THE ROLE OF ION-NEUTRAL REORIENTATION IN THE UNIMOLECULAR REACTIVITY OF E-IONIZED AMINES - THE INFLUENCE OF THE SIZE OF THE FRAGMENTS

During the unimolecular decomposition of an ion IN --> I(ion) + N(neut ral), the fragments may mutually reorient before decomposition, when t he energy has reached the reorientation critical energy. After I-N reo rientation, I and N have only a very low probability of recombining in to the original IN structure, but can only decompose (or occasionally follow a different reaction pathway, if thermochemically allowed). As a consequence, decompositions of metastable ions are irreversibly deci ded as soon as their corresponding I-N reorientation threshold has bee n reached. It is shown, from the mass-analysed ion kinetic energy of s everal series of homologous primary amines, that the competition betwe en alpha-cleavages (and the occasionally accompanying losses of alkane s) is governed by the relative I-N reorientation-(Father than decompos ition)-critical energies. When the neutral has no permanent dipole, th e I-N reorientation critical energy depends on the sizes of the fragme nts, especially that of the fragment ion. An interpretation is propose d related to the McAdoo and Hudson hypothesis.