Effect of variations of the size and structure of the principal alkyl group on alkene elimination from the immonium ions CH3CH2CH=N+(CH3)R, CH3CH2CH2CH=N+(CH3)R and CH3CH2CH2CH2CH=N+(CH3)R

The reactions of three homologous series of metastable immonium ions of gen eral structure, CH3(CH2)(m)CH=N+(CH3)R, [m 1-3; R = n-CnH2n+1, n = 1-10, 12 , and 14, iso-C5H11, CH2CH(CH3)C2H5, neo-C2H5, acid CH2CH3C(CH3)(3)] are re ported and discussed. The first member of each homologous series is anomalo us in undergoing several reactions, including elimination of molecular hydr ogen and a methyl radical. In contrast, higher members of all three series expel either or both alkenes, CnH2n or Cn-1H2n-2 derived from the principal alkyl group, R. The influence of the size and structure of R on these alke ne eliminations is analysed. Loss of CnH2n produces an approximately Gaussi an metastable peak; the associated kinetic energy release is relatively sma ll and varies only slightly as the homologous series are ascended. In contr ast, Cn-1H2n-2 elimination releases a much larger amount of kinetic energy, particularly for n = 3, when the associated metastable peak is dished; but the kinetic energy release declines dramatically on ascending the homologo us series. Loss of Cn-1H2n-2 favoured by branching at the gamma-carbon atom , provided there is a gamma-hydrogen atom, but it does not occur when CH2CH 2C(CH3)(3) because gamma-hydrogen transfer is impossible. (Int J Mass Spect rom 199 (2000) 189-200) (C) 2000 Elsevier Science B.V.