Three Major Electronic States of Dioxygen (O2); Relevance to Differences in the Reactivities of Iron(IV)-Hemes

These diagrams show the three major electronic states of dioxygen (O2) [note: I can't change pictures, in the blogger software, after I upload them, without dismantling the whole posting, basically, but the final diagram should show either no single-electron-transfer arrow or a two-electron-transfer arrow on zwitterion II (showing transfer to zwitterion I), not a single-electron-transfer arrow]. I'll put up sketches of the molecular orbitals soon, here, but one of the main points of this article [see Yamaguchi et al., 2009: (http://linkinghub.elsevier.com/retrieve/pii/S0277538709000515)] and of many other articles is that triplet dioxygen exists in a robustly-diradical state, with regard to the 2pi*x and 2pi*y antibonding molecular orbitals that are singly-occupied by electrons of the same spin, and that, in all of the three major electronic states of dioxygen, the 2pz(sigma) (a.k.a. 2sigma(2pz) a.k.a. 3sigma(gerade)) bonding molecular orbital exhibits a less-robust, 2pz-2pz diradical character with "singlet coupling" between the two "partially unpaired" electrons [the 2pz atomic orbitals are oriented along the z-axis, which is the internuclear axis that intersects both oxygen nuclei, and form a "partial" 2sigma(2pz) (a.k.a. 3sigma(gerade)) sigma bond]. That's shown in one of the diagrams below. Also, the highest-energy singlet excited state (singlet sigma) exists as a pair of zwitterionic "resonance" forms (still within the same electronic state), and the first excited state (singlet delta) exists as four resonance forms (two zwitterions and two (2pi*x and 2pi*y) diradicals in which there is "sigma coupling" between the two orbitals that are singly-occupied by electrons of opposite spin). The other articles cited are these [Lyne et al., 1993: (http://pubs.acs.org/doi/abs/10.1021/ic00074a022); Bytautas and Ruedenberg, 2010: (http://link.aip.org/link/?JCPSA6/132/074109/1)]. The other point is that the FeO moieties of some of the electronic states of perferryl and ferryl heme (and other oxidation states of heme) exist as "singlet delta"-like diradicals or zwitterions, "singlet sigma"-like zwitterions alone, or triplet-O2-like diradicals [Yamaguchi et al., 2009; Filatov et al., 1999: (http://www.rsc.org/delivery/_ArticleLinking/DisplayArticleForFree.cfm?doi=a809385g&JournalCode=P2)(http://cat.inist.fr/?aModele=afficheN&cpsidt=10072829)].