time / ms

0 10 20 30 40

global fitindividual fit

0.6 HMB (1012 cm-3)

0

1

OH + HMB HMB–OH

T = 324 K

Straight lines - fine, but not what I was eager to find

no (ipso-)adduct formed (or very unstable)

no hope to find it at lower temperatures, since HMB would condense

High slopes even at low concentrations

kOH too high (1.4.10-10) for abstraction

addition channel may be included (stable adduct?)

rise the temperature!

?

8 s-1

91 s-1

141 s-1

First

? experimental evidence of ipso-additionof OH to methyl-substituted aromatics andO2 reactivity of hexamethylbenzene–OH

Rainald Koch and Cornelius ZetzschAtmospheric Chemistry Research Lab., Uni Bayreuth

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.

Naphthalene–OH, + O2 or NO2 ?(wanted: source of ambient nitro-PAH)

from very recent FP/RF experiments:

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?MotivationIn 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.

time / ms

0 50 100 150

global fitindividual fit

Yes, very fast (looks like toluene at 1016 O2)

kO2 = 1.2.10-13 cm3s-1, 200x that of toluene–OH

1.35·1012 cm-3

0

2

T = 360 K

HMB: none

34

OH + HMB HMB–OH ... +O2?

With the (highly uncertain) assignment of all loss to the OH-side of

the equilibrium (to the abstraction channel), the rate constant for the

back-decomposition is k-1 = 38 s-1 (like toluene at 320 K) while the

abstraction channel, kabstr = 38·10-12 cm3s-1, is still too large (it should

be < 5·10-12 cm3s-1, since abstraction from toluene is < 0.7 at 340 K [5])

rise the temperature (let k-1 dominate adduct loss)

1 ·1014 O2 cm-3

69 s-1 56 s-1 43 s-1

28 s-1

15 s-1

6 s-1

time / ms

0 40 80

global fit individual fit

The global fit fits all

1 ·1012 cm-3

0 HMB

up to 4 ·1014 O2

(twice)

4.8

T = 385 K

?

Faster equilibration at 385 K and 5x HMB distinctly biexponential decays, even at 4·1014 of O2

kO2 = 1.8 ·10-13 cm3s-1, 6x that of phenol-OH

"kabstr " (see above) = 20·10-12 cm3s-1

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.

Technical support by Heinz-Ulrich Krüger and funding through the Federal Ministry of Environment is gratefully acknowledged.