Biradicaloid form

B and C do not. There is very little reduction in the ring current of D compared to the parent compound A [263,264], whereas B and C show marked reductions in ring current [264,265], Tn the ca.ses of compounds C and D, a detailed consideration of tlic possible Kckul forms suggests that bond localisation should be more marked in C [251] this is in accord with the observed n.m.r. spectra. It has been suggested that in the case of C a biradicaloid form, in which both henzo-rings are delocalised, may also contribute to the overall structure [264],... 

The boron-centred biradicaloid (R2PBR )2 (5.17, R = Pr, R = Bu) is an isoelectronic analogue of the (RPCR)2 biradicaloids, 5.14 and 5.15. Both of these four-membered rings contain 22 valence electrons for R=H. The P2B2 system 5.17 is synthesised by the reaction of a 1,2-dichlorodiborane with two equivalents of LiP Pr2. The P2B2 ring is presumably formed by rearrangement of the initially formed acyclic P-B-B-P skeleton (Scheme 5.2). 

Interestingly, for the parent molecule (H2PBH)2 calculations show that the planar form would be the transition state (at 16.4kcalmol ) for the inversion of the l,3-diphospha-2,4-diborabicyclo[1.1.0]butane 5.17. Hence the choice of the sterically demanding substituents on P and B ( Pr and Bu, respectively) appears to be the determining factor in the isolation of the biradicaloid 5.17. A more detailed discussion of the influence of the exocyclic substituents on the relative stabilities of cyclic B2P2 biradicaloid systems can be found in Section 9.3.2. 

Heterosymmetric Biradicaloids The Hamiltonian matrix has the following form ... 

The photochemical cycloreversion of SCBs is known to be initiated from the lowest (a, a ) excited singlet state by cleavage of one of the ring Si—C(2) bonds to form a biradicaloid intermediate. This excited-state intermediate cleaves to silene and alkene, recloses to starting material, or undergoes an intramolecular disproportionation if an alkyl substituent is present at C-2 <1999CJC1136, 19890M1112>. 

Xi - Xi + Xt)- The most stable MO , = j( f, + Xi Xi X ) >s neglected in this example it is always doubly occupied and forms a nonpolar-izable core. At square or nearly square geometries the system is a perfect biradical or a homosymmetric biradicaloid, respectively, and the energy ordering of the three singlet states may be obtained from Figure 4.19 or 4.20 ... 

The formation of benzvalene is formally an x[2 + 2] cyclo-addition. The S, (Bju) reaction path from benzene toward prefulvene starts at an excited-state minimum with symmetry and proceeds over a transition state to the geometry of prefulvene, where it enters a funnel in S, due to an S,-So conical intersection and continues on the Sg surface, mostly back to benzene, but in part on to benzvalene (Palmer et al., 1993 Sobolewski et al., 1993). At prefulvene geometries, Sg has a flat biradicaloid region of high energy with very shallow minima whose exact location depends on calcula-tional details (Kato, 1988 Palmer, et al., 1993, Sobolewski et al., 1993). Fulvene has been proposed to be formed directly from prefulvene or via secondary isomerization of benzvalene (Bryce-Smith and Gilbert, 1976). Calculations support the former pathway with a carbene intermediate (Dreyerand Klessinger, 1995). 

Initially, one of the geometrical isomers, presumably Z, forms faster. The competing thermal E-Z interconversion complicates the situation sufficiently that it is not yet clear whether the transposition reaction forms this geometrical isomer exclusively. If so, the mechanism is likely to be of the dyoiropic kind, involving a shift of both migrating aryl groups and a biradicaloid transition state with a bent or planar bicyclobutane-like structure 24. An alternative that cannot yet be ruled out with confidence is a disilene-to-silylsilylene shift followed by a silylsilylene-to-disilene shift (cf. Sections II.A.3 and II.B.l.b). Both possibilities are displayed in equation 19. 

The intermediacy of a species analogous to (i) (Scheme 1.7) that some described as a resonance hybrid of a radical pair and an ion pair and others as an ultrafast equilibrium between radical and ion pair has been proposed in the photoionization reaction of benzyl derivatives [172, 174-176]. Also, interestingly, to explain the driving force behind the rotational motion that leads to an extremely rapid deactivation of S of TAM cations, Vogel and Rettig [177] proposed that SI promptly forms a biradicaloid charge-transfer state. 

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