Horst Hippler

High pressure range of addition reactions of HO. II. Temperature and pressure dependence of the reaction HO+CO⇔HOCO→H+CO2

Thermal rate constants of the complex‐forming bimolecular reaction HO+CO■HOCO→H+CO2 were measured between 90 and 830 K in the bath gas He over the pressure range 1–700 bar. In addition, the vibrational relaxation of HO in collisions with CO was studied between 300 and 800 K. HO was generated by laser photolysis and monitored by saturated laser‐induced fluorescence. The derived second‐order rate coefficients showed a pronounced pressure and complicated non‐Arrhenius temperature dependence. Above 650 K, the disappearance of HO followed a biexponential time law, indicating thermal instability of collisionally stabilized HOCO. By analyzing the corresponding results, an enthalpy of formation of HOCO of ΔHof,0=−(205±10) kJ mol−1 was derived. On the basis of energy‐ and angular‐momentum‐dependent rates of HOCO formation, activated complex properties for the addition reaction HO+CO→HOCO were derived from the limiting high‐pressure rate constants; with the limiting low‐pressure rate constants, activated complex pr...

The thermal unimolecular decomposition rate constants of ethoxy radicals

We experimentally determined complete falloff curves of the rate constant for the unimolecular decomposition of ethoxy radicals. Two different techniques, laser flash photolysis and fast flow reactor were used both coupled to a detection of C2H5O radicals by laser induced fluorescence. Experiments were performed at total pressures between 0.001 and 60 bar of helium and in the temperature range of 391–471 K. Under these conditions the β-C–C scission (1a) CH3CH2O+M→CH2O+CH3+M is the dominating decomposition channel. From a complete analysis of the experimental falloff curves the low and the high pressure limiting rate constants of k1a,0=[He] 3.3×10-8 exp(-58.5 kJ mol-1/RT) cm3 s-1 and k1a,∞=1.1×1013 exp(-70.3 kJ mol-1/RT) s-1 were extracted. We estimate an uncertainty for the absolute values of these rate constants of ±30%. Preexponential factor and activation energy are significantly lower than previous estimations. The rate constants are discussed in terms of statistical unimolecular rate theory. Excellent agreement between the experimental and the statistically calculated rate constants has been found. BAC-MP4, QCISD(T), or higher level of theory provide a reliable picture of the energy and the structure of the transition state of this radical bond dissociation reaction. On the same theoretical basis we predict the high pressure limiting rate constant for the β-C–H scission (1b) CH3CH2O+M→CH3CHO+H+M of k1b,∞=1.3×1013 exp(-84 kJ mol-1/RT) s-1. Atmospheric implications are discussed.

TEMPERATURE AND PRESSURE DEPENDENCE OF THE ADDITION REACTIONS OF HO TO NO AND TO NO2. IV. SATURATED LASER-INDUCED FLUORESCENCE MEASUREMENTS UP TO 1400 BAR

The recombination reactions HO+NO+M⇒HONO+M(1) and HO+NO2+M⇒HNO3+M(2) have been investigated over an extended pressure (1–1000 bar) and temperature (250–400 K) range. HO radicals were generated by laser flash photolysis of suitable precursors and their decays were monitored by saturated laser-induced fluorescence (SLIF) under pseudo-first-order conditions. The measured rate constants were analyzed by constructing falloff curves which provide the high pressure limiting rate constants k∞. In the given temperature range, these rate constants are k1,∞=(3.3±0.5)×10−11×(T/300 K)−(0.3±0.3) and k2,∞=(7.5±2.2)×10−11 cm3 molecule−1 s−1.

The β C–C bond scission in alkoxy radicals: thermal unimolecular decomposition of t-butoxy radicals

The temperature and pressure dependence of the unimolecular decomposition of t-butoxy radicals was studied by the laser photolysis/laser induced fluorescence technique. Experiments have been performed at total pressures between 0.04 and 60 bar of helium and in the temperature range 323–383 K. The low and the high pressure limiting rate constants as well as the broadening factor Fc have been extracted from a complete falloff analysis of the experimental results: k0=[He]×1.5×10−8 exp(−38.5 kJ mol−1/RT) cm3 s−1, k∞=1.0×1014 exp(−60.5 kJ mol−1/RT) s−1, and Fc=0.87−T/870 K. We anticipate an uncertainty for these rate constants of ±30%. Important features of the potential energy surface have been computed by ab initio methods. The Arrhenius parameters for the high pressure limiting rate constant for the β C–C bond scission of t-butoxy radicals have been computed from the properties of a transition state based on the results of G2(MP2) ab initio calculation. The results from density functional theory (DFT) with a small basis set (B3LYP/SVP) are very similar. Excellent agreement between the calculated and the experimental rate constants has been found. We suggest a common pre-exponential factor for β C–C bond scission rate constants of all alkoxy radicals of A=1014±0.3 s−1. Thus we express the high pressure limiting rate constant for ethoxy and i-propoxy radicals by k∞=1.0×1014 exp(−78.2 kJ mol−1/RT) and 1.0×1014 exp(−63.1 kJ mol−1/RT) s−1, respectively. For the reverse reactions, the addition of CH3 radicals to CH2O, CH3CHO, and (CH3)2CO, we obtained activation enthalpies of 32, 42, and 52 kJ mol−1, respectively.

HIGH-PRESSURE RANGE OF THE ADDITION OF HO TO HO. III. SATURATED LASER-INDUCED FLUORESCENCE MEASUREMENTS BETWEEN 200 AND 700 K

The addition of HO to HO was studied by saturated laser induced fluorescence at temperatures between 200 and 700 K and at pressures of the bath gas helium up to 100 bar. In combination with earlier measurements at 298 K, a set of falloff curves is constructed for the given temperature range. The limiting high‐pressure rate constant for the reaction HO+HO(+He)→H2O2(+He) follows as k1,∞=(2.6±0.8)×10−11 (T/300 K)0±0.5 cm3 molecule−1 s−1, practically independent of the temperature between 200 and 400 K. At higher temperatures, k1,∞ decreases. These results serve as a reference for statistical adiabatic channel model calculations of the recombination rate.

A detailed experimental and theoretical study on the decomposition of methoxy radicals

We present a detailed experimental and theoretical study on the pressure and temperature dependence of the rate constant for the thermal unimolecular decomposition of methoxy radicals, according to CH3O + M → CH2O + H + M. Experimentally, we studied the decomposition of the methoxy radical at temperatures between 680 and 810 K and pressures ranging from 1 to 90 bar helium. The methoxy radicals have been generated by laser flash photolysis of methylbenzoate [C6H5C(O)OCH3] at 193 nm and detected by laser-induced fluorescence. Additionally, we characterized the important features of the potential energy surface by ab initio calculations. The results of these calculations were used to analyze the thermal rate constant applying both the Troe formalism as well as a master equation approach. The following falloff parameters have been extracted: k1,∞ = 6.8 × 1013 exp(−109.5 kJ mol−1/RT) s−1, k1,0 = [He] 1.9 × 10−8(T/1000 K)−2.4 exp(−101.7 kJ mol−1/RT) cm3 s−1 and FC(He) = 0.715–T/4340 K. Additionally, we reanalyzed the literature data for N2 as bath gas and we recommend the following falloff parameters for this: k1,0 = [N2] 3.1 × 10−8(T/1000 K)−3.0 exp(−101.7 kJ mol−1/RT) cm3 s−1 and FC(N2) = 0.97–T/1950 K. In contradiction to earlier studies we did not find any indications that tunneling markedly contributes to the thermal rate constant under our experimental conditions. We calculated the specific and the high-pressure limiting rate constants using RRKM theory and obtained satisfactory agreement with experimental results. We attribute the strong fluctuations of the specific rate constants to be essentially caused by the properties of the density of states. For the β C–H scission reactions in alkoxy radicals we suggest for the high-pressure limiting rate constants a common A factor and activation energy of A = 1013.8 ± 0.3 s−1 and Ea = 94 ± 6 kJ mol−1. Consequently, the reverse reactions, i.e. the H-atom additions to the carbon site of the CO π bond in aldehydes and ketones, always compete with the direct H-atom abstraction.

Transient spectra, formation, and geminate recombination of solvated electrons in pure water UV-photolysis: an alternative view

The temporal evolution of the optical absorption of solvated electrons in a neat water jet has been investigated in two pulse femtosecond experiments. A 90 fs (FWHM) UV pulse at 267 nm directly ionized the neat water and the subsequent absorption has been probed by a white light continuum at 10 different wavelengths in the range between 450 and 1000 nm. Due to the thickness of the water jet the time-resolution is limited to about 150 fs. The transient absorption contains not only information on the temporal evolution of the absorption spectrum but also data on the formation and geminate recombination of the solvated electrons. We have used the optical sum rules to separate the temporal evolution of the absorption spectrum from the concentration of the solvated electrons in the time interval between 300 and 100 ps after to photoionization pulse. At ultrashort times the absorption spectra are found in the infrared and undergo a substantial blue-shift during the first few picoseconds. After about 5 ps the absorption spectrum of thermally equilibrated solvated electrons is obtained. Within our time-resolution the data show no evidence of transient electronically excited states of solvated electrons. We interpret the temporal evolution of the absorption spectrum using the optical sum rules and deduce the time dependent decrease of the mean squared dispersion in position (〈Δr2(t)〉) of the electrons. Unmistakably, 〈Δr2(t)〉 is related to the solvation process of electrons in polar fluids and therefore contains the solvation dynamics. In addition, we clearly see for the first time the delayed formation of solvated electrons followed by geminate recombination.

Complete falloff curves for the unimolecular decomposition of i-propoxy radicals between 330 and 408 K

The temperature and pressure dependence of the rate constant for the unimolecular decomposition of i-propoxy radicals has been determined using the laser photolysis/laser induced fluorescence technique. Important features of the potential energy surface have been calculated by abinitio methods. Experiments have been performed at total pressures between 0.01 and 60 bar of helium and in the temperature range 330–408 K. The low and the high pressure limiting rate constants have been extracted from a complete falloff analysis: k0=[He]×1.0×10-8 exp(-43.8 kJ mol-1/RT) cm3 s-1 and k∞=1.2×1014 exp(-63.7 kJ mol-1/RT) s-1. We estimate an uncertainty for these rate constants of ±30%. Both rate constants have been discussed in terms of statistical unimolecular rate theory. Very good agreement between the calculated and the experimental rate constants has been found.

Incubation times, fall-off and branching ratios in the thermal decomposition of toluene: Experiments and theoryElectronic supplementary information (ESI) available: Molecular parameters used for SACM calculations and the master equation analysis, as well as correlation schemes for vibrations and rotations, used in the SACM calculations. See http://www.rsc.org/suppdata/cp/b2/b202999e/

We have investigated the thermal unimolecular decomposition of toluene in a low-pressure shock tube (0.1 bar<p<2 bar) in the temperature range 1350 K to 1900 K. The decomposition of toluene proceeds in two competing parallel reactions C6H5CH3+M→C6H5CH2+H+M (R1) and C6H5CH3+M→C6H5+CH3+M (R2). Reaction (R1) generally dominates under all conditions. The rate constant k1 has been determined using H-atom detection via calibrated atomic resonance absorption spectroscopy (ARAS) at 121.6 nm. At the highest temperatures a slight pressure dependence of k1 was noticed while at lower temperatures k1 was found to be almost pressure independent. At the lowest pressures and highest temperatures we additionally identified significant induction times. We analyzed induction times, fall-off, and rate constants with a two-channel master equation model. Collisional energy transfer probabilities for toluene/Ar were taken as determined by Luther and co-workers (, U. Hold, T. Lenzer, K. Luther, K. Reihs and A. C. Symonds, Ber. Bunsen-Ges. Phys. Chem., 1997, 101, 552; T. Lenzer, K. Luther and K. Reihs, J. Chem. Phys., 2000, 112, 4090) and specific rate constants from simplified SACM calculations, which have been confirmed by experiments (, U. Brand, H. Hippler, L. Lindemann and J. Troe, J. Phys. Chem., 1990, 94, 6305; , H. Hippler, Ch. Riehn, J. Troe and K.-M. Weitzel, J. Phys. Chem., 1990, 94, 6321). The numerical solution of the master equation was obtained considering a maximum energy of 80 000 cm−1 and a matrix dimension of 1000. The lower 500 energy levels have been considered as discrete levels taking into account the molecular specific structure in the density of states. The unimolecular pressure dependent rate constants k1(M) and k2(M) were determined from the eigenvalues of the system. The induction time was identified as the delay obtained from back extrapolation of the stationary reaction rate. The agreement between experimental and modeled pressure and temperature dependent rate constants k1 and k2 was excellent. The experimental incubation times were predicted within a factor of three indicating three times slower energy transfer rates at the ‘bottleneck’ than used in the model.

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).