Low-Temperature Reactions of OH Radicals with Propene and Isoprene in Pulsed Laval Nozzle Expansions

Abstract

The kinetics of the reactions of OH radicals with propene and isoprene in N 2 have been studied in the temperature range of 58-300 K in a Laval nozzle expansion. Laser-induced fluorescence of the OH radical that is formed in the photolysis of H 2 O 2 at 193 nm has been detected. The determined rate constants (2 × 10 -11 to 2 × 10 -10 cm 3 molecule -1 s -1 ) for the OH radicals reacting with excess propene and isoprene (2methyl-1,3-butadiene) have been found to increase when the temperature is decreased. The room temperature rate constants are in agreement with the literature data. Below 120 K, a saturation of the rate constant for the case of propene and a turnover to a formally positive temperature dependence for isoprene have been observed. The observed negative temperature dependence and the course of the temperature dependence at very low temperatures for both reactions are discussed within the framework of the loose transition-state theory and a simple two-transition-state model.

Pulsed Laval nozzle study of the kinetics of OH with unsaturated hydrocarbons at very low temperatures

“…The overall design is based on the pulsed version of a CRESU 165 apparatus pioneered by M. Smith and co-workers 24,28 , and subsequently adopted by the groups of Leone 22,23 and Abel. 25 The design is influenced by experience gained by the authors during the operation of continuous 6 and pulsed 23 Laval nozzle systems. A schematic of the overall experimental arrangement 170 of the apparatus is shown in Figure 1, consisting of a vacuum chamber and pumps, gas delivery system and reservoir, Laval nozzle, photolysis and probe laser systems, fluorescence collection system and data acquistion system.…”

“…19, 20 130 The CRESU technique uses a continuous flow of gas and although able to produce temperatures down to 7 K 1 requires very large pumps and enormous flows of gases. A continuous system with a modest pumping capacity using short Laval nozzles has recently been developed and able to generate 135 temperaures as low as 77 K. 21 In order to reduce pumping capacity and gas flow considerably further, a number of pulsed CRESU systems have been developed, in which a short pulse of gas is expanded through the Laval nozzle, [22][23][24][25][26] achieving temperatures as low as 58 K, 25 and the reaction 140 initiated and monitored using LFP-LIF. In this paper we describe the pulsed Laval nozzle apparatus developed in Leeds and the results of experiments to characterise the spatial and temporal variation of temperature and density within the supersonic flow.…”

Gas phase reactive collisions, experimental approach

“…Uniform supersonic flows were initially developed in a continuous flow version for the study of ion-molecule reactions [35]. Later, the methodology was adapted to the study of reactions between neutral species in a continuous flow version [36][37][38] and also in a pulsed flow version [39][40][41][42][43].…”

A chirped-pulse Fourier-transform microwave/pulsed uniform flow spectrometer. I. The low-temperature flow system

“…Uniform supersonic flow systems have been developed and applied with great success for kinetics studies at low temperatures, as demonstrated by the CRESU technique (a French acronym for Reaction Kinetics in Uniform Supersonic Flows), 7,[14][15][16] and several subsequent instruments modeled after it. [17][18][19][20][21] At the heart of these systems is a Laval nozzle, an axisymmetric convergent-divergent nozzle recognizable as the familiar rocket nozzle design, from which emerges a flow uniform in temperature, velocity, and density along the propagation axis. The density in these flows (∼10 16 -10 17 cm −3 ) is sufficiently high to offer the reactants and products a relatively dense, inert, and thermalized medium.…”

“…17,23 Advantages of these pulsed systems have been demonstrated for many applications. [18][19][20][24][25][26][27] One shortcoming of these designs, however, is that they have all been based on the use of solenoid actuated valves that deliver limited gas throughput. Because establishing a stable flow requires reaching stable target pressures upstream of the flow, typically ≥50 mbar in the stagnation region, small reservoir volumes have been employed and in some cases the valve is fired directly in the nozzle throat.…”