Gernot Friedrichs

Quantitative detection of HCO behind shock waves: The thermal decomposition of HCO

Using FM spectroscopy formyl radicals were detected for the first time behind shock waves. HCO radicals have been generated by 308 nm photolysis of mixtures of formaldehyde in argon. The HCO spectrum of the (A2A″ ← 2A′) (0900 ← 0010) transition was measured at room temperature with high resolution and the predissociative linewidths Γ of the individual rotational lines were fitted to Γ = X + ZN′2(N′ + 1)2, where X = 0.22 cm−1 and Z = 1.0 × 10−5 cm−1. Since FM spectroscopy is very sensitive to small line shape variations the spin splitting in the Q-branch could be resolved. Time resolved measurements of HCO profiles at temperatures below 820 K provided the temperature independent rates of reaction (4), H + HCO → H2 + CO, and reaction (5), HCO + HCO → CH2O + CO, k4 = 1.1 × 1014 cm3 mol−1 s−1k5 = 2.7 × 1013 cm3 mol−1 s−1and the low pressure room temperature absorption cross section of the Q(9)P(2) line at 614.872 nm, αc = (1.5 ± 0.4) × 106 cm2 mol−1 (base e). Measurements of the unimolecular decomposition of HCO, reaction (3) HCO + M → H + CO + M, were performed at temperatures from 835 to 1230 K and at total densities from 3.3 × 10−6 to 2.5 × 10−5 mol cm−3. They can be represented by the following Arrhenius expression. k3 = 4.0 × 1013·exp(−65 kJ mol−1/RT) cm3 mol−1 s−1 (Δ log k3 = ±0.23)The corresponding RRKM fit, 4.8 × 1017·(T/K)−1.2·exp(−74.2 kJ mol−1/RT) cm3 mol−1 s−1 (600 < T/K < 2500), supports the lower range of previously reported high temperature rate expressions.

Bismuth Tri- and Tetraarylcarboxylates: Crystal Structures, In Situ X-ray Diffraction, Intermediates and Luminescence

A systematic investigation of the systems Bi(3+)/carboxylic acid/HNO3 for the tri- and tetracarboxylic acids pyromellitic acid (H4Pyr), trimellitic acid (H3Tri) and trimesic acid (H3BTC) acid led to the discovery of five new bismuth carboxylates. Structural characterisation allowed the influence of the linker geometry and the Bi(3+):linker molar ratio in the starting solution on the crystal structure to be determined. The crystallisation of three selected compounds was investigated by in situ energy-dispersive X-ray diffraction. Three new crystalline intermediates were observed within minutes, and two of them could be isolated by quenching of the reaction mixture. Their crystal structures were determined from laboratory and synchrotron X-ray powder diffraction data and allowed a possible reaction pathway to be established. In depth characterisation of the luminescence properties of the three bismuth pyromellate compounds was carried out. Fluorescence and phosphorescence could be assigned to (mainly) ligand- and metal-based transitions. The polymorphs of Bi(HPyr) exhibit different luminescence properties, although their structures are very similar. Surprisingly, doping of the three host structures with Eu(3+) and Tb(3+) ions was only successful for one of the polymorphs.

Dimerization of HNO in Aqueous Solution: An Interplay of Solvation Effects, Fast Acid–Base Equilibria, and Intramolecular Hydrogen Bonding?

The recent unraveling of the rather complex acid-base equilibrium of nitroxyl (HNO) has stimulated a renewed interest in the significance of HNO for biology and pharmacy. HNO plays an important role in enzymatic mechanisms and is discussed as a potential therapeutic agent against heart failure. A cumbersome property for studying HNO reactions, its fast dimerization leading to the rapid formation of N(2)O, is surprisingly far from being well understood. It prevents isolation and limits intermediate concentrations of nitroxyl in solution. In this study, a new mechanism for the HNO dimerization reaction in aqueous solution has been theoretically derived on the basis of DFT calculations. Detailed analysis of the initial reaction step suggests a reversal of the cis-trans isomer preference in solution compared to the corresponding gas phase reaction. In contrast to a gas phase derived model based on intramolecular rearrangement steps, an acid-base equilibrium model is in agreement with previous experimental findings and, moreover, explains the fundamental differences between the well studied gas phase reaction and the solvent reaction in terms of polarity, cis-trans isomerizations, and acidities of the intermediates. In the case of cis-hyponitrous acid, the calculated pK(a) values of the acid-base equilibria were found to be significantly different from the corresponding experimental value of the stable trans isomer. Under physiological conditions, N(2)O formation is dominated by the decomposition of the unstable monoanion cis-N(2)O(2)H(-) rather than that of the commonly stated cis-HONNOH.

Design and field application of a UV-LED based optical fiber biofilm sensor.

Detecting changes in the formation dynamics of biofilms stemming from bacteria and unicellular microorganisms in their natural environment is of prime interest for biological, ecological as well as anti-fouling technology research. We developed a robust optical fiber-based biofilm sensor ready to be applied in natural aquatic environments for on-line, in situ and non-destructive monitoring of large-area biofilms. The device is based on the detection of the natural fluorescence of microorganisms constituting the biofilm. Basically, the intrinsic fluorescence of the amino acid tryptophan is excited at a wavelength of λ=280 nm and detected at λ=350 nm utilising a numerically optimized sensor head equipped with a UV-LED light source and optical fiber bundles for efficient fluorescence light collection. Calibration was carried out with tryptophan solutions and two characteristic marine bacteria strains revealing linear signal response, satisfactory background suppression, wide dynamic range, and an experimental detection limit of 4 × 10(3)cells/cm(2). Successful field experiments in the Baltic Sea accomplished over a period of twenty-one days provided for the first time continuous observation of biofilm formation dynamics in a natural habitat. Starting from the first adhering bacteria, the measurement yielded the characteristic three phases of biofilm formation up to a fully developed biofilm. The sensor system holds potential for applications in aquatic sciences including deep sea research and, after further miniaturisation, in the industrial and biomedical field.

Revealing structural properties of the marine nanolayer from vibrational sum frequency generation spectra

Natural nanolayers originating from sea surface and subsurface water samples collected in the Baltic Sea have been investigated using surface-sensitive vibrational sum frequency generation (VSFG) spectroscopy. Distinct spectral signatures of CH and OH bond stretch vibrations have been detected at wavenumbers ranging from 2700 to 3900 cm(-1). Measured water-air interface spectra as well as observed signal intensity trends are discussed in terms of composition and structure of the natural organic nanolayer. Reasoning was based on the comparison with reference spectra, spectral trends inferred from previous VSFG studies, reported average composition of dissolved organic matter in seawater, and simplified assumption that surfactants can be classified as soluble (wet) and insoluble (dry) surfactants. Wet surfactants have been found to be dominant, and often lipid-like compounds form a very dense surfactant nanolayer. Supported by comparison spectra of xanthan gum solutions, the observed VSFG spectral signatures were tentatively assigned to lipopolysaccharides or other lipid-like compounds embedded in colloidal matrices of polymeric material. In addition, VSFG spectra of a polluted harbor water sample and a water sample covered with diesel oil are reported.

Glyoxal Oxidation Mechanism: Implications for the Reactions HCO + O2 and OCHCHO + HO2.

A detailed mechanism for the thermal decomposition and oxidation of the flame intermediate glyoxal (OCHCHO) has been assembled from available theoretical and experimental literature data. The modeling capabilities of this extensive mechanism have been tested by simulating experimental HCO profiles measured at intermediate and high temperatures in previous glyoxal photolysis and pyrolysis studies. Additionally, new experiments on glyoxal pyrolysis and oxidation have been performed with glyoxal and glyoxal/oxygen mixtures in Ar behind shock waves at temperatures of 1285-1760 K at two different total density ranges. HCO concentration-time profiles have been detected by frequency modulation spectroscopy at a wavelength of λ = 614.752 nm. The temperature range of available direct rate constant data of the high-temperature key reaction HCO + O2 → CO + HO2 has been extended up to 1705 K and confirms a temperature dependence consistent with a dominating direct abstraction channel. Taking into account available literature data obtained at lower temperatures, the following rate constant expression is recommended over the temperature range 295 K < T < 1705 K: k1/(cm(3) mol(-1) s(-1)) = 6.92 × 10(6) × T(1.90) × exp(+5.73 kJ/mol/RT). At intermediate temperatures, the reaction OCHCHO + HO2 becomes more important. A detailed reanalysis of previous experimental data as well as more recent theoretical predictions favor the formation of a recombination product in contrast to the formerly assumed dominating and fast OH-forming channel. Modeling results of the present study support the formation of HOCH(OO)CHO and provide a 2 orders of magnitude lower rate constant estimate for the OH channel. Hence, low-temperature generation of chain carriers has to be attributed to secondary reactions of HOCH(OO)CHO.

Sensitive absorption methods for quantitative gas phase kinetic measurements. Part 2: Cavity ringdown spectroscopy

Cavity ringdown spectroscopy (CRDS) and frequency modulation spectroscopy (FMS) are sensitive absorption based detection methods that have found widespread applications in gas phase reaction kinetics. In part 2 of this review, the theoretical foundations of CRDS are addressed with a special emphasis on quantitative time-resolved measurements of concentration profiles. A complementary review of FMS can be found in part 1 (Z. Phys. Chem. 222 (2008) 1–30). Practical aspects, possible pitfalls, attainable sensitivities, and modern trends are discussed. Recent kinetic studies based on CRDS measurements as a time-resolved detection tool are briefly reviewed and a bibliography with 136 entries is included to facilitate the access to the large body of original literature.

The Products of the Reactions of o-Benzyne with Ethene, Propene, and Acetylene: A Combined Mass Spectrometric and Quantum Chemical Study.

Abstract The primary products of the bimolecular reactions of ortho-benzyne, o-C6H4 (1,2-dehydrobenzene), with ethene, propene, and acetylene have been detected by molecular beam mass spectrometry at a combustion relevant temperature of T = 1475 K. o-Benzyne was produced by flash pyrolysis of phthalic anhydride in the absence and presence of the respective reactant. Potential reaction pathways of the addition reactions were investigated by quantum chemical calculations. Channels with biradical intermediates were found to be energetically more favorable than alternative quasi-concerted [2+1] cycloaddition and concerted H-transfer pathways. Bicyclic benzocyclobutene and benzocyclobutadiene were identified as the main products of the reactions with C2H4 and C2H2, respectively. At combustion temperatures, however, these cyclic products are likely to undergo sequential ring opening. In the case of propene, the presence of an allylic H atom initiates a favorable ene-type reaction sequence yielding the open-chain product allylbenzene. Overall, hydrocarbon reactivity was found to increase in the order C2H2, C2H4 to 3H8. The range of the estimated bimolecular rate constants is comparable to the rate constants of the corresponding phenyl radical reactions and hence point out a potentially important role of o-C6H4 reactions in flame and soot formation chemistry.

Sensitive absorption methods for quantitative gas phase kinetic measurements. Part 1: Frequency modulation spectroscopy

Frequency modulation spectroscopy (FMS) and cavity ringdown spectroscopy (CRDS) are sensitive absorption based detection methods that have found widespread applications in gas phase reaction kinetics. In part 1 of this review, the theoretical foundations of FMS are addressed with a special emphasis on time-resolved quantitative measurements of concentration profiles. A complementary review of CRDS can be found in part 2 (Z. Phys. Chem. 222 (2008) 31–61). Practical aspects, possible pitfalls, attainable sensitivities, and modern trends are discussed. Kinetic studies based on FMS measurements as a time-resolved detection tool are briefly reviewed and a bibliography with more than a hundred entries is included to facilitate the access to the large body of original literature.

Investigation of the Thermal Decomposition of Ketene and of the Reaction CH2 + H2 ⇔ CH3 + H

Using frequency modulation (FM) spectroscopy singlet methylene radicals have been detected for the first time behind shock waves. The thermal decomposition of ketene served as source for metylene radicals at temperatures from 1905 to 2780 K and pressures around 450 mbar. For the unimolecular decomposition reaction, (1) CH2CO + M → CH2 + CO + M, the rate constants obtained are: k 1 = (9.5 ± 5.7) · 1015 · exp[(−244 ± 25) kJ mol−1/RT] cm3 mol−1 s−1. As a first study of a methylene reaction at high temperatures by diretly tracing methylene the reaction of methylene with hydrogen, (8 + 9) 1,3CH2 + H2 → CH3 + H, was investigated at temperatures from 1930 to 2455 K and pressures around 500 mbar. For the total rate constant of the singlet and triplet methylene reaction a temperature independent value was obtained: log(k8+9/(cm3 mol−1 s−1)) = 13.89 ± 0.26. A comparison with low temperature literature data and the systematics of activation energies of triplet methylene reactions allowed a consistent description of singlet and triplet contributions and of the forward and reverse reaction.