Diradical character

Very recent calculations within the unrestricted ZORA-SORCI formalism on complexes indicated the neutral fe(l,2-dithiolene) complexes as consisting of two S,S-coordinated n radical monoanions strongly antiferro-magnetically coupled via the central diamagnetic Mn ion with a not negligible diradical character. In this context, the low-energy NIR transition should be considered as a LLCT band. See ref. 53. 

In addition to the tert-butyl enol ethers mentioned above (15% yield), the action of KOtBu on l-iodo-4-methylcyclohexene in DM SO furnished the dimers 85 and tri-mers of 81 in 30 and -25% yield (Scheme 6.24). As in the case of 6 (see Scheme 6.10), the formation of oligomers of 81 was completely suppressed on performance of this reaction in the presence of (tBu)2NO, whereas theenol ethers (86 and its 5-methyl isomer, with the former originating in part and the latter totally from 4-methylcydohex-yne) were observed as in the reaction in the absence of the stable radical. Instead of the dimers 85 and the trimers of 81, a mixture of the hydroxylamine derivatives 87 was isolated in 38% yield. These findings indicate that 81 has no diradical character, in contrast to its immediate dimer 84, which is hence trapped quantitatively by (tBu)2NO [61]. 

Because of the weight of 43, the system 41/43 undergoes an easier rotation about its formally Ge = C double bond than about its formally C-C single bond. The diradical character of 41 should favor any addition with radical intermediates and polymerization reactions.45... 

This rearrangement (equation 66), discovered independently by several groups, was perhaps the first reaction widely accepted as proceeding through a diradical intermediate. " " Recently, diradical character has also been... 

Photochemistry. One might deduce that since the lowest electronic transition corresponds to transfer of an electron from an oxygen atom to a carbon atom, the nn state should have substantial diradical character and should react also by a McLafferty-type rearrangement or a cleavage, as in the mass spectrometer. This is indeed the case. The photochemical a cleavage is called the Norrish type I reaction, and the rearrangement is called the Norrish type II reaction. Both are discussed in Chapter 15. 

An analysis in terms of VB structures (see exercise 3) shows that this configurational mixing corresponds to approximately 40% diradical character in the wave function for ozone. The RHF wave function, on the other hand, contains only 12% of the diradical VB structure (the result was obtained using Hiickel values for the coefficients of the orbitals (2 11)). It is clear from these considerations that a correct treatment of the electronic structure for the ozone molecule must be based on a multiconfigurational wave function. 

The experimental data are consistent [116] with the intermediate depicted in Scheme 38 or at least with a corresponding polarized exciplex capable of acting as a proton acceptor. For a singlet species, as in the present case, little or no diradical character is likely. The delocalization shown in Scheme 38 involves an interaction between centers formally separated by an sp3 hybridized carbon atom. The tendency for a hypothetical diradical to polarize in a preferred sense is also shown in the scheme. The stability of such a singlet intermediate species of the major left-hand canonical type (Scheme 38) would be favored by dimethylation at the positive pole, and the apparent failure of propene and the but-2-enes to un-... 

The problem with ozone probably arises at least partly from the fact that this molecule has singlet diradical character (Section 8.2) it is approximately a species in which two electrons, although having opposite spin, are not paired in the same orbital [125] ... 

Since there have been some confusions in the nomenclature <1990AGE344, 1991CRV335>, it must be emphasized that (1) 1,2,3-trioxolanes are primary ozonides or moloxides (2) 1,2,4-trioxolanes are secondary or final ozonides and (3) Criegee s carbonyl oxide intermediate 1, 2 has been found theoretically to have a pronounced diradical character 3, not only in the gas phase, but also in solution in nonpolar solvents only its reaction with carbonyl compounds in solution has a polar character. Nevertheless, the name carbonyl oxide is so well entrenched that it will continue to be used for intermediates 1-3. 

It is widely believed that enone-alkene photoadditions proceed through an exciplex (excited complex). For cyclopentenone and 7, the exciplex forms from the photoexcited enone in its triplet state and the glycal in its ground-state. The regiochemistry of addition probably reflects a preferred alignment of the addends in the exciplex. Because a photoexcited enone probably has considerable diradical character, it is reasonable to assume that the more electrophilic a-keto radical would prefer to bind to the more nucleophilic portion of the alkene, since this would maximise attractive interactions within the complex. The reaction of 7 with cyclopentenone would thus favour the formation of diradical 22, which would then ring-close to cyclobutane 14 (Scheme 6.7). 

Aryl cations are highly reactive intermediates with two possible electronic configurations, the singlet and triplet states. The former is a localized cation with a vacant a orbital at the dicoordinated carbon atom, whereas the latter has a diradical character with single occupancy of the g orbital and the charge being delocalized in the... 

This control, however, can be tested only in acyclic dienes, which lack of the conformational rigidity of cyclic substrates. Loss of the stereochemical information could be due to isomerization of the starting diene [18b,19c], to two-step processes [18], or to the reversible formation of an exciplex which, depending on the substrate structure, may have a zwitterionic and/or diradical character [19]. 

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