Vicinal Elimination from 2-Fluoroisopropyl Cation in the Gas Phase

Article abstract of DOI:10.1021/ja00211a006

An electron bombardment flow (EBFlow) reactor has been used to assay C3H5⁺ isomer distribution from different sources.Quenching the ions by fluoride abstraction followed by 19F NMR of recovered neutral C3H5F permits analysis of the ratio of CH3C=CH2⁺ to allyl cation.Electron impact on 1-bromopropene yields predominantly allyl cation, in agreement with a previously reported mass spectrometric measurement.Ionization of isobutene yields an 80:20 mixture of CH3C=CH2⁺ and allyl cation.Electron impact on tert-butyl fluoride yields only CH3C=CH2⁺.The difference between the two latter results means that ionized isobutene is not an intermediate in production of C3H5⁺ from tert-butyl fluoride.Therefore, the major fragment of tert-butyl fluoride, (CH3)2CF⁺ (1), must be undergoing vicinal elimination of hydrogen fluoride to yield C3H5⁺.Theoretical calculations provide evidence that this does not take place via an isolated, electronically excited state of 1.Ab initio calculations on the ground-state C3H6F⁺ potential energy surface reveal six stable isomers of 1,3-8, all of which have stable, isoelectronic C3H6O analogues.Two of these, 7 and 8 (isoelectronic to allyl alcohol and acetone enol, respectively), correspond to ion-molecule complexes of C3H5⁺ and hydrogen fluoride.More extensive calculations on 8 illustrate the difference between an ion-molecule complex and a hydrogen-bonded complex.Other isomers of 1, CH3CHFCH2⁺ (2) and CH3CH⁺CH2F (11), do not have stable, isoelectonic C3H6O analogues.Neither are they stable C3H6F⁺ isomers.Ab initio calculations show that 2 undergoes barrier-free rearrangement to CH3CH2CHF⁺ (3), while 11 represents one of the resonance structures of 2-methylfluoriranium, 4.Ab initio results show good agreement with empirical estimates for ΔH_f(stand.) of 3, of methylvinylfuoronium (5), and of methyl-2-propenylfluoronium (9).These lead to a calculated heat of formation of 8 that is 19 kcal/mol higher than that of 1.Ion 1 corresponds to the global energy minimum, and its two lowest energy isomers are calculated to be 3 and fluoretanium (6).Together the experimental and theoretical results show that 1 undergoes a 1,3-hydrogen shift to 8 enroute to elimination in preference to rearrangement to 6.