Singlet normally forbidden

The blue colour of oxygen in the liquid and solid phases is due to electronic transitions by which molecules in the triplet ground state are excited to the singlet states. These transitions are normally forbidden in pure gaseous oxygen and, in any case, they occur in the infrared region of the spectrum at 7918 cm" ( Ag) and 13 195 cm" ( ]+). However, in the condensed phases a... 

The first selection rule concerns the multiplicity of states Since the components of p are ungerade or odd, the integrand becomes gerade or even only provided the product of the two spin functions is ungerade, i.e., if their multiplicity or the total spin quantum number S does not change, or in other words if AS=0. Thus singlet-triplet transitions are normally forbidden. 

The ortho cycloaddition is thermally forbidden in a suprafacial-suprafacial manner and the photochemical reaction is forbidden with S benzene and ground-state alkene. On the basis of these considerations, it could be understood that the ortho addition had only been observed with systems where the alkene is the lowest excited singlet species (as with maleimides [37,74,75] or where either the alkene or the arene has marked acceptor properties (the only examples known at that time were benzene-acrylonitrile [127] and benzonitrile + a mono-olefin [1,73], Benzene-acrylonitrile and benzonitrile-olefin systems do not display charge-transfer absorption, but charge transfer could well follow excitation. Bryce-Smith further stated that irradiation of benzene in the presence of simple mono-olefins normally provides B2u (Si) benzene as the lowest excited singlet species, which leads to meta rather than ortho addition, but the latter process might, in principle, be able to occur under conditions where a Biu (S2) state of benzene is populated. 

The theory underlying this effect depends critically on two selection principles the nuclear spin dependence of intersystem crossing in a radical pair, and the electron spin dependence of the rates of radical pair reactions. Combined, these selection principles cause a sorting of nuclear spin states into different products and result in characteristic nonequilibrium populations in the nuclear spin levels of geminate reaction products (whose formation is allowed for singlet pairs but spin forbidden for triplet pairs) and in complementary nonequilibrium populations in the spin levels of free-radicals ( escape ) products (whose formation is electron spin independent). The transitions between these levels will be in the direction towards restoring the normal Boltzmann population their intensities will depend on the extent of nonequilibrium population. The observed effects are... 

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