First ? experimental evidence of ipso -addition of OH to methyl-substituted aromatics and

1. First? experimental evidence of ipso-additionof OH to methyl-substituted aromatics and O2 reactivity of hexamethylbenzene–OH Rainald Koch and Cornelius Zetzsch Atmospheric Chemistry Research Lab., Uni Bayreuth Naphthalene–OH, + O2 or NO2 ? (wanted: source of ambient nitro-PAH) from very recent FP/RF experiments: Þ kO2 may well be < 10-17 cm3s-1 at 408 K(PAH–OH adducts may instead react with NO2 in Diesel engines). However, kO2 is of the order of 10-15 cm3s-1 at 298 K [1], too fast for ambient NO2 to be competitive. However, the measurements on naphthalene–OH + NO2, planned for this poster, fell victim to a more interesting question… OH addition at the ipso-position? Motivation In FP/RF-data on OH addition to 1,3,5-trimethylbenzene, obtained by Birger Bohn in ’98 [2], I detected systematic deviations from the biexponential model, hinting at a third radical species involved. Since the symmetrical 1,3,5-TMB allows only two isomers, ortho and ipso, this may be the first experimental evidence for addition of OH at the ipso-position in the gas-phase oxidation of aromatics. However, addition of OH at the ipso-position is generally not considered [3] due to the lack of experimental evidence and recent DFT calculations [4]. Therefore, measurements with hexamethylbenzene (HMB), where only ipso-addition is possible, should be made. Here they are... Experimental References [1] R Koch, and C Zetzsch: Cycling of OH radicals in the system naphthalene/ O2/NO studied by FP/RF and LP/RF, presented at the 13th Int. Sympos. on Gas Kinetics, Dublin, 1994. [2] B Bohn and C Zetzsch: Abbaumechanismen von Aromaten nach Anlagerung von OH, Präsentation zum TFS/LT3-D3 Statusseminar in Mainz, Dez. 1998. [3] JG Calvert, R Atkinson, KH Becker, RM Kamens, JH Seinfeld, TH Wallington, and G Yarwood: The mechanisms of atmospheric oxidation of aromatic hydro-carbons, Oxford Univ Press, 2002. [4] D Johnson, S Raoult, R Lesclaux, and LN Krasnoperov: UV abs. spectra of methyl-subst. hydroxy-cyclohexdienyl radicals in the gas phase. J. Photo-chem. Photobiol. A, 176, 98-106, 2005. [5] R Koch: Kinetische Untersuchung der Folgereaktionen der OH-Addukte von Aromaten mit NO, NO2 und O2 mit simultaner Auswertung von Kurvenscharen, PhD thesis, Univ. Hannover, 1992. [6] R Koch,R Knispel, M Elend, M Siese, and C Zetzsch: Consecutive reactions of aromatic–OH adducts with NO, NO2 and O2 in the gas phase: Benzene, toluene, m- and p-xylene, hexamethylbenzene, phenol, m-cresol, and aniline, ACPD manuscript in preparation. • Conclusions • · There is relatively stable addition of OH to hexamethylbenzene. • · ipso-addition is real. • · The third species deduced from triexponential decays of OH in the presence of 1,3,5-trimethylbenzene is probably the ipso-adduct. • · The evaluation of all that data (p,T-variation, O2 added, same for the 1,2,3- and 1,2,4-isomer) with a still to develop global fit (necessary because of the weak third exponential) would be a project of its own. • The very high O2 reactivity of HMB–OH is in line with the factor-of-3-per-methyl-substituent observed between benzene-, toluene-, and m-xylene–OH [6]. • The global fit [5, 6] is helpful to check models for consistency and to include decays for which an individual fit won‘t be stable. Flash Photolysis/Resonance Fluorescence (FP/RF, flash lamp, microwave lamp, photomultiplier) employing photolysis of 0.07 mbar H2O in 250 mbar of He. Flash energy is 0.6 J (electrical input). OH fluorescence is recorded up to the next flash (4 s). 50 to 200 recordings are summed up. The typical signal and background amplitudes are 2.5 and 2.0 ·105 photon counts/s, respectively, leading to S/N ratios of some 103. Technical support by Heinz-Ulrich Krüger and funding through the Federal Ministry of Environment is gratefully acknowledged.