Fe+-mediated interconversion of n- and i-C3H7OH preceding their gas-phase dehydrations:: Experimental and computational evidence for memory effects and inherent asymmetry of constitutionally equivalent methyl groups

The n-propyl/isopropyl isomerization operative in Fe+-mediated dehydrations of n-propyl and isopropyl alcohols has been rigorously analyzed by tandem mass spectrometry and density functional calculations. Metastable ion studies of complexes of atomic Fe+ with a set of selectively deuterated propyl alcohols address structural details of the reversibility of beta-hydrogen transfer steps. The labeling distributions in the ionic products reveal that the transiently focused, constitutionally equivalent methyl groups remain distinguishable in the course of n-C3H7 --> i-C3H7 isomerizations. This asymmetry causes a memory effect operative in the Fe+-mediated dehydration of n-propyl alcohol. While density functional calculations provide detailed insight into the nature of the reaction intermediates and, inter alia, suggest that the experimentally deduced asymmetry can be attributed to agostic interactions of the iron center with a beta-hydrogen atom of the emerging methyl group, a comprehensively consistent, quantitative explanation of the experimentally observed effects cannot be provided by the calculations.