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| Titel |
Rate and pathways for the reaction of OH with the biogenic p-cymene, an alkylated aromatic |
| VerfasserIn |
R. Strekowski, M.-T. Rayez, J.-C. Rayez, C. Zetzsch |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250030800
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| Zusammenfassung |
| Aromatics are known to contribute strongly to tropospheric formation of ozone, and
p-cymene (4-isopropyltoluene) is one of only a few biogenic, volatile aromatic hydrocarbons.
In spite of its symmetry, this molecule (CH3 - Ï-CH(CH3)2) has a multitude of potential
pathways of its reaction with OH radicals. Addition of OH is well-known to be the
predominating primary step in the tropospheric transformation of aromatic hydrocarbons.
The addition is expected to occur preferably at a non-occupied position, where four positions
are available: two equivalent ones ortho to the methyl group and two equivalent ones
ortho to the isopropyl group. Furthermore, various C-H bonds (4 aromatic and
10 aliphatic) are available for abstraction, leading to benzyl-type radicals in two
cases.
The present study combines theoretical calculations with kinetic experiments in the gas
phase. The theoretical calculations are based on electronic quantum chemistry DFT method
for the investigation of the possible pathways in the potential energy surface of the reaction.
The experiments are carried out by the flash photolysis/resonance fluorescence
technique.
OH radicals are produced by pulsed vacuum-UV photolysis of H2O (> 115 nm) in the
presence of p-cymene in a slow flow of He as carrier gas. Their pseudo-first-order decays are
monitored by resonance fluorescence, storing the photon counts by multichannel scaling
in a PC and accumulating 50 decays each; see Koch et al. (2007) for details of
the technique and evaluation of data. The temperature was varied between room
temperature (295 K) and 345K, the He pressure was 250 mbar, and the level of p-cymene
was increased stepwise, up to 3 x 1013 molecules/cm3. The decays of OH were
observed to be exponential at room temperature, becoming clearly biexponential at
higher temperatures, thus indicating reversible addition of OH according to the
equilibration
OH + p-cymene - p-cymene-OH (1, -1)
These reactions might be accompanied by various abstraction channels, summarized
as
OH + p-cymene - alkylbenzyl + H2O (2)
A value of 1.4 x 10-11 cm3 s-1 at 295 K is obtained for the sum k1 + k2, in good
agreement with a value of 1.51 x 10-11 cm3 s-1determined by Corchnoy and Atkinson
(1990) in a smog chamber at 295 K. The sum k1 + k2 decreases slightly with increasing
temperature, falling below 10-11 cm3 s-1 at 345 K. The Arrhenius plot reveals
a curved behaviour with a negative activation energy, approximately 1 x 10-12
exp (60 K/T) cm3 s-1. The biexponential behaviour corresponds to an apparent
equilibrium constant of k1/k-1 = 8 x 10-25 exp [(-8500 ± 400) K/T] cm3 s-1. On
the other hand, the bond energy of OH in the adduct can hardly be obtained from
this biexponential behaviour alone since the abstraction of H atoms from the alkyl
groups of p-cymene can be estimated to contribute markedly. Extrapolating the
respective abstraction channels of toluene and the xylenes to two methyl substituents
would yield k2= 1.6 x 10-18 T2exp (-38 K/T) cm3 s-1 (Atkinson, 1989). This
amounts to 1.2x10-12 cm3 s-1 at 295 K (about 9% of the observed reactivity) and
1.7x10-12 cm3 s-1 at 345K (>17% of the observed reactivity) and does not even
take the possibly largerreactivity of the isopropyl group (as compared to CH3) into
account.
The abstraction channel has been found to predominate in the analogous reaction of
atomic Cl with p-cymene (Finlayson-Pitts et al, 1999), and further experiments by other
methods are required to clarify the reaction channels for OH radicals.
References
Atkinson, R. (1989) Kinetics and Mechanisms of the Gas-Phase Reactions of the
Hydroxyl Radical with Organic Compounds. J. Phys. Chem. Ref. Data, Monograph 1, Am.
Chem.
Soc./Am. Inst. Phys./NIST, p. 229.
Corchnoy, S.B., Atkinson, R. (1990) Kinetics of the gas-phase reactions of OH and NO3
radicals with 2-Carene, 1,8-CineoIe, p-Cymene, and Terpinolene. Environ. Sci. Technol. 24,
1497-1502.
Finlayson-Pitts, B. J., Keoshian, C.J., Buehler, B., Ezell, A.A. (1999) Kinetics of
reaction of chlorine atoms with some biogenic organics. Int . J. Chem. Kinet. 31,
491-499.
Koch, R., Knispel, R. Elend, M., Siese, M., Zetzsch, C. (2007) Consecutive reactions of
aromatic-OH adducts with NO, NO2 and O2: benzene, naphthalene, toluene, m- and
p-xylene, hexamethylbenzene, phenol, m-cresol and aniline. Atmos. Chem. Phys.
7, 2057-2071. |
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