Quintet state

With these assignments at hand the analysis of the hyperfine shifts became possible. An Fe(III) in tetrahedral structures of iron-sulfur proteins has a high-spin electronic structure, with negligible magnetic anisotropy. The hyperfine shifts of the protons influenced by the Fe(III) are essentially Fermi contact in origin 21, 22). An Fe(II), on the other hand, has four unpaired electrons and there may be some magnetic anisotropy, giving rise to pseudo-contact shifts. In addition, there is a quintet state at a few hundred cm which may complicate the analysis of hyperfine shifts, but the main contribution to hyperfine shifts is still from the contact shifts 21, 22). 

Consider the simple model complex [FeO(NH3)5] in its lowest triplet and quintet states (Fig. 5.9). 

The shape of these orbitals does not change strongly between the lowest triplet state ( 2g) with the configuration. .. b2) 3e) (2b f and the lowest quintet state ( Aig) with the configuration. .. (Ifi2) 3e) (2fii) ... 

The high-spin quintet states (S = 2) can be achieved only in s1 or s° configurations,... 

It is surprising that the quintet state for iron(II) is appreciably populated in the derivatives of the amidine system 65 (Dq(Ni2+)=1170 cm-1), despite the absence, in this instance, of any apparent steric barrier to coordination from substituents and the formation of five-membered chelate rings. 

In the case of a quintet state consisting of two weakly coupled triplets and populating with the triplet and singlet manifolds, the signal intensity of the quintet will be given by (13). 

Encouraged by the experimental finding (Itoh, 1967 Wasserman et al., 1967) that the dicarbene [15 m = 2] ( = [3]) had a ground quintet state, i.e. all four spins were ferromagnetically coupled, Iwamura and Itoh have been engaged in a project directed towards the construction of the higher series of poly(carbenes) [15], [23] and [24] (Iwamura et al., 1985 Teki et al., 1983, 1985, 1986). 

Epr signals due to the triplet and quintet states were obtained when single crystals of diacetylene were photolysed (Huber and Schwoerer, 1980 Schwoerer et al., 1981 Sixl et ai, 1986). The quintet states are considered to arise from substructures such as [25]. 

Similarly, when diphenyldiacetylenes having nitreno-groups at the m,p -and w,m -positions [30i] were generated, only the former showed a ground quintet state (Iwamura and Murata, 1989). 

When two triplet species are allowed to interact weakly, singlet, triplet and quintet states are produced as shown in Fig. 18. Since the exchange integral... 

Fig. 22 (a) Epr spectrum obtained by photolysis of the didiazo-compound o-[38a] of a [2.2]paracyclophane. (b) The signals due to unoriented dicarbene o-[38] in the quintet state simulated by a perturbation calculation. 

In the case of the pseudo-me/o-isomer (m-[38]), signals due to a quintet state were not observed. Instead, a set of signals due to the thermally populated triplet Tj appeared at 25 K. The temperature dependence data were analysed in terms of an equation similar to (13) [3 exp —AEjRT) in place of 5 exp (-iAEIRT) in the numerator] to give AE j = 98cm" ( = 280 cal mol" ). The behaviour of p-[38] was more like o-[38], although the former was more reactive and did not allow a satisfactory temperature-dependence study. 

The IR spectra of several electronic states of 16 were computed (Fig. 6). It is interesting to note that the major IR bands of the A" state of 16 are predicted to be about three times stronger compared to those of the other states. The predicted IR intensities of the quintet state are quite similar to those of the A and A states, but the pattern is sufficiendy different. On the other hand, the latter two states cannot be discriminated easily by IR because their spectra are very similar, something that is intimately related to the similar geometries of the two states. 

Biscarbenes 30 (n = 0) and 31 (Structures 31-33) were the first hydrocarbons for which quintet states were demonstrated [79,95]. Because the species were... 

Figure 11 shows some of the geometrical parameters (computed at the MCSCF level of theory) for the five electronic states of 33. The local geometries of the carbene and nitrene subunits (bond angle at the carbene center, C—bond length of the carbene and the bond lengths between the diradical centers and the benzene ring) of the quintet state ( A ) are very similar to that of triplet phenylcar-bene and phenynitrene computed at the same level of theory (Fig. 12). The situation is less clear for the other two A states ( A and A ), but the geometry of...

Figure 1 shows the one-electron energies of iron(II) porphine in both spin-restricted and spin-unrestricted (different orbitals for different spins) form. The geometry used has a planar porphine with an Fe-N distance of 2.01 A and no axial ligands. This calculation assumes a high-spin quintet state with the configuration (dxy) ... 

C can then undergo further intersystem crossing to the T state, D. The final intermediate produced in the photodissociation of the synthetic hemes, D, has been shown to be, within experimental error, equivalent to the ground deoxy state. D correlates with the production of the quintet state, both directly from B and through further reaction of the deoxy state C. Therefore we assign D to... 

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