"Proton-transport" catalysis in the gas phase. Keto-enol isomerization of ionized acetaldehyde

Fourier transform ion cyclotron resonance experiments show that a variety o f molecules catalyze the hydrogen transfer which converts ionized acetaldeh yde CH3CHO.+ 1 to its vinyl alcohol counterpart CH2CHOH.+ 2. Each of these ions has been characterized by its specific bimolecular reactions with sele cted reactants. Calculations show that two pathways, for which the rate det ermining barriers have almost the same energy, are feasible. The first tran sition state involves a direct catalyzed 1,3-H transfer, while the second i nvolves two successive 1,2-H transfers. A detailed experimental study, usin g methanol as a catalyst as well as labeled reactants, indicates that only the first pathway operates in the isomerization process. The different step s of these two independent pathways were elucidated. The first begins with the formation of a highly stabilized complex 3, involving a two-center-thre e-electron interaction between the two oxygen atoms and an interaction betw een a hydrogen of the methyl group of 1 and the oxygen of methanol. This co mplex isomerizes into a complex 4, which in rum gives the complex 5, via a transition state located 6.3 kcal mol(-1) below the energy of the reactants . This complex 5 corresponds to ionized vinyl alcohol hydrogen bonded to th e oxygen of methanol, which dissociates to yield ion 2. The second pathway begins with the interaction between the hydrogen of the CHO group and the o xygen of methanol and gives the complexes 6 and then 7, which correspond to protonated methanol hydrogen bonded to a CH3CO. radical. Dissociation of 7 to give protonated methanol is favoured with respect to further isomerizat ion leading to ionized vinyl alcohol. Compared to the unimolecular conversi on between energetic ions 1 and 2, which can occur either by a direct 1,3-H transfer or by a double 1,2-H transfer, the reaction of 1 with methanol ca talyzes the first pathway while inhibiting the second one. In the case stud ied, catalysis is perhaps better described as a hydrogen atom transport. (I nt J Mass Spectrom 202 (2000) 161-174) (C) 2000 Elsevier Science B.V.