Ultra-low temperature kinetics of neutral–neutral reactions: rate constants for the reactions of OH radicals with butenes between 295 and 23 K

Abstract

The first experiments on the kinetics of reactions of the OH radical at temperatures below 80 K are reported. They have been carried out by applying t h e pulsed laser photolysis (PLP), laser-induced fluorescence (LIF) technique for studying the kinetics of free radical reactions in t h e ultracold environment provided by the gas flow in a CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus. This method has yielded rate constants for the reactions of OH with but-1-ene, (Z)-but-2-ene and (€)-but-2-ene at temperatures down to 23 K. The rate constants for all three reactions increase monotonically as the temperature is lowered and this dependence of the rate constants on temperature can be fitted to an empirical expression of t h e form (k/tO-" cm3 molecule-' s-') = a exp[b(T/298K)] with a and b equal to 5.2 and -2.8 for but-1-ene, 4.7 and -2.1 for (Z)-but-2-ene and 5.4 and -2.1 for (€)-but-2-ene.

Reactions between neutral species at low temperatures: Laboratory results and astrophysical modelling

“…Reactions of the CN radical were the first to be investigated in the CRESU apparatus in Rennes (Sims et al 1994b). The rate coefficients obtained (Sims et al 1993b) for their reaction with C2H2 are shown in Fig.…”

“…Over the past 4 years, the PLP-LIF technique has been implemented in the super-cold environment provided in a CRESU apparatus (Sims & Smith 1995, Sims et al 1994b. The CRESU apparatus, originally devised to study ion-molecule reactions under thermally equilibrated conditions at temperatures down to 8 K , Rowe et al 1987), has at its heart a Laval nozzle (see Fig.…”

“…Downstream from the nozzle a relatively dense (η « 10 16 -10 17 molecule cm -3 ) supersonic flow of gas is generated which remains uniform for several tens of centimetres, equivalent to a few hundred microseconds. The gas is generally supersaturated but, with suitable choice of the carrier and the concentration of the coreagent, the formation of dimers and higher clusters can be avoided (Sims et al 1994b). …”

“…The reactions involving CN and CH (like that between CN and C2H2) probably all proceed via an energised addition adduct which then dissociates to a lower energy pair of products. (Sims et al 1993b, Canosa et al 1996 and for OH reacting with 1-butene (Sims et al 1994b). For the first two reactions, the rate coefficients appear to reach an essentially limiting value of (4 -5) χ 10~1 0 cm 3 molecule -1 s -1 below ca.…”

“…As the data in Table 2 show, low temperature rate coefficients have been measured for a number of reactions between the diatomic radicals CN, OH or CH and small alkenes or acetylene (Sims et al 1993b, Sims et al 1994b, Canosa et al 1996. In the case of OH with butènes, the reaction proceeds by collisionally-aided stabilisation to a hydroxy-butyl radical, the reaction being in its high pressure 'saturated' limit.…”