1. First ? experimental evidence of ipso-addition of OH to methyl-substituted aromatics and O 2 reactivity of hexamethylbenzene –OH Rainald Koch and Cornelius Zetzsch Atmospheric Chemistry Research Lab., Uni Bayreuth Flash Photolysis/Resonance Fluorescence (FP/RF, flash lamp, microwave lamp, photomultiplier) employing photolysis of 0.07 mbar H 2 O 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 ·10 5 photon counts/s, respectively, leading to S/N ratios of some 10 3. Naphthalene–OH, + O 2 or NO 2 ? (wanted: source of ambient nitro-PAH) from very recent FP/RF experiments: k O2 may well be < 10 -17 cm 3 s -1 at 408 K (PAH–OH adducts may instead react with NO 2 in Diesel engines). However, k O2 is of the order of 10 -15 cm 3 s -1 at 298 K [1], too fast for ambient NO 2 to be competitive. However, the measurements on naphthalene–OH + NO 2, 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/ O 2 /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, NO 2 und O 2 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, NO 2 and O 2 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, O 2 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 O 2 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 wont be stable. Technical support by Heinz-Ulrich Krüger and funding through the Federal Ministry of Environment is gratefully acknowledged.