Kinetik der Reaktionen des Hydroxylradikals mit ungesättigten Kohlenwasserstoffen in einer Lavaldüsenexpansion

Abstract

The investigation of hydroxyl radical reactions with unsaturated hydrocarbons like ethyne, ethylene, propene and isoprene at low temperatures plays a significant role for the understanding of tropospheric chemistry. The temperature dependence of rate constants provides information about mechanistic details regarding the observed reaction. Bimolecular rate constant are then very sensitive to details of the potential energy surface at chemical significant energies.This thesis presents experimental investigations of the hydroxyl radical reactions with several unsaturated hydrocarbons at low temperatures. The carrier gas (nitrogen) is cooled down to 58 K using the expansion through a Laval nozzle. The Laval nozzle produces a uniform flow (temperature, density and Mach number are constant) of the carrier gas. Hydroxyl radicals are generated by means of laser photolysis of hydrogen peroxide at 193 nm and 248 nm. The time dependent hydroxyl radical concentration is observed via laser induced fluorescence (LIF). To estimate the rate constants of the observed reactions calculations according to the statistical adiabatic channel model SACM) are performed, the required educt and product properties are obtained by means of DFT methods.The determined rate constants between 60 and 300 K range from 10-12 to 10-10 cm3s-1. A systematic increase of the rate constants from ethyne to ethylene, propene and isoprene is observed. Moreover there is (at least up to 100 K) a negative temperature dependence of the rate constants. This feature appears due to long range electrostatic forces directly connected to transition states located at great reactant distances. The vibrational relaxation of the hydroxyl radical is used for the determination of high pressure limiting rate constants of the hydroxyl radical reaction with ethyne and ethylene. The OH recombination reaction with propene and particularly isoprene distinguish themselves by a decrease of the rate constants below 100 K. This effect may be due to the existence of intermediate isomers. Analysis of the rate constants reveals a significant barrier for the OH ethyne reaction, which dominates the negligible temperature dependence between 300 and 60 K, whereas the respective ethylene reaction shows a negative temperature dependence within the measured range.