FIELD-IONIZATION KINETICS - UNIMOLECULAR DECOMPOSITION OF THE HEXANAL RADICAL CATION OVER THE TIME RANGE FROM 10-PS TO 10-MUS

Rates of formation of the major product ions from decomposition of hexanal, [3,3-2H2] hexanal, [4,4-2H2] hexanal, and [5,5-2H2] hexanal following field ionization have been determined over the time range 10 ps to several microseconds. These ions can be attributed to decomposition processes initiated by β-, γ-, δ-, or ε-hydrogen transfer to the oxygen via five-, six-, seven-, or eight-membered cyclic transition states. It is suggested that the McLafferty rearrangement at short times (<10-10 s) forms a vinyl alcohol ion and but-1-ene, and at longer times (10-10 to 10-6 s) a but-2-ene ion and vinyl alcohol. The observations at times <1 ns of m/e 57, m/e 73, and m/e 82 with [3,3-2H2]hexanal and m/e 73 with [4,4-2H2]hexanal are explained by decompositions initiated by β-hydrogen transfer. The intermediate so formed rearranges by a 1,2-hydrogen shift to (CH= O+βHCH2CβH γHĊγHCH2CH3), which then decomposes along its usual pathways (i.e., as if it were formed directly by γ-hydrogen transfer). © 1979 American Chemical Society.