Energy dependencies of the proton transfer reactions H3O++CH2O↔CH2OH++H2O

The proton transfer reaction system H3O+ + CH2O double left right arrow CH2OH+ + H2O has been investigated in both directions as a function of the mean relative kinetic energy, KEcm, 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 x 10(-12) cm(3) s(-1) at KEcm = 0.05 eV to k(r) = 2 x 10(-10) cm(3) s(-1) at KEcm = 1 eV. This endoergic reaction is paralleled by an associative channel forming CH2OH+.H2O, which undergoes ligand switching with water molecules to produce H3O+.H2O, yielding a bond energy BE(CH2OH+-H2O) = 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 reaction-mass spectrometry (PTR-MS). (C) 1997 Elsevier Science B.V.