Rearrangement heavy atom

Interestingly, it was possible to probe the spin-forbidden component of the tunneling reaction with internal and external heavy atom effects. Such effects are well known to enhance the rates of intersystem crossing of electronically excited triplets to ground singlet states, where the presence of heavier nuclei increases spin-orbit coupling. Relative rates for the low-temperature rearrangements of 12 to 13 were... 

As a result of these heavy-atom KIE experiments the principal features of the benzidine rearrangements have now been firmly established. The two main products arise from two parallel reactions one of which is concerted and the other is not. Other concerted processes have been identified and all of the concerted processes can be readily classified in the terminology of sigmatropic rearrangements within the general class of percyclic reactions. 

The benzidine rearrangements can also be brought about thermally, but very few mechanistic studies have been carried out. One set of heavy-atom KIE measurements has been made in the reaction of 2,2 -hydrazonaphthalene (18)21. Substantial nitrogen (1.0611 for the [15N, 15N ]) and carbon (1.0182 for the [1,1 -13C2]) KIE values were obtained showing that, just as for the acid catalysed reaction, this is a [3,3]-sigmatropic rearrangement, this time presumably of the non-protonated reactant. 

In terms of photophysics, electron transfer reactions create an additional non-radiative pathway, so reducing the observed emission lifetimes and quantum yields in A-L-B dyads in comparison with a model compound. However, there are other processes, such as molecular rearrangements, proton transfer and heavy-atom effects, which may decrease the radiative ability of a compound. One of the most important experimental methods for studying photoinduced processes is emission spectroscopy. Emission is relatively easy to detect and emission intensities and lifetimes are sensitive to competing processes. Studying parameters such as emission quantum yields and lifetimes for a given supramolecular species and associated... 

Sadeghpoor, R., Ghandi, M., Najafi, H.M., and Farzaneh, F. (1998) The oxa-di-jt-methane rearrangement of p.y-unsaturated ketones induced by the external heavy atom cation effect within a zeolite. Chemical Communications, (3), 329-330. 

Interestingly, some systems, such as (1) itself, show a tautomeric equilibrium between an H2 complex and a classical dihydride form others show a stretched H2. In the first case we have a double minimum on the potential energy surface (PES) (H2 and dihydride) and in the other a single minimum. The difference appears to lie in the motion of the heavy ligands that produces a barrier to the dihydrogen/dihydride tautomerism and where there are no such heavy atom rearrangements, the barrier disappears and a stretched H2 becomes possible. 

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