Heavy atoms spin-orbit coupling

The heavy atom effect is traditionally studied by physicists as redistribution of electron transition rates in the presence of bromine or iodine atoms. In the presence of heavy atoms spin-orbit coupling in the molecules is substantial.1,2 The spin-orbit interaction fulfils the role of the disturbance factor that removes the multiplicity prohibition. 

Nag-Chaudhuri J, Stoessell L, McGlynn S.P. External heavy-atom spin-orbital coupling effect. II. Comments on the effect of ferric acetylacetonate on the spectra of polyacenes. J Chem Phys 1963 38 2027-8. 

For compounds containing heavy atoms, spin-orbit and electron correlation energies are approximately of the same size, and one cannot expect these effects to be independent of each other. A variational approach that treats both interactions at the same level is then preferable to a perturbation expansion. Special care is advisable in the choice of the spin-orbit operator in this case. The variational determination of spin-orbit coupling requires a spin-orbit... 

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... 

In this section the effect of spin-orbit coupling on radiative and radiationless intercombinational transitions (transitions occurring between states of different multiplicity) will be investigated. We will be particularly concerned with the use of internal and external heavy atoms to induce spin-orbit coupling. The effect of heavy atoms on intercombinational processes occurring in aromatic hydrocarbons, carbonyl compounds, and heterocyclic compounds will be discussed. 

The amount of trans dimer formed upon the photolysis of acenaphthylene in heavy-atom solvents could be correlated with the square of the spin-orbit coupling constant for the heaviest atom in the solvent (see Table 5.11) ... 

From measurements of this type Thomaz and Stevens found a linear relationship for a graph of log(kJrPC,2) vs. where n is the number of halogen atoms in the molecule, is the spin-orbit coupling constant, and Em is the polarographic half-wave reduction potential of the heavy-atom quencher (Figure 5.16). This correlation suggests that an exciplex is formed by partial... 

A third possible channel of S state deexcitation is the S) —> Ti transition -nonradiative intersystem crossing isc. In principle, this process is spin forbidden, however, there are different intra- and intermolecular factors (spin-orbital coupling, heavy atom effect, and some others), which favor this process. With the rates kisc = 107-109 s"1, it can compete with other channels of S) state deactivation. At normal conditions in solutions, the nonradiative deexcitation of the triplet state T , kTm, is predominant over phosphorescence, which is the radiative deactivation of the T state. This transition is also spin-forbidden and its rate, kj, is low. Therefore, normally, phosphorescence is observed at low temperatures or in rigid (polymers, crystals) matrices, and the lifetimes of triplet state xT at such conditions may be quite long, up to a few seconds. Obviously, the phosphorescence spectrum is located at wavelengths longer than the fluorescence spectrum (see the bottom of Fig. 1). 

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