ORTHO EFFECTS - A MECHANISTIC STUDY

The fragmentation mechanism of the loss of a water molecule from the r adical cations of 2-methylbenzylalcohol 1, 2-methylbenzoic acid 2, and some related compounds 3-9 by an ortho effect is studied. The analysi s of the MIKE spectra together with specific deuterium labelling studi es and assisted by semi-empirical calculations reveal a continuous spe ctrum of mechanisms of the ortho effect ranging from a two-step mechan ism with a rate determining final loss of the neutral fragment and a r ate determining 1,5-hydrogen transfer in the first step entailing a la rge reverse activation energy to a presumably concerted 1,4-eliminatio n with a more or less asymmetric high energy transition state, These m echanisms cause a different behavior of metastable ions with respect t o the kinetic energy release (KER) during the fragmentation by an orth o effect, In the case of a rate determining last step as established f or the 1,4-elimination of NH3 from the molecular ions of 2-methylbenzy lamine 8 the absence of a reversed activation energy ensures ''normal' ' KER behavior and narrow Gaussian shaped peaks in the MIKE spectra, T he 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 a re observed for this elimination process in the MIKE spectra of the pr ecursor ions, However, the kinetic energy release distribution (KERD) during the dissociation depends on the stability of the intermediate d istonic 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 o f the intermediate ion generated by the initial 1,5-hydrogen shift the mechanism can not be discriminated from a concerted 1,4-elimination o f H2O with a more or less asymmetric transition state. This situation is typically for the Fragmentation of ionized 2-methylbenzoic acid 2 a nd related 2-alkylbenzoic acids, and a broad, nearly triangular signal is observed in the MIKE spectrum of these molecular ions.