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| Titel |
Experimental and theoretical study of the reaction of OH with the aromatic molecule p-cymene, proceeding via two adducts. |
| VerfasserIn |
Paulo Alarcon, Cornelius Zetzsch, Birger Bohn, Marie-Thérèse Rayez, Jean-Claude Rayez |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250052607
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| Zusammenfassung |
| Nonmethane volatile organic compounds (NMVOCs) play a major role in the photochemical formation of ozone and secondary aerosols, and both processes are initiated by OH radicals, which are important in combustion as well. We studied the reaction of the biogenic p-cymene with OH radicals at Bayreuth in the temperature range between 300 and 413 K at a total pressure of 195 mbar in He by vacuum UV flash photolysis of H2O and time resolved resonance fluorescence of OH. The addition of OH predominates at room temperature and we can distinguish two adducts being formed. The observed decays of the OH signal are triexponential at elevated temperatures and consistent with the mechanism:
OH + p-cymene <--> adduct 1 (1, -1)
OH + p-cymene <--> adduct 2 (2,-2)
OH + p-cymene <--> products (3)
The sum (k1 + k2 + k3) was found to be (1.50 ± 0.05) x 10-11 cm3 s-1, in very good agreement with a value of 1.51 x 10-11 cm3 s-1 determined by Corchnoy and Atkinson (1990) in a chamber study at T=295K, and decrease slightly with increasing temperature up to T=330K, where it starts to fall to around 4 x10-11 cm3 s-1. Assuming that abstraction (k3) is negligible against addition, the equilibrium constant K1 of the loosest adduct (k1/k-1) can be estimated from the expression:
K1 = (k1 + k2 + k3)2/(k1 x k-1) ~ k12/(k1 x k-1)
DFT calculations performed at Bordeaux using the M06-2X functional shows a prereactive complex (PRC), 6.2 kcal/mol below the entrance channel followed by a transition state between -0.5 and 0.6 kcal/mol according to the addition site. If entropy is taken into account by calculating the activation free energy, we have found that the ortho and meta sites with respect to the CH3 substituted site are the most reactive. More theoretical studies are in progress. |
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