Reactions of OH radicals with aromatics were investigated in the presence of He at pressures ranging from 5 to 800 mbar using pulsed vacuum uv photolysis of H2O for the production of OH and time resolved resonance fluorescence for the detection of OH. At room temperature (296 K) in the presence of 133 mbar He the rate constants, k, were observed to be in the high pressure limit. The rate constants, (k ± Δk)/10−12 cm3 s−1, were determined to be: (1.0 ± 0.2) for benzene; (120 ± 24) for aniline; (28 ± 6) for phenol, (0.50 ± 0.10) for 1,2,4‐trichlorobenzene. — For the temperature dependence of the rate constant for 1,2,4‐trichlorobenzene the Arrhenius expression (k ± 3σ) = (2.3 ± 1) · 10−12 exp [ — (430 ± 125) K/T] cm3 s−1 is obtained from measurements at 273 to 368 K. — The rate constants at room temperature can be correlated with the ionization potential, I, of the aromatics, yielding the equation: logk/cm3s−1 = 0.74–1.36 · I/eV. This equation is seen to represent the rate data on 18 aromatics to within a precision of a factor of two; with some precautions it might be used to predict rate constants for further substituted benzene aromatics. Outliers in such a diagram are benzaldehyde and the cresols (being more reactive than predicted), and diphenyl and chlorinated aromatics (being less reactive).
The influence of the transition metal (Ti, V, Zr, W) doping on the carbon matrix nanostructuring during the thin film growth and subsequent annealing is investigated. Pure and metal-doped amorphous carbon films (a-C, a-C:Me) were deposited at room temperature by non-reactive magnetron sputtering. The carbon structure of as-deposited and post-annealed (up to 1300 K) samples was analyzed by X-ray diffraction (XRD) and Raman spectroscopy.The existence of graphene-like regions in a-C is concluded from a (10) diffraction peak. A comparison of the XRD and Raman results suggests that XRD probes only the small amount of 2-3 nm large graphene-like regions, whereas the majority of the sp 2 phase is present in smaller distorted aromatic clusters which are probed only by Raman spectroscopy. Annealing leads to an increase of the graphene size and the aromatic cluster size. During the carbon film growth the addition of metals enhances ordering of sp 2 carbon in sixfold aromatic clusters compared to a-C, Ti and Zr showing the strongest effect, W the lowest. This order qualitatively corresponds with the catalytic activity of the respective carbides found during graphitization of carbide-doped graphites published in the literature. With annealing, carbide crystallite formation and growth occurs in a-C:Me films, which destroys the initial carbon structure, reduces the size of the initially formed aromatic clusters and the differences in carbon structure introduced by different dopants. For high annealing temperatures the carbon structure of a-C:Me films is similar to that of a-C, and is determined only by the annealing temperature.2
In order to obtain a better understanding of the physical and chemical processes within micromixers or microreactors and to optimize these devices it is necessary to monitor the concentrations within the microchannels. To get chemical information, laser Raman spectroscopy can be used. This method is very selective for individual chemical compounds, allows a spatial resolution of 10 lm within fluids and a quantitative analysis. We examined the hydrolysis of the acetal 2,2-dimethoxypropane to acetone and methanol in the presence of hydrogen ions as catalyst. This reaction can be used to characterize micromixers. The aim of this work is the in situ monitoring of concentrations of acetal and its products, acetone and methanol, during the hydrolysis of acetal within a T-shaped micromixer with a channel width of 0.4 mm and a channel depth of 0.2 mm. In these experiments a continuous-wave argon ion laser was used as an excitation source. The laser radiation was coupled into a microscope and into the micromixer covered with a quartz plate. A special microscope objective was used. It allows the correction of optical aberrations resulting from quartz plates up to a thickness of 2 mm. Concentration profiles of acetal, methanol, and acetone were measured across the width of the microchannel.
In the bulk gas, the rp(O) value monotonically changes from about 200 to 100 ns with increasing excitation energy (Table I). These values agree fairly well with the fluorescence lifetimes obtained in the supersonic jet.11,12 Accordingly, the slow fluorescence in the bulk gas obviously corresponds to the fluorescence which shows a single exponential decay in the jet. In the bulk gas also, the fast component is very difficult to detect in the decay curve for 338.7-nm excitation (see Figure 6a). However, the molecular level structure even at a very low vibrational level of St cannot be identified as the small molecule limit. As already mentioned, the minute analysis of the experimental results on the quantum yield and decay of the fluorescence at various pressures in the bulk gas indicates that the fluorescence characteristics of acetaldehyde vapor for region-I excitation belong to the intermediate case.The electronic relaxation in St of acetaldehyde vapor after region-I excitation is thus characterized by reversible intersystem crossing to T¡. The Sj -* T, intersystem crossing yield in lower vibrational levels of Si acetaldehyde was reported by Gandini and Hackett5 to be unity at moderately high pressures. Therefore, the lifetime of the fast fluorescence can reasonably be related to the T¡ intersystem crossing rate. The possibility, suggested by Noble and Lee,11•12 that isolated acetaldehyde molecules excited to lower vibrational levels in Sj undergo internal conversion to S0 may be ruled out.Noble and Lee11,12 suggested further that the fast fluorescence, observed in a bulk gas by Atkinson et al.,9 is to be emitted from levels with considerably high rotational quantum numbers. As mentioned previously, however, both the quantum yield and the decay of fluorescence seem to be almost independent of the rotational level excited. It is, therefore, very improbable that the fast component which can be observed in a bulk gas corresponds to fluorescence from high rotational levels.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.