Biradical magnetized

Magnetic field effects on the reaction kinetics or yields of photochemical reactions in the condensed phase have been studied [20-23]. They have proved powerful for verifying the mechanism of photochemical reactions including triplet states. Previously, we obtained photogenerated triplet biradicals of donor-acceptor linked compounds, and found that the lifetimes of the biradicals were remarkably extended in the presence of magnetic fields up to 1T [24]. It has been reported that Cgo and its derivatives form optically transparent microscopic clusters in mixed solvents [25,26]. The clustering behavior of fullerene (C o) is mainly associated with the strong three-dimensional hydrophobic interactions between the C o units. Photoinduced... 

An early attempt to test the disjoint hypothesis compared the magnetic properties of two isomeric tricyclic m-quinonoid non-Kekule molecules 17, formally a biradical with tetraradical resonance structures, and 18, formally a tetraradical (Section 2.3). These molecules belong to the point groups C2 and C2v, respectively, and it will be mnemonically convenient to use those descriptors in what follows. The test derives from the recognition that the connectivities of the two molecules... 

With the singlet and triplet states of TME now rather firmly established to be close in energy, the opportunity presents itself to tune the gap and thereby exert control over the magnetic and chemical properties of a biradical series. This modulation now has been smdied in the series of heterocyclic TME derivatives (46) from the corresponding diazenes (45, Scheme 5.10). Reviews of this work are avail-able. 2 - 2 ... 

In some systems, triplet BET can occur, as deduced from time-resolved optical spectroscopy, magnetic field effects, CIDNP, or optoacoustic calorimetry. Triplet BET is governed by energetic factors, which determine rates, and by the relative topologies of the potential surfaces of parent molecule, radical ions, and of accessible triplet or biradical states. Divergent topologies for different states may cause rearrangements. 

Nitroxides are the most common of the oxygen-centred biradicals to be reported. As a model for spin-crossover molecules, the nitronyl nitroxide (105) was prepared and by oxidation with PbC>2 afforded the triplet biradical (106) which was characterized by ESR (Scheme 15).242 The one-electron oxidation of (105) afforded the singlet cation (107) which was seen to exist in equilibrium with (106) in solution. The authors claim that pH-controlled interconversion between two species of different spin multiplicities in this way may provide die basis for novel magnetic switches or pH sensors. The N.N-dialkylamino nitronyl nitroxides (108) were prepared and afforded die diplet-state biradical cation species by one-electron oxidation with iodine.243 The authors propose that, by the similarity of die electronic structures, diese structures can be regarded as hetero-analogues of trimethylenemethane. A paper confirms die conversion of 3,3-dimethyldioxetane into die corresponding ring-opened 1,2-diol but refutes the... 

Magnetic susceptibilities of 10a and 10b were measured on a SQUID suscep-tometer in microcrystalline form. %T-T plots are shown in Fig. 9.5. The data were analyzed in terms of a modified singlet-triplet two-spin model (the Blea-ney-Bowers-type), in which two spins (S = V2) couple antiferromagnetically within a biradical molecule by exchange interaction J. The best-fit parameters obtained by means of a least-squares method were 2J/kB = -2.2 + 0.04 K for 10a and -11.6 + 0.4 K for 10b. Although the interaction (2J/kB = -2.2 K) between the two spins in the open-ring isomer 10a was weak, the spins of 10b showed a remarkable antiferromagnetic interaction (2J/kB = -11.6 K). 

As described earlier, when two nitronyl nitroxides are magnetically coupled via an exchange interaction, the biradical gives a nine-line ESR spectrum. If the exchange interaction is weaker than the hyperfine coupling, the two nitroxide radicals are magnetically independent and give a five-line spectrum. In intermediate situations, the spectrum becomes complex. 

Afree radical (with just one unpaired electron) is described as an electronic doublet because, in an external magnetic field, the electron can only exist in one of two possible spin states ( up or down ). By contrast, a pair or radicals, or a biradical (a species with two unpaired electrons in the same molecule) can exist in either of two electronic states singlet or triplet. In the singlet state the electrons are paired (opposite spin) a singlet radical pair (or biradical) is thus diamagnetic (W5 = I -1 = 0) and not observable by EPR. The radical pair above is shown in the singlet state. 

Figure 11.1. (a) Singlet and (/>) triplet electronic states of a radical pair or biradical. Boldface (vertical) arrow indicates the direction of B0 dashed arrows represent the net magnetic vector of the electrons. 

Several silole bridged nitroxyl biradicals were prepared and characterized by X-ray diffraction, and their magnetic properties were investigated both experimentally and theoretically <2006MI386, 2006MI1319, 2006CEJ5547>. 

When the primary acceptor is prereduced, electron spin polarization can transfer by exchange interaction from BPh to Q, leading to an inversion of the EPR line of in RCs where was magnetically uncoupled from Fe [112] (Fig. 8). From a phenomenological treatment [112-114] it was concluded that the exchange interaction /(BPh QA) was 3 - 5 G, whereas /(P BPh ) was between 1 and 5 G. A more sophisticated treatment of the three-spin system P BPh QX [115] led to 7(P BPh ) between 0 and +8 G. (Note that for / = 0 polarization may develop if D = 0.) A positive value of / for a biradical state is unusual it might be explained by some form of superexchange via an intermediate (possibly one of the accessory bacteriochlorophylls). 

Chiarelli, R. and Rassaat, A. (1991). Magnetic properties of some biradicals of D2d symmetry. In Magnetic molecular materials (ed. Gatteschi, et al.), pp. 191-202. Kluwer Academic Publishers, Dordrecht, The Netherlands. [198]... 

Magnetic Field Effects on Chemical Reactions through Biradicals 8.1 Historical Introduction... 

---