Triplet open-shell ground states

Both l3A"-4b and 23A"-4b are predicted to be minima on the potential surface at the CASSCF level.61 This is in contrast with the situation for 4a, where one triplet state is calculated to be a transition state for pseudorotation of the other.55-57 A derivative of triplet 4b has been implicated in solution trapping experiments,49 but there have been no reports of the direct observation of 4b. Facile intersystem crossing from the triplet ground state (13A") to the open-shell singlet state ( A"), followed by vibrational relaxation to one of the enantiomers of 3b (Fig. 12), would be expected to rapidly depopulate the 13A" state of 4b. Thus, direct detection of triplet 4b may prove to be much more difficult than detection of 4a was.lla... 

These methods can, in principle, be used to study all types of electronic states. For excited states, usually the reference state is the ground state and is closed-shell. The excited states are then singlets or triplets and are properly spin- and symmetry-adapted, and open-shell singlet states are obtained. The reference state can be an open-shell, but not all excited states obtained are spin eigenfunctions and results are of mixed quality. The electron attachment- (EA) and IP-EOM schemes are also most profitably applied when the reference state is a closed-shell. 

It is found that triplet (S = l) is the ground state for a linear Fe-O-O conformation, with the same electronic configuration as found for the open-shell singlet ... 

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