ENERGY DEPENDENCIES OF THE PROTON-TRANSFER REACTIONS H3O(-DOUBLE-LEFT-RIGHT-ARROW-CH2OH++H2O()+CH2O)

The proton transfer reaction system H3O+ + CH2O double left right arro w 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 cal culated collisional limiting value, k(c), showing a slightly negative energy dependence. The rate constant, k(r), for the reverse channel, w hich 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) a t KEcm = 1 eV. This endoergic reaction is paralleled by an associative channel forming CH2OH+.H2O, which undergoes ligand switching with wat er molecules to produce H3O+.H2O, yielding a bond energy BE(CH2OH+-H2O ) = 27.7 kcal mol(-1) in agreement with previous data. The present res ults are important requisites to monitor the formaldehyde concentratio ns in air using proton transfer reaction-mass spectrometry (PTR-MS). ( C) 1997 Elsevier Science B.V.