Das Projekt "Struktur-Wirkung-Beziehungen fuer Reaktionen waehrend des Abbaus von biogenen leichtfluechtigen Kohlenwasserstoffen" wird vom Umweltbundesamt gefördert und von Universität-Gesamthochschule Essen, Fachbereich 8 Chemie, Institut für Physikalische und Theoretische Chemie durchgeführt. In the present project the oxidation reactions will be initiated via the photolytic production of OH radicals or by photolysis of specific alkylhalides in the presence of O2 and NO. The RO reactivity can be related to the time dependence of (OH) and (NO2) which are monitored simultaneously using laser spectroscopic methods. The technique has already been validated. During this project it will be extended to investigate C3, C4 and C5 alkoxy radicals. The method will provide information on all over-all reactivities of alkoxy radicals generated in the OH initiated oxidation of the alkanes mentioned above, and is therefore useful in deriving absolute information on the NO2 (and therefore O3) yield per hydrocarbon oxidized to a carbonyl compound under tropospheric conditions (NOCON factors). Moreover, in a detailed analysis of the (NO2) and (OH) data, making use of infomation on the position of the initial OH attack and using different isomers of the alkane, reactivities of specific RO radicals will be obtained. The experiments will be performed under variation of (O2) and (NO) leading to a determination of the erlative importance of nitrit formation versus decomposition/isomerisation and reaction with O2. Moreover, the effect of the variation of total pressure (10-100 mbar) in the decomposition and isomerisation rate coefficients will be investigated. Theoretical studies of the unimolecular isomerisation and decomposition will be performed using RRKM methods. A major information essential for the performance of these calculations ist the thermochemistry of the different reaction pathways. Starting from detailed knowledge of the C2 and C3 systems a semiempirical extrapolation method is developed for extracting the reaction enthalpies as a function of the chain length of the RO radical. Moreover, intrinsic activation barries for the decomposition and isomerisation reactions of different RO radicals are extracted using ab inition calculations on RO radical geometries using GAUSSIAN programme. The results of these calculations will be compared with experimental data where those are available with reasonably high reliability (C2-C5). Data from the experimental and theoretical studies by the various memebers of this work package will be used to develop SARs for alkoxy reactions. So far our group has made preliminary inverstigations for the n-alkanes up to the C6 system. The results obtained represent weighted averages for the different individual members of alkoxy that are formed in the oxidation of larger alkanes as a result of different CI and OH attack. They indicate that the rate coefficients of RO decomposition or isomerisation approach a limiting value of 10(xp=5) 1/s at room temperature and 50 mbar total pressure.
Das Projekt "Direkte Untersuchungen troposphaerischer Reaktionen von Alkoxy-Radikalen" wird vom Umweltbundesamt gefördert und von Universität-Gesamthochschule Essen, Fachbereich 8 Chemie, Institut für Physikalische und Theoretische Chemie durchgeführt. The oxidation of hydrocarbons in the troposhere is of major importance when dealing with photochemical ozone formation. Therefore this project is focussed on the reactivity of RO-radicals of C3-C6-hydrocarbons towards O2 and NO2 as well as the thermal decomposition and the isomerization of these RO-radicals. The received data shall be evaluated by simulation under consideration of complete reaction mechanisms. The NO/NO2 condersion factors (NOCON-factors) obtained by this method reflect the mechanistic quota of the ozone formation potential of these hydrocarbons. The present investigations are limited to select Alkoxy-radicals with typical structural differences, i.e. 1-Alkoxy, 2-Alkoxy, i-Alkoxy, cycl.-Alkoxy. For these species absolute and relative rate constants can be determined by time-resolved studies of OH and NO2. In order to strengthen the comprehension of the measured rate constants and in order to transfer these data to other RO-radicals that are not yet investigated theoretical studies will be made. These studies are based on RRKM-calculations with determination of structural parameters of transition states by Gaussian 92 ab inition calculations.
Das Projekt "Laboruntersuchungen zur Kinetik stratosphaerisch relevanter Reaktionen von BRO- und IO-Radikalen" wird vom Umweltbundesamt gefördert und von Universität-Gesamthochschule Essen, Fachbereich 8 Chemie, Institut für Physikalische und Theoretische Chemie durchgeführt. Fuer die og Untersuchungen ist geplant, eine kombinierte LIF/UV-VIS-Absorptions-Anordnung aufzubauen. Die Erzeugung der verschiedenen Radikale soll durch Excimer-Laserphotolyse geeigneter Radikalvorlaeufer erfolgen. IO-Radikale koennen sehr empfindlich mit Hilfe der Laser-induzierten Fluoreszens-(LIF)-Technik nachgewiesen werden. Die Anregung erfolgt dabei im A2II3/2 (v'=2) kleiner -- X2II3/2 (v''=0) - Uebergang mit Fluoreszenz aus v'' = 3. BrO bzw CIO-Radikale besitzen ausgepraegte UV-Bandenspektren mit relativ grossen Absorptionskoeffizienten im nahen UV. Damit ist es moeglich, diese Spezies durch zeitaufgeloeste UV-Absorptionsspektroskopie zu detektieren. Durch Einsatz einer 'gatebaren' CCD-Kamera wird erreicht, dass die Absorption der Edukte (CI2, Br2 etc) separiert werden koennen.