Energy stabilization, oxygen species

The introduction of heteroatoms into the hydrocarbon diradicals is a frequently applied strategy to tune the spin preference and relative stabilities of diradicals. The heteroatoms may change the energies of donor or acceptor orbitals, and consequently affect the donor-acceptor interaction involved in the cyclic orbital interaction. Take 2-oxopropane-l,3-diyl, or so-called oxyallyl (OXA, 18) as an example [29]. It is a hetero analog of TMM, as shown in Fig. 14. The replacement of CH with oxygen in the central fl unit leads to a decrease in energies of Jt and k orbitals. This may enhance the orbital interaction through one path (denoted by bold lines) and weaken that via the other (denoted by wavy lines) relative to the continuous cyclic orbital interaction in the parent species 1 (Fig. 14). As a result, the p-Jt -q... 

Zimmerman et al.(i2y pointed out that the bridged phenonium ion from (48) is more stable (lower energy route) than is the phenonium ion from (47), while the phenonium ions from (49) and (50) should be of about equal stability. An alternate explanation has been proposed by Kropp,a) who suggests that preferred migration to the position adjacent to the electron-rich oxygen in aprotic solvents arises from a preferred minimization of charge separation in in the dipolar species. 

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