Ortho effects: A mechanistic study

The fragmentation mechanism of the loss of a water molecule from the radical cations of 2-methylbenzylalcohol 1, 2-methylbenzoic acid 2, and some related compounds 3-9 by an ortho effect is studied. The analysis of the MIKE spectra together with specific deuterium labelling studies and assisted by semi-empirical calculations reveal a continuous spectrum of mechanisms of the ortho effect ranging from a two-step mechanism with a rate determining final loss of the neutral fragment and a rate determining 1,5-hydrogen transfer in the first step entailing a large reverse activation energy to a presumably concerted 1,4-elimination with a more or less asymmetric high energy transition state, These mechanisms cause a different behavior of metastable ions with respect to the kinetic energy release (KER) during the fragmentation by an ortho effect, In the case of a rate determining last step as established for the 1,4-elimination of NH3 from the molecular ions of 2-methylbenzylamine 8 the absence of a reversed activation energy ensures ''normal'' KER behavior and narrow Gaussian shaped peaks in the MIKE spectra, The other mechanisms of the ortho effect exhibit a significant reverse activation energy originating in a barrier of the initial 1,5-hydrogen transfer of the ortho effect, Consequently, large values of the KER are observed for this elimination process in the MIKE spectra of the precursor ions, However, the kinetic energy release distribution (KERD) during the dissociation depends on the stability of the intermediate distonic ion created by the 1,5-hydrogen migration, A well-defined and stable distonic ion as an intermediate leads to flat topped peaks and a large KER which is typically observed for the elimination of H2O in the MIKE spectra of the molecular ions of 2-methylbenzylalcohol 1 and related 2-alkylbenzylalcohols. In the case of a very short life time of the intermediate ion generated by the initial 1,5-hydrogen shift the mechanism can not be discriminated from a concerted 1,4-elimination of H2O with a more or less asymmetric transition state. This situation is typically for the Fragmentation of ionized 2-methylbenzoic acid 2 and related 2-alkylbenzoic acids, and a broad, nearly triangular signal is observed in the MIKE spectrum of these molecular ions.