Triradical

For a threefold degenerate MO occupied by three electrons, a configuration of a triradical having a quartet multiplicity is required hy Hund s rule. In organic chemistry, systems of this type occur rarely but with transition element complexes, quartets, quintets, and sextets, they are common. 

The e.s.r. spectrum of the phosphonium radical (88, Y == H) resembles that of PhgCH except that the splitting produced by the phosphorus atom is over three times that produced by the methine proton. The spectrum of (88, Y = COMe) was also determined. The formation of the triradical... 

The ntility of the experimental methods are illnstrated in this chapter by considering their applications to the stndy of reactive molecules, including radicals, car-benes and diradicals, carbynes and triradicals, and even transition states. These are provided in Section 5.4, which inclndes resnlts for representative bond dissociation energies and an extensive list of thermochemical results for carbenes, diradicals, carbynes, and triradicals. Section 5.5 provides a comparison and assessment of the resnlts obtained for selected carbenes and diradicals, whereas spectroscopic considerations are addressed in Section 5.6. 

The following sections describe applications of these methods to the investigation of the thermochemical properties of radicals, carbenes, biradicals, and triradicals. 

TABLE 5.3. Enthalpies of formation and BDEs for formation of carbenes, diradicals, carbynes, and triradicals. ... 

The Squires reaction can also be used to generate radical anions of triradicals by using polysilylated precursors. Therefore, sequential reaction of 3,5-/7w-trimethylsi-lylmethylbenzyl or 3,5-few-trimethylsilylphenyl anions with 2 molecules of F2 leads to the formation of 1,3,5-trimethylenebenzene (Eq. 5.18a) or 1,3,5-tridehydrobenzene (Eq. 5.18b) radical anion. " ... 

Wenthold and co-workers have used this approach to generate the negative ion of 5-dehydro-m-xylylene (Eq. 5.19). ° Using bracketing experiments to determine the gas-phase acidity at the 5-position of m-xylylene, and the kinetic method to determine the triradical electron affinity, the BDE at the 5-position of m-xylylene was measured (Eq. 5.20). 

Few positive ion methods are generally applicable to the investigation of triradicals because of the problem of measuring ionization energies for di- and triradicals and... 

H. A. Lardin, The Thermochemistry of Aromatic Radicals, Biradicals, and Triradicals, PhD Thesis, Purdue University, 2003. 

Krylov AI (2006) Spin-flip equation-of-motion coupled-cluster electronic structure method for a description of excited states, bond breaking, diradicals, and triradicals. Acc Chem Res 39 83-91... 

A second class of molecules with two unpaired electrons has the two electrons localized, to a first approximation, in different parts of the molecule. We refer to such molecules as biradicals (or, with three or more unpaired electrons, triradicals, etc.). Examples are the dinitroxides (e.g. 1), and certain binuclear vanadium(iv) and copper(n) complexes. 

In this section we consider the spin Hamiltonian appropriate to a biradical with weak dipolar coupling and see how ESR spectra of such species should appear. Obviously, it is possible to find triradicals, tetraradicals, etc. treatment of such species is similar, though of course somewhat more complicated. 

Fig. 4 Esr spectra of high-spin trianthrylene [21a] at 120 K in MTHF/K A, biradical triplet state [21a]2- (labelled O) B, triradical quartet state [21a]3- (labelled ) C, biradical triplet state [21a]4- (labelled ).

Addition of the Grignard reagent, generated in situ from (375), to nitrone (373) or to 2,5-dimethyl-l-pyrroline-A-oxide, affords biradical (379) or nitrone containing monoradical (380). Furthermore, (380) can be transformed into biradical (381) and triradical (382) (Scheme 2.165) (620). 

In the trication-triradical of 1,3,5-triaminobenzene, quantum-chemical calculations indicate that the positive charges are very much delocalized into the benzene core, whereas the nitrogen atoms bear negative charges (Nguyen et al. 2005). 

The stability of 2-oxepinoxy radical qualified it as a target for further theoretical and experimental study. The calculations of Barckholtz et al. allowed the refinement of a feasible energetic pathway toward 2-oxepinoxy radical these DFT calculations supplemented the semiempirical work of Carpenter and also proposed a triradical intermediate between the dioxiranyl and oxepinoxy species (Fig. 9a). 

Depending on the amount of thianthrenium perchlorate, /V,/V,/V, /V, /V",/V"-hexakis (anisyl)-l,3,5-triaminobenzene gives its cation radical, dication diradical, and trication triradical as well (Stickley et al. 1997). These species are stable in methylene chloride at low temperatures (at 298 K they can exist for several days). Spin and charge are localized at each oxidized nitrogen atom. The dication diradical and trication triradical structures are ground-state triplet and quartet molecules (Sato et al. 1997). 

A monoradical is a doublet and a diradical can be a singlet or a triplet. How many spin states are possible for a triradical ... 

A simple series of 1 —> 2 A-branching cascade molecules based on 4,4 ,4 -tris(A,A-diphenylamino)triphenylamine has been prepared and characterized.1181 The ESR spectrum of the cationic triradical of the related l,3,5-tris(diphenylamino)benzene has been studied 19 in detail. 

Scheme 4.19. Preparation of a perchlorinated triradical via A1C13 activated arylation. The triradical was isolated in two isomeric forms, D3 and C2.

The radical anion and the triradical anion of 43 are both paramagnetic species and can be detected by EPR. The EPR spectrum of the radical anion of 43 shows an 11-line hyperfine pattern for the ten equivalent protons76. The trianion radical of 43 shows a highly resolved EPR spectrum where additional 7Li (and 6Li) couplings are evident74,76b. 

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