approximately 7.7 x 10(-9) exp(-33,110 K/T) and k0,Rad/[Kr] approximately 4.1 x 10(-9) exp(-36 910 K/T) cm3 molecule(-1) s(-1). While the threshold energy E0,2 for channels 2 and 3 is taken from spectroscopic measurements, the threshold energy E0,1 for channel 1 is fitted on the basis of experimental ratios k0,Rad/k0,Mol in combination with photolysis quantum yields. The derived value of E0,1(1) = 81.2 (+/-0.9) kcal mol(-1) is in good agreement with results from recent ab initio calculations, 81.9 (+/-0.3) kcal mol(-1), but is higher than earlier results derived from photophysical experiments, 79.2 (+/-0.8) kcal mol(-1). Rate coefficients for the high-pressure limit of the reaction are also modeled. The results of the present work markedly depend on the branching ratio between channels 2 and 3. Expressions of this branching ratio from classical trajectory calculations and from photolysis quantum yield measurements were tested. At the same time, a modeling of the photolysis quantum yields was performed. The formaldehyde system so far presents the best characterized multichannel dissociation reaction. It may serve as a prototype for other multichannel dissociation reactions.

This article, the second in the series, presents kinetic and photochemical data evaluated by the IUPAC Subcommittee on Gas Kinetic Data Evaluation for Atmospheric Chemistry. It covers the gas phase and photochemical reactions of Organic species, which were last published in 1999, and were updated on the IUPAC website in late 2002. The article consists of a summary sheet, containing the recommended kinetic parameters for the evaluated reactions, and eight appendices containing the data sheets, which provide information upon which the recommendations are made.

The asymptotic interactions at large intermolecular distances are determined for two open-shell systems, H(X2S1/2) + O2 (X3∑{skg/−}) and \( O({}^3P_{j_O } ) + OH(X^2 \Pi _{\tilde \Omega } )\) for fixed values of intramolecular distances r. The electronic diabatic Hamiltonians are set up for two purposes: i) the direct diagonalization of the electronic Hamiltonian yielding two-dimensional potential energy surfaces (PES) which depend on the intermolecular distance R and the angle γ. between R and r, and ii) the incorporation of the diabatic electronic basis into the diabatic roronic basis which can be used in the construction of the roronic Hamiltonian in the total angular momentum representation. The former procedure allows one to compare the asymptotic PES with their ab initio counterparts, while the latter provides the input data for the calculation of low temperature capture rate constants within the statistical adiabatic channel model (SACM).

Limiting high pressure rate constants for the recombination reaction H+O(2)--HO(2) are modeled between 0 and 5000 K on an ab initio potential. Quantum capture theory is employed for the temperature range from 0 to about 1 K, while classical trajectory calculations are suitable for covering temperatures above about 200 K. The intermediate temperature range is analyzed by adiabatic channel capture theory. The system is characterized by transition-state switching from outer transition states in the long-range-C(6)R(6) potential to inner transition states in the range of a "shoulder" of the potential. The limiting high pressure rate constants from the trajectory calculations are sufficient for comparison with the experimental data which are available over the range from 300 to 900 K. Specific rate constants k(E,J) for HO(2) dissociation are also given and analyzed with respect to internal consistency with capture cross sections.