Field dissociation rotational effect

Relaxation of complicated ligands may occur as a step in both pathways. Diebler and Eigen 461 indicated the ways in which such mechanisms could be analysed using fast reaction methods. Several studies of Ln(III) complex formation and of the formation of Ln(III) mixed complexes have been analysed. Generally the dissociative mechanism is considered to dominate and we are then concerned with the water exchange rate. Several studies have shown that the rate decreases from La(III) to Lu(III) but there seems to be a minimum rate around Tm(III). This is also seen in the rate of rotation of ligands on the surface of the ions, Fig. 7. There may be a small crystal field term, or another contribution to a tetrad -like effect from the 4f electron core. However in the hydrate the precise relationship between the inner and outer sphere water may also be important as we saw when we discussed the heat and entropy of complex ion formation. 

H. Abou-Rachid, T. Nguyen-Dang, O. Atabek, Dynamical quenching of laser-induced dissociations of diatomic molecules in intense infrared fields Effects of molecular rotations and misalignments, J. Chem. Phys. 114 (2001) 2197. 

Thus far, mnaway was predicted or observed only for atomic ions in atomic gases. For polyatomic ions and/or gases, the decrease of sD,(s) at high s required for mnaway appears less likely because of vibrational and/or rotational energy loss channels that are usually effective in the relevant s range (2.5 and 2.6.1). For polyatomic ions, field heating may cause dissociation at E/N below the mnaway threshold, leaving the issue moot. 

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. 

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