Field dissociation isotope effects

Fig. 2.26 When 4He is replaced with 3He, the secondary Rh2+ peak disappears even though 3HeRh2+ ions are still formed. At first glance, this strong isotope effect is most surprising since one would expect 3HeRh2+ to field dissociate more easily than 4HeRh2+ because of its smaller reduced mass. This peculiar isotope effect is the result of a center of mass transformation in the applied field, as can be understood from the Schroedinger equation of eq. (2.63), already explained in...

The phenomena studied in spin chemistry (magnetic field effects, magnetic isotopic effect, spin polarization of electrons and nuclei, effects of resonance microwave pumping, etc.) are based on the peculiarities of reactions of spin-correlated radical or radical ion pairs in condensed media. These pairs arise from dissociation (ionization) of molecules under heat, light or ionizing radiation, preserving spin multiplicity of their precursors. 

For intramolecular isotope effects, as one starts from a unique precursor, the two processes correspond to the same internal energy distribution, so that useful information can be inferred even from ionabun-dances measured without internal energy selection (like metastable dissociations or field ionization kinetics). This is not the case for intermolecular effects, where there is no warranty that the two internal energy distributions are the same. Within the RRKM framework, the intramolecular kinetic isotope effect depends only on the transition state properties (critical energy, rotational constants and vibrational frequencies) and not on the reactant properties. 

Generally speaking, the dissociation modes are established by photolysis, isotope studies, the electric field effect, and, to some extent, by special mass spectrometric methods. In addition, polymerization and isomerization studies have been helpful. 

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