Selection rules atom transfer reactions

Insufficient experimental data is available to demonstrate either the occurrence or lack of selection rules. In Table 15, some of the fastest reactions do correspond to transitions allowed for both atoms. Singlet helium transfers to Ne at a rate consistent with the data in Fig. 26, whereas triplet helium transfers comparatively slowly neither of the He transitions is allowed. However, there appear to be other cases, discussed earlier, where a forbidden transition is preferred to an allowed transition. Stepp and Anderson146 have suggested that there is partial conservation of electronic angular momentum accompanying energy transfer between atoms, and interpreted experiments on mercury fluorescence by means of the steps... 

It should be clear that the selection rules apply to concerted processes between spatially accessible parts of a molecule. Antarafacial [1,3], [1,5], or [3,3] reactions involving u-orbital transfers between carbon atoms are not possible. Antarafacial migrations do become feasible when the transition state forms a cycle of at least 7 or 8 atoms Woodward and Hoffmann (1965b) cite the [1,7] example (76). 

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