Volume 218, Issue 4

The electronic and geometric structure, stability and molecular properties of the cationic van-der-Waals complex Ar 2 H + in its ground electronic state are studied by means of two ab-initio quantum-chemical approaches: conventional configuration interaction (multi-reference and coupled-cluster methods) and a diatomics-in-molecules model with ab-initio input data. To ensure consistency between the two approaches, one and the same one-electron atomic basis set (aug-cc-pVTZ by Dunning) is employed in both. The topography of the ground-state potential-energy surface is examined with respect to the nature of the binding and the stability of structures corresponding to stationary points. In accordance with most earlier theoretical work, there are two local minima at linear arrangements: a strongly bound centro-symmetric moiety, (Ar–H–Ar) + , and a weakly bound van-der-Waals complex, Ar···ArH + . These are separated by a low barrier. Only the centro-symmetric molecule is significantly stable ( D e = 0.68eV) against fragmentation into Ar + ArH + and should have structural and dynamical relevance. A fairly simple diatomics-in-molecules model taking into account only the few lowest electronic fragment states yields a qualitatively correct description of the ground state but shows quantitative deviations from the more accurate configuration-interaction data in detail. Nevertheless, it should provide a good starting point for the treatment of larger complexes Ar n H + with n > 2.

The gas-phase reaction of O( 3 P ) atoms with benzene was investigated in a flow system in the pressure range of 50–100mbar and a temperature of 295 ± 2K with a focus on the product formation. O 2 concentrations in the carrier gas were in the range of (7.7–84) × 10 14 molecule cm −3 . The primary stable products detected were phenol, benzene oxide/oxepin and a not identified compound with the probable composition C 5 H 6 O. The yields of phenol and benzene oxide/oxepin were 0.12 ± 0.02 and 0.26 ± 0.06, respectively, being not affected by the experimental conditions. For benzene oxide/oxepin, a rapid consecutive reaction with O( 3 P ) atoms was observed with a rate coefficient of k (295K) = (1.1 ± 0.1) × 10 −10 cm 3 molecule −1 s −1 measured at a pressure of 100mbar. A substance with the formula C 6 H 6 O 2 (likely oxepin 4,5-epoxide or oxepin 2,3-epoxide), the isomers of muconaldehyde, as well as formic acid, acrolein and trans -butenedial were identified as products of the reaction of O( 3 P ) atoms with benzene oxide/oxepin.

The hydrogenation of acrolein has been studied over various Ag/SiO 2 catalysts in a pressure range from 50mbar to 20bar. Increasing partial pressures of acrolein and/or hydrogen lead to increasing selectivities of allyl alcohol. The selectivity towards allyl alcohol also depends on the structural features of the Ag/SiO 2 catalyst. Larger particles seem to favour the production of propanal. This observation is discussed in view of the different plane-to-edge-ratio of the different catalysts. The interaction of hydrogen with silver samples has been studied with TAP at very low pressures as well as with calorimetry at ambient pressures. Both methods indicate, that also the hydrogen adsorption is structure-sensitive. No interaction of hydrogen with electrolytic silver or the support material alone was observed. IR spectroscopy has been used to elucidate the interaction of acrolein with Ag/SiO 2 catalysts. A strong interaction with the support material was found, hindering the observation of probable silver-acrolein interaction.

The aggregation behavior of 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) in the nonaqueous solvents formamide (FA) and 3-hydroxypropionitrile (HPN) has been studied. UV/Vis absorption maximum λ max and thermal cis → trans isomerization rate constant k iso of an azo dye, as well as fluorescence intensity ratios I M / I E of monomer and excimer, and the excimer lifetime τ E of two dipyrenyl compounds as probe molecules have been determined. For comparison, aggregation of the amphiphile POPC molecules was followed “probe-free” by means of dynamic light scattering (DLS) measurements. Combining the results of probe molecule studies and DLS it turned out that the critical aggregation concentration of POPC in FA < 0.05mmol dm −3 . At ≥ 0.2mmol dm −3 , aggregates have hydrodynamic radii 30 < R h < 80nm and high microviscosity in their hydrocarbon interior. A vesicle-like structure of the aggregates can therefore be assumed. Aggregation behavior of POPC in HPN is much less pronounced, essentially small aggregates with radii of a few nanometer have been detected which include solvent molecules.

The reactions NH( a ) + NH 3 ( X̃ ) → products (1) ND( a ) + NH 3 ( X̃ ) → products (2) NH( a ) + ND 3 ( X̃ ) → products (3) ND( a ) + ND 3 ( X̃ ) → products (4) were studied in a quasi-static reaction cell at room temperature and pressures of 10 and 20mbar with He as the main carrier gas. The electronically excited reactants NH( a ) and ND( a ) were generated by laser-flash photolysis of HN 3 and DN 3 , respectively, at λ = 308nm and detected by laser-induced fluorescence (LIF). Also the ground state species NH( X ) and ND( X ) as products were detected by LIF. From the measured concentration-time profiles of NH( a ) and ND( a ) under pseudo-first order conditions, the following rate constants were obtained: NH( a ) + NH 3 ( X̃ ) → products (1) ND( a ) + NH 3 ( X̃ ) → products (2) NH( a ) + ND 3 ( X̃ ) → products (3) ND( a ) + ND 3 ( X̃ ) → products (4) k 1 = (9.1 ± 0.9) × 10 13 cm 3 mol −1 s −1 k 2 = (9.6 ± 1.0) × 10 13 cm 3 mol −1 s −1 k 3 = (8.0 ± 1.0) × 10 13 cm 3 mol −1 s −1 k 4 = (7.2 ± 0.8) × 10 13 cm 3 mol −1 s −1 . The major products are the corresponding NH i D 2 − i ( X̃ ) radicals ( i = 0, 1, 2), whereas quenching processes such as NH( a ) + ND 3 → NH( X ) + ND 3 are of minor importance (1%). The isotope exchange NH( a ) + ND 3 → ND( X ) + NHD 2 is negligible, and the corresponding channel on the singlet surface NH( a ) + ND 3 ( X ~ ) → ND( a ) + NHD 2 ( X̃ ) contributes with 1% to the overall NH( a ) depletion in that reaction. The experimental findings are discussed in terms of a chemical activation mechanism by means of statistical rate theory.

Variational transition state theory (VTST) is widely used for the modelling of barrierless radical-radical recombination reactions. In this application, VTST suffers from a number of limitations some of which are of more technical, others of more fundamental nature. The former are caused by inappropriate averaging over individual adiabatic channel potentials or by the neglect of quantum effects, the latter are due to deviations from adiabatic dynamics. It is shown that most radical-radical recombination reactions are characterized by Massey parameters which are smaller than unity such that the dynamics is nonadiabatic. VTST treatments which generally assume adiabatic dynamics, therefore, have a fundamental problem. Calculations of rate constants by VTST often exceed classical trajectory results by about 10 to 20percent. This is normally attributed to “recrossing trajectories”. In the present work it is shown, however, that deviations of this magnitude also have to be expected for nonadiabatic dynamics in comparison to adiabatic dynamics. It is, therefore, suggested that “recrossing” at least in part has to be attributed to nonadiabatic dynamics. A way out of the dilemma is the use of a combination of statistical adiabatic channel and classical trajectory concepts.

The reversible addition reaction 2- Ċ 3 H 7 + i -C 4 H 8 ⇔ (CH 3 ) 2 CHCH 2 Ċ (CH 3 ) 2 was studied in the temperature range 491–543K by product analysis in experiments in which 2- Ċ 3 H 7 radicals were generated by the thermal decomposition of azoisopropane. The enthalpy of formation of the radical (CH 3 ) 2 CHCH 2 Ċ (CH 3 ) 2 was determined, the value obtained being Δ f H ° = -21.8 ± 5.2kJ mol -1 . From this result, the following group value was deduced: Δ f H °[ Ċ –(C) 3 ] = 177.1kJ mol -1 . From the latter, Δ f H °( t - Ċ 4 H 9 ) = 51.6kJ mol -1 was estimated. The rate constant of self-combination of the radical (CH 3 ) 2 CHCH 2 Ċ (CH 3 ) 2 was determined for the first time: log( k 12 /(dm 3 mol -1 s -1 )) = 7.3.

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 −13 cm 3 molecule −1 s −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.

The mechanisms and the rates of the reactions of the fluorinated hydrocarbon radical CH 2 F with the fluorinated and pure hydrocarbon radicals CHF 2 , CH 3 , and C 2 H 5 , respectively, have been studied at low pressure (around 2 mbar) and room temperature using the discharge-flow reactor technique. Mass spectrometry either with electron impact or laser induced multiphoton ionization was applied for the detection of labile and stable species. The cross combination of the radicals is mainly followed by HF elimination from the chemically activated adduct. The overall rate coefficients at 298K for CH 2 F + CHF 2 → C 2 H 2 F 2 + HF (1) CH 2 F + CH 3 → C 2 H 4 + HF (2) CH 2 F + C 2 H 5 → C 3 H 6 + HF (3) were derived from the analysis of the radical-time profiles by fit procedures; the following values were obtained: CH 2 F + CHF 2 → C 2 H 2 F 2 + HF (1) CH 2 F + CH 3 → C 2 H 4 + HF (2) CH 2 F + C 2 H 5 → C 3 H 6 + HF (3) k 1 = (3.0 + 1.5/ − 0.7) × 10 12 cm 3 mol −1 s −1 k 2 = (4.0 ± 1.5) × 10 13 cm 3 mol −1 s −1 k 3 = (9.0 ± 3) × 10 12 cm 3 mol −1 s −1 . The results are discussed in terms of chemical activation mechanisms.