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

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 the pulsed laser photolysis (PLP), laser-induced fluorescence (LIF) technique for studying the kinetics of free radical reactions in the ultra-cold environment provided by the gas flow in a CRESU (Cinétique de Réaction in Ecoulement Supersonique Uniforme) apparatus. This method has yielded rate constants for the reactions of OH with but-1-ene, (Z)-but-2-ene and (E)-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 the form (k/10–10 cm3 molecule–1 s–1)=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 (E)-but-2-ene.