Freitag vormittags Vorträge - Bunsentagung - Technische Universität ...

Freitag 21.05.2004, 17:30 Uhr
Vortrag - Kinetik
B
19
The thermal unimolecular decomposition of
HCO
N Krasteva, H Hippler , F Striebel
Universität Karlsruhe, Lehrstuhl f. Molekulare Physikalische
Chemie, Fritz-Haber-Weg 4, 76131 Karlsruhe
The formyl radical(HCO) is an important intermediate in hydrocarbon
combustion and its fate under combustion conditions
is determined by the competition between the unimolecular
decomposition and bimolecular reactions with H, OH and
O2. The unimolecular decomposition, which yields H and CO,
has also attracted researcher’s interest due to its non-statistical
behavior (sometimes also called non-RRKM-behavior). Due to
the low density of states of HCO at energies near the threshold
energy (ρ(E0)¡ 1 / cm − 1) and a weak coupling between
the vibrational modes of the HCO the specific rate constants
(k(E)) show marked fluctuations (see e.g. H.-M. Keller et. al,
J Chem. Phys. 105 (1996), 4983). While k(E) has been investigated
both experimentally and theoretically, the influence of
the fluctuations of k(E) on the thermal rate constants (k(T,p))
has so far not been investigated in detail. This fact motivated us
to investigate the title reaction under high pressure conditions.
We did experiments at temperatures between 590 K and 800 K
and pressures ranging from 1 to 150 bar helium. Formyl radicals
have been generated by laser photolysis of acetaldehyd and
we detected HCO using laser induced fluorescence. An analysis
of the pressure dependence of the rate constants show that
standard treatments like a RRKM-model or a master equation
formalism are not able to describe the experimentally observed
pressure dependence adequately. The experimentally observed
pressure dependence is, however, consistent with the result of
a Lindemann-Hinshelwood-type calculation based on the resonance
widths calculated by Keller et al (see above). These calculations
show that the falloff curves are much broader than
expected: at pressures as low as a few mbar the rate constant is
no more in the low pressure regime while at 150 bars it is still
more than one order of magnitude smaller than the predicted
high pressure limiting rate constant.