Energy dependencies of the proton transfer reactions [formula omitted

Abstract

The proton transfer reaction system H 3O + + CH 2O ⇎CH 2OH + + H 2O has been investigated in both directions as a function of the mean relative kinetic energy, KE cm, between the reactants from 0.05 eV to 1 eV in a selected ion flow drift tube (SIFDT) experiment. The rate constant k f for the forward channel follows closely the calculated collisional limiting value, k c, showing a slightly negative energy dependence. The rate constant, k r, for the reverse channel, which is endoergic by 5.2 kcal mol −1, increases from k r = 2.3 × 10 −12 cm 3 s −1 at KE cm = 0.05 eV to k r = 2 × 10 −10 cm 3 s −1 at KE cm = 1 eV. This endoergic reaction is paralleled by an associative channel forming CH 2OH +·H 2O, which undergoes ligand switching with water molecules to produce H 3O +·H 2O, yielding a bond energy BE(CH 2OH +−H 2O) = 27.7 kcal mol −1 in agreement with previous data. The present results are important requisites to monitor the formaldehyde concentrations in air using proton transfer reactionmass spectrometry (PTR-MS).