AbstractsChemistry

Until a few years ago it was believed that OH radicals would only add to the aromatic molecule at non-substituted sites and that the ipso addition (i.e., addition to a substituted site) was unimportant. However, kinetic studies on hexamethylbenzene indicated that the ipso addition was not only possible but can occur much faster than what could be explained by the H-atom abstraction alone. In this work, we investigated a series of alkylated benzenes with two to five substituents, using flash photolysis of water vapour for production of OH radicals and resonance fluorescence for their time resolved detection. Among these methylated compounds there are three that can form only 2 adducts (ortho at an unoccupied site and ipso at an occupied site): 1,4-dimethylbenzene, 1,3,5-trimethylbenzene, and 1,2,4,5-tetramethylbenzene. For these compounds two different mechanisms were investigated: (i) formation of two adducts by direct addition and (ii) formation of a second adduct by isomerization of the first adduct. For the other methylated compounds: 1,2,3,4-, 1,2,3,5-tetramethylbenzene, and pentamethylbenzene, several adducts are expected, and therefore only the rate constants for the overall reaction were investigated. Moreover, the mechanism for the reaction of the aromatic biogenic p-cymene (4-isopropyl-toluene) with OH radicals was investigated. In this case, theoretical predictions helped to reduce the number of possible adducts from four to two, which allowed applying the two-adduct-formation model. OH rate constants for all the studied aromatics at temperatures between 300 K and 340-360 K, equilibrium constants and formation yields for each adduct, as well as formation entropies and enthalpies were determined. Formed adducts were identified based on thermodynamic data and comparison with published theoretical predictions and experimental results. Nevertheless, it was not possible to determine if the formation of both adducts occurs via direct formation or via isomerization, being more likely that the real mechanism lies between these two extremes. Addition von OH radikale an aromatischen Moleküle ist seit mehrere Jahrzehnten untersucht worden. Es wurde angenommen dass die Addition nur an freien Stellen des Aromaten stattfindet und die ipso Addition (d.h., OH Addition an einer bereits besetzen Stelle) wurde immer vernachlässigt. Jahre später, kinetische Untersuchung des voll besetzten Aromat Hexamethylbenzol haben bewiesen dass die ipso Addition nicht nur möglich ist sondern viel schneller verläuft als erwartet nur von der H-atom Abstraktion. Eine Folge von alkylierten Benzolen mit zwischen zwei und fünf Substituenten wurde für diese Arbeit untersucht. OH Radikale wurden mit Wasserdampfphotolyse erzeugt und ihre relative Konzentration wurde mit Resonanzfluoreszenz zeitlich verfolgt. Drei der untersuchten Aromaten können nur zwei Addukte bilden (jeweils ein ortho and ein ipso): 1,4-Dimethylbenzol, 1,3,5 Trimethylbenzol und 1,2,4,5-Tetramethylbenzol. Für diese drei Substanzen zwei Mechanismen wurden untersucht: (i)… Advisors/Committee Members: Zetzsch, Cornelius (advisor).