Matthias Olzmann

Infrared Detection of Criegee Intermediates Formed during the Ozonolysis of β‐Pinene and Their Reactivity towards Sulfur Dioxide

Recently, direct kinetic experiments have shown that the oxidation of sulfur dioxide to sulfur trioxide by reaction with stabilized Criegee intermediates (CIs) is an important source of sulfuric acid in the atmosphere. So far, only small CIs, generated in photolysis experiments, have been directly detected. Herein, it is shown that large, stabilized CIs can be detected in the gas phase by FTIR spectroscopy during the ozonolysis of β-pinene. Their transient absorption bands between 930 and 830 cm(-1) appear only in the initial phase of the ozonolysis reaction when the scavenging of stabilized CIs by the reaction products is slow. The large CIs react with sulfur dioxide to give sulfur trioxide and nopinone with a yield exceeding 80%. Reactant consumption and product formation in time-resolved β-pinene ozonolysis experiments in the presence of sulfur dioxide have been kinetically modeled. The results suggest a fast reaction of sulfur dioxide with CIs arising from β-pinene ozonolysis.

Shock-Tube Study of the Thermal Decomposition of CH3CHO and CH3CHO + H Reaction

The thermal decomposition of acetaldehyde, CH3CHO + M --> CH3 + HCO + M (eq 1), and the reaction CH3CHO + H --> products (eq 6) have been studied behind reflected shock waves with argon as the bath gas and using H-atom resonance absorption spectrometry as the detection technique. To suppress consecutive bimolecular reactions, the initial concentrations were kept low (approximately 10(13) cm(-3)). Reaction was investigated at temperatures ranging from 1250 to 1650 K at pressures between 1 and 5 bar. The rate coefficients were determined from the initial slope of the hydrogen profile via k1 = [CH3CHO]0(-1) x d[H]/dt, and the temperature dependences observed can be expressed by the following Arrhenius equations: k1(T, 1.4 bar) = 2.9 x 10(14) exp(-38 120 K/T) s(-1), k1(T, 2.9 bar) = 2.8 x 10(14) exp(-37 170 K/T) s(-1), and k1(T, 4.5 bar) = 1.1 x 10(14) exp(-35 150 K/T) s(-1). Reaction was studied with C2H5I as the H-atom precursor under pseudo-first-order conditions with respect to CH3CHO in the temperature range 1040-1240 K at a pressure of 1.4 bar. For the temperature dependence of the rate coefficient the following Arrhenius equation was obtained: k6(T) = 2.6 x 10(-10) exp(-3470 K/T) cm(3) s(-1). Combining our results with low-temperature data published by other authors, we recommend the following expression for the temperature range 300-2000 K: k6(T) = 6.6 x 10(-18) (T/K) (2.15) exp(-800 K/T) cm(3) s(-1). The uncertainties of the rate coefficients k1 and k6 were estimated to be +/-30%.

Accurate computational determination of the binding energy of the SO3∙H2O complex

Reliable thermochemical data for the reaction SO3 + H2O<-->SO3 x H2O (1a) are of crucial importance for an adequate modeling of the homogeneous H2SO4 formation in the atmosphere. We report on high-level quantum chemical calculations to predict the binding energy of the SO3 x H2O complex. The electronic binding energy is accurately computed to De = 40.9+/-1.0 kJ/mol = 9.8+/-0.2 kcal/mol. By using harmonic frequencies from density functional theory calculations (B3LYP/cc-pVTZ and TPSS/def2-TZVP), zero-point and thermal energies were calculated. From these data, we estimate D0 = -Delta H(1a)0(0 K) = 7.7+/-0.5 kcal/mol and Delta H(1a)0(298 K) = -8.3+/-1.0 kcal/mol.

Reaction of Dimethyl Ether with Hydroxyl Radicals: Kinetic Isotope Effect and Prereactive Complex Formation

The kinetic isotope effect of the reactions OH + CH3OCH3 (DME) and OH + CD3OCD3 (DME-d6) was experimentally and theoretically studied. Experiments were carried out in a slow-flow reactor at pressures between 5 and 21 bar (helium as bath gas) with production of OH by laser flash photolysis of HNO3 and time-resolved detection of OH by laser-induced fluorescence. The temperature dependences of the rate coefficients obtained can be described by the following modified Arrhenius expressions: k(OH+DME) = (4.5 ± 1.3) × 10(-16) (T/K)(1.48) exp(66.6 K/T) cm(3) s(-1) (T = 292-650 K, P = 5.9-20.9 bar) and k(OH+DME-d6) = (7.3 ± 2.2) × 10(-23) (T/K)(3.57) exp(759.8 K/T) cm(3) s(-1) (T = 387-554 K, P = 13.0-20.4 bar). A pressure dependence of the rate coefficients was not observed. The agreement of our experimental results for k(OH+DME) with values from other authors is very good, and from a fit to all available literature data, we derived the following modified Arrhenius expression, which reproduces the values obtained in the temperature range T = 230-1500 K at pressures between 30 mbar and 21 bar to better than within ±20%: k(OH+DME) = 8.45 × 10(-18) (T/K)(2.07) exp(262.2 K/T) cm(3) s(-1). For k(OH+DME-d6), to the best of our knowledge, this is the first experimental study. For the analysis of the reaction pathway and the kinetic isotope effect, potential energy diagrams were calculated by using three different quantum chemical methods: (I) CCSD(T)/cc-pV(T,Q)Z//MP2/6-311G(d,p), (II) CCSD(T)/cc-pV(T,Q)Z//CCSD/cc-pVDZ, and (III) CBS-QB3. In all three cases, the reaction is predicted to proceed via a prereaction OH-ether complex with subsequent intramolecular hydrogen abstraction and dissociation to give the methoxymethyl radical and water. Overall rate coefficients were calculated by assuming a thermal equilibrium between the reactants and the prereaction complex and by calculating the rate coefficients of the hydrogen abstraction step from canonical transition state theory. The results based on the molecular data from methods (I) and (II) showed a satisfactory agreement with the experimental values, which indicates that the pre-equilibrium assumption is reasonable under our conditions. In the case of method (III), the isotope effect was significantly underpredicted. The reason for this discrepancy was identified in a fundamentally differing reaction coordinate. Obviously, the B3LYP functional applied in method (III) for geometry and frequency calculations is inadequate to describe such systems, which is in line with earlier findings of other authors.

The rate coefficient of the C3H3 + C3H3 reaction from UV absorption measurements after photolysis of dipropargyl oxalate

The kinetics of the C3H3 + C3H3 reaction was investigated by using dipropargyl oxalate (DPO) as a new, halogen-free photolytic source for propargyl radicals in the gas phase. After laser-flash photolysis of DPO at 193 nm, the initial absorbance was determined at different wavelengths, and the results were compared with values obtained in analogous experiments using propargyl halides as precursors. A satisfactory agreement of the absorbances was found between 295 and 355 nm but differences were observed near 242 nm. The latter wavelength has also been proposed for C3H3 detection. Our results, however, indicate that this absorption is probably due to halogen-containing species. The rate coefficient of the C3H3 + C3H3 reaction was then determined from time-resolved absorption measurements at 332.5 nm with DPO as precursor. Values of (2.7 ± 0.6) × 10−11 cm3 molecule−1 s−1 at 373 K, (2.8 ± 0.6) × 10−11 cm3 molecule−1 s−1 at 425 K, (3.5 ± 0.8) × 10−11 cm3 molecule−1 s−1 at 500 K, and (4.1 ± 0.8) × 10−11 cm3 molecule−1 s−1 at 520 K were obtained with no significant pressure dependence between 1 and ca. 100 bar (140 bar for T = 373 K).

The Reaction Rates of O 2 with Closed-Shell and Open-Shell Al x − and Ga x − Clusters under Single-Collision Conditions: Experimental and Theoretical Investigations toward a Generally Valid Model for the Hindered Reactions of O 2 with Metal Atom Clusters

In order to characterize the oxidation of metallic surfaces, the reactions of O2 with a number of Al(x)(-) and, for the first time, Ga(x)(-) clusters as molecular models have been investigated, and the results are presented here for x = 9-14. The rate coefficients were determined with FT-ICR mass spectrometry under single-collision conditions at O2 pressures of ~10(-8) mbar. In this way, the qualitatively known differences in the reactivities of the even- and odd-numbered clusters toward O2 could be quantified experimentally. To obtain information about the elementary steps, we additionally performed density functional theory calculations. The results show that for both even- and odd-numbered clusters the formation of the most stable dioxide species, [M(x)O2](-), proceeds via the less stable peroxo species, [M(x)(+)···O2(2-)](-), which contains M-O-O-M moieties. We conclude that the formation of these peroxo intermediates may be a reason for the decreased reactivity of the metal clusters toward O2. This could be one of the main reasons why O2 reactions with metal surfaces proceed more slowly than Cl2 reactions with such surfaces, even though O2 reactions with both Al metal and Al clusters are more exothermic than are reactions of Cl2 with them. Furthermore, our results indicate that the spin-forbidden reactions of (3)O2 with closed-shell clusters and the spin-allowed reactions with open-shell clusters to give singlet [M(x)(+)···O2(2-)](-) are the root cause for the observed even/odd differences in reactivity.

Mechanisms and rates of the reactions C2H5+O and 1-C3H7+O

The mechanisms and rates of the reactions of the primary alkyl radicals ethyl and l-propyl with oxygen atoms at room temperature and low pressure (around 5 mbar) have been studied using two independent experimental arrangements. The reactants were generated by UV-laser flash photolysis with different precursors (C 2 H 5 COC 2 H 5 , C 2 H 6 +CFCl 3 , C 2 H 5 I, C 3 H 7 COC 3 H 7 , SO 2 ). Stable species concentrations were measured quantitatively by Fourier transform IR and OH radical concentrations of the ground ( v =0) and first vibrational ( v =1) state by time-resolved laser-induced fluorescence. For both reaction 1 and reaction 2, the mechanism is explained in terms of the formation and subsequent decomposition of a chemically activated alkoxy radical and a competing abstraction channel leading directly to OH and the alkene: C 2 H 5 +O→C 2 H 5 (reaction Ia)/C 2 H 5 O→HCHO+CH 3 (reaction la 1 )/CH 3 CHO+H (reaction 1a 2 )//C 2 H 5 +O→C 2 H 4 +OH (reaction 1b). The absolute branching ratio was determined preferentially using diethyl ketone as the C 2 H 5 radical source leading to (1a 2 )/(1a 2 )/(1b), 32/44/24. Relative branching ratios for the C 2 H 5 radical sources C 2 H 6 +Cl and C 2 H 5 I were derived as (1a 1 /(1a 2 )=1/1.5 and 1/1.55, respectively. The overall rate coefficient of the reaction C 2 H 5 +O was measured as k 1 =(1.04±0.1)×10 14 cm 3 mol −1 s −1 and in addition k (C 2 H 5 +OH)=(7.0±1)×10 13 cm 3 mol −1 s −1 . The mechanism and the rate of reaction 2 were found as 1-C 3 H 7 +O→1-C 3 H 7 O (reaction 2a)/1-C 3 H 7 O→HCHO+C 2 H 5 (reaction 2a 1 )/C 2 H 5 CHO+H (reaction 2a 2 )//1.C 3 H 7 +O→C 3 H 6 +OH (reaction 2b) (branching ratio (2a 1 )/(2b), 44/32/24 and k 2 =(8.2±1)×10 3 mo −1 s −1 The results are discussed in terms of statistical rate theory.

Heat of formation of the HOSO2 radical from accurate quantum chemical calculations

The reaction HOSO(2)+O(2)-->HO(2)+SO(3) (2) is of crucial importance for sulfuric acid formation in the atmosphere, and reliable thermochemical data are required for an adequate modeling. The currently least well known thermochemical quantity of reaction (2) is the enthalpy of formation of the hydroxysulfonyl radical (HOSO(2)). We report on high-level quantum chemical calculations to predict the binding energy of the HO-SO(2) bond and deduce a value for the enthalpy of formation of HOSO(2) using the most reliable thermodynamic data of OH and SO(2). On the basis of anharmonic vibrational frequencies from calculations at the fc-CCSD(T)/cc-pV(T+d)Z level of theory, the enthalpy of reaction at 0 K for the reaction OH+SO(2)-->HOSO(2) (1) was computed to be Delta(R)H(0 K)(1)=-109.4+/-2.0 kJ/mol and the thermal corrections result in Delta(R)H(298 K)(1)=-114.7+/-3.0 kJ/mol. From these values, we obtain Delta(f)H(0 K)(HOSO(2))=-366.6+/-2.5 and Delta(f)H(298 K)(HOSO(2))=-374.1+/-3.0 kJ/mol, respectively. Accordingly, Delta(R)H(0 K)(2)=-8.5+/-3.0 and Delta(R)H(298 K)(2)=-9.5+/-3.0 kJ/mol.

Shock-Tube Study of the Reactions of Hydrogen Atoms with Benzene and Phenyl Radicals

Abstract The reactions of hydrogen atoms with phenyl radicals, H + C6H5 → products (1), and with benzene, H + C6H6 → products (2), have been studied behind reflected shock waves in the temperature range 1200–1350 K with argon as the bath gas. H-atom resonance absorption spectrometry at 121.6 nm was used as detection technique. Hydrogen atoms and phenyl radicals were produced by thermal decomposition of C2H5I and C6H5I, respectively. Low initial concentrations (~1012–1015 cm-3) were employed to suppress consecutive bimolecular reactions as far as possible.The rate coefficients were determined from fits of the H atom concentration-time profiles in terms of a small mechanism. For reaction (1), a temperature-independent rate coefficient k1 = 1.3×10–10 cm3 s–1 was obtained at pressures around 1.3 bar. For the rate coefficient of reaction (2), the temperature dependence can be expressed as k2(T) = 5.8×10–10 exp(–8107 K/T) cm3 s–1, and a pressure dependence was not observed between 1.3 and 4.3 bar. The uncertainties of k1 and k2 were estimated to be ±40%.

On the role of bimolecular reactions in chemical activation systems

The kinetics of chemically activated intermediates in complex-forming bimolecular reactions can often be described in terms of the branching ratio between their unimolecular decomposition channels and the collisional stabilization under steady-state conditions (D/S). This requires that the stabilized part of the population does not contribute to the decomposition rate, which is true only for reaction times shorter than the thermal lifetime of the intermediate. As these intermediates, however, are often radicals or other highly reactive species, they can undergo consecutive bimolecular reactions. Here two limiting cases are conceivable: (i) If the bimolecular steps are slow, mainly stabilized intermediates react, and their population is prevented from being built up, i.e. their thermal decomposition is suppressed. In this case the steady-state description in terms of D/S remains adequate also for reaction times longer than the thermal lifetime of the intermediate. (ii) If the bimolecular reactions are extremely fast, even highly excited intermediates react and are removed from the population. Consequently, the observed relative yield of the unimolecular channels, Φ, drops below the steady-state value Φss=[1+(D/S)−1]−1. The branching between decomposition and stabilization of a chemically activated intermediate, therefore, can depend on the rate of its bimolecular consecutive reactions, a fact which is often overlooked in the kinetic analysis of such systems. In the present work we investigate the dependence of Φ on the rate of these bimolecular steps in a general way by means of a master equation. A simple method is presented to estimate the parameter range, where the steady-state approach in terms of D/S remains adequate if bimolecular reactions of the intermediate occur. The problem is exemplarily treated for the reaction between chemically activated but-2-yl radicals and hydrogen atoms.