Keto−Enol Isomerization of Gas-Phase 2‘-Methylacetophenone Molecular Ions Probed by High-Temperature Near-Blackbody-Induced Dissociation, Ion−Molecule Reactions, and ab Initio Calculations

Abstract

The thermal dissociation of several substituted acetophenone molecular ions induced by infrared radiation from a hot wire has been studied in a Fourier transform ion cyclotron resonance spectrometer. The temperature dependence of the dissociation rate constants reveals that the 2'-methylacetophenone molecular ion is characterized by a much higher activation energy for dissociation than other acetophenones. This molecular ion also exhibits a very different behavior with respect to charge-transfer reactions. Unlike molecular ions obtained from other isomeric acetophenones, the 2'-methylacetophenone M + ° ion does not promote charge exchange with dimethyl disulfide but does undergo relatively slow electron transfer with ferrocene (IE = 6.74 eV). Ab initio calculations at the MP2/6-31G(d) level predict that the 2-MeC 6 H 4 COCH 3 +° ion (1) can undergo facile tautomerization to the much more stable enol ion 2, 2'-CH 2 C 6 H 4 C + (OH)CH 3 , by a 1,4-hydrogen migration (calculated energy barrier of 20 kJ mol - 1 ). The calculated recombination energy of this ion is in good agreement with the observations from the charge-exchange experiments. A full analysis of the potential energy surface suggests that, at low ionizing energies (≤11.5 eV), essentially all of the long-lived molecular ions have isomerized to 2. The present example reveals the versatility and some of the advantages of the high-temperature near-blackbody-induced dissociation (hot wire emission) for probing structural problems in ion chemistry.