Diradicals localized

Bicyclic localized singlet 1,3-a-diradicals were theoretically designed by the orbital phase theory (Scheme 28c) [54]. 

Extensive experimental studies of localized singlet diradicals have been made by Abe and co-workers [55-60]... 

At the equilibrium inter-atomic distance R, two paired electrons of occupy the bonding orbital with a closed-shell low-spin singlet (S = 0). When the bond length is further increased, the chemical bond becomes weaker. The dissociation limit of corresponds to a diradical with two unpaired electrons localized at each atom (Fig. 1). In this case, the singlet (S spin-antiparaUel) and triplet (T spin-parallel) states are nearly degenerate. Different from such a pure diradical with... 

If the radical orbitals are mainly of the p-character (e.g., a p-orbital of carbon in TMM), they interact with each other through the n bond, labeled fl (Fig. 4a). This kind of ir-conjugated diradicals with p-type radical orbitals is classified as the ir-type diradicals. Interestingly, in the language of orbital phase theory, the simplest 1,3-localized diradical, TM (2) also belongs to the ir-type family provided its radical orbitals are dominated by the p-character. The only difference between the TMM (Fig. 4a, left) and TM (Fig. 4a, right) lies in the fact that the radical orbitals of TM interact with each other through the a bond (labeled X) instead of the n bond in TMM. 

Despite the success of these simple rules in the Jt-conjugated diradicals, most of them cannot be directly applied to the localized diradicals within the o-framework. In Sect. 5, we will demonstrate that the orbital phase theory works effectively for the localized 1,3- and 1,4-diradicals as well. 

Fig. 18 Typical conformations of acyclic localized 1,3-diradicals, including disrotatory conformers a and c, and conrotatory conformer b...

Stable localized singlet 1,3-diradicals are bnilt on 2,4-disilacyclobntane-l,3-diyls with electron-withdrawing o-bonds on the silicon atoms and jt-electron... 

Fig. 21a, b Orbital phase properties on singlet and triplet localized sUicon-centered 1,3-diradicals. a o-Type. b ir-Type... 

Singlet diradicals are usually extremely short-lived intermediates. For example, trimethylene (TM, 2) was observed to have a fast decay time of 120 fs by femtosecond spectroscopy [84, 85]. Since the localized 1,3-cyclopentanediyl diradical (62) was characterized by Buchwalter and Closs in 1975 [81, 82], experimental efforts have been made to prepare and characterize the persistent, localized singlet 1,3-diradicals. Some experimental achievements of the localized diradicals are collected in Fig. 25 and Table 3. It should be mentioned that the literature of experimental studies selected here is not exhaustive and more related references can be found in [83-115] and others. 

Table 3 A brief collection of some experiments for localized 1,3-diradicals...

Substantial advances have been made in understanding, preparing, and detecting the carbon-centered delocalized or localized diradicals. But the silicon-centered diradicals are much less explored. Search for other stable localized singlet diradicals remains a goal of experimental and theoretical scientists. 

On the basis of the orbital phase continuity/discontinuity in the involved cyclic orbital interactions, some general rules were drawn for the Jt-conjugated and localized diradicals ... 

In a similar fashion the bonding in H2 might be formally regarded as a complementary pair of one-electron donor-acceptor interactions, one in the ot (spin up ) and the other in the 3 (spin down ) spin set.8 In the long-range diradical or spin-polarized portion of the potential-energy curve, the electrons of ot and (3 spin are localized on opposite atoms (say, at on HA and 3 on HB), in accordance with the asymptotic dissociation into neutral atoms. However as R diminishes, the ot electron begins to delocalize into the vacant lsB(a) spin-orbital on HB, while (3 simultaneously delocalizes into Isa on HA, until the ot and (3 occupancies on each atom become equalized near R = 1.4 A, as shown in Fig. 3.3. These one-electron delocalizations are formally very similar to the two-electron ( dative ) delocalizations discussed in Chapter 2, and they culminate as before (cf. Fig. 2.9) in an ionic-covalent transition to a completely delocalized two-center spin distribution at... 

The anion-diradical (BQNN) has two nondegenerated SOMOs. One is delocalized over the entire molecule and the other (SOMO ) is localized within the NN group. 

The distance between the two spin-bearing centers is mnch greater in the structure shown in Scheme 1.39 than that shown in Scheme 1.38. Note that in this case, a control of spin coupling via control of electron localization is attained. The straightforward control of electron localization was implemented via modification of the snbstitnents at the center site (compare Schemes 1.38 and 1.39). The mentioned anion-diradicals were prepared by partial oxidation of relative polyanions with iodine (Rajca and Rajca 1995). When the carbanion-stabilized 4-biphenyl, in the place of 4-tert-butylphenyl, was employed as a snbstitnent at the center site, the negative charge was confined to the center site and the spin density to the terminal (nonadjacent) site. 

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