Two different pathways for unimolecular and for catalyzed keto-enol isomerization of ionized acetophenone

Studies of the unimolecular reactions in the gas phase of the C6H5COCH3+ (1 ) and C6H5C(OH)CH2+ (2) ions have shown (1) that ion 1 does not convert to ion 2 prior to methyl radical loss, (2) that ion 2 isomerizes into ion 1 pr ior methyl radical loss, and (3) that this keto-enol isomerization does not occur by a direct 1,3-H transfer but by two successive 1,4-H transfers. Fo urier transform ion cyclotron resonance experiments show that acetone catal yses the isomerization 1 --> 2. Further, by using labeled reactants, it is demonstrated that this isomerization occurs by a direct catalyzed 1,3-H tra nsfer whereas the less energy demanding pathway connecting bare ions 1 and 2 is a double 1,4-H transfer. This represents the first description of a sy stem for which the pathways connecting two isomeric ions are different for the unimolecular and for the catalyzed isomerizations. (C) 2000 Elsevier Sc ience B.V.