Spin quartet

Figure 4-6. Partial correlation diagram for the spin-quartets of d ions in octahedral symmetry.

Photoionization can also access excited electronic states of the ion that are difficult to study by optical methods. The photoionization yield of FeO increases dramatically 0.36 eV above the ionzation energy. This result corresponds to the threshold for producing low spin quartet states of FeO. These states had not been previously observed, as transitions to them are spin forbidden and occur at inconveniently low energy. Because the FeO + CH4 reaction occurs via low spin intermediates, accurately predicting the energies of high and low spin states is critical. 

Fig. 4.6 Quadrupole splitting of the excited state of Fe with I = 3/2 and the resulting Mossbauer spectrum. Quadrupole interaction splits the spin quartet into two degenerate sublevels 7, OT/) with energy separation A q = 2 q. The ground state with I = 1/2 remains unsplit. The nuclear states are additionally shifted by electric monopole interaction giving rise to the isomer shift 8...

The ligand field of a quasi-octahedral coordination sphere acting on the (i-orbitals lifts the degeneracy of the excited spin quartet (and others) as sketched in tier (3). The nature of the resulting states and their energy depends on the strength... 

In a simple crystal-field description, a spin-quartet ground state (S = 3/2) of iron(III) is energetically stabilized over the spin-sextet (S = 5/2) when the energy separation... 

The temperature dependence (296-330 K) of the ring methyl proton resonances of these monomeric heme complexes in the hydrophobic micellar cavity shows [22] a small deviation from the Curie law as in the low-spin complexes in organic and simple aqueous solvents [1, 52]. The origin of such deviation has been variously ascribed [3, 1, 53] either to aggregation or second order Zeeman (SOZ) effect or presence of low-lying spin-quartet state. Since these low-spin hemes in micellar solutions are in deaggregated form, the deviation may be due to the SOZ and/or presence of low-lying excited state. 

We now choose a Hamiltonian H for which the wave function (60) is an exact ground state wave function. As in the one-dimensional case, we seek the required Hamiltonian in the form of a sum of cell Hamiltonians acting in the space of two nearest neighbor spin quartets ... 

When the site spinor vj/V - p is a symmetric fourth-rank spinor QA u/p (corresponding to the two-dimensional AKLT model[13]), only the quintet component out of the six multiplets on each spin quartet is present in the wave function (60). The sixth-rank spinors (62) are symmetric with respect to two triplets of indices and, hence, contain four multiplets with S = 0, 1, 2, 3 formed from two quintets. Consequently, the cell Hamiltonian (Hi and H coincide in this case) has the form... 

Based on DFT calculations, Shaik and coworkers proposed a two-state reactivity process. The doublet and quartet states of P450 Compound I (see Figure 17) have virtually identical reactivity but follow very different pathways. The low-spin doublet state leads to a substrate radical transition state with no barrier to the rebound step, whereas the high-spin quartet state generates a radical with a significant barrier to rebound. In this model, predominance of the low-spin pathway explains the apparently short ( 100 fs) radical lifetimes in some radical clock experiments, while the high-spin pathway accounts for the finite radical lifetimes in others. The calculations further suggested that the doublet... 

In each multiplet structure, the data are separated according to the spin degeneracy spin quartet state on the right hand and spin doublet state on the left. The electron configuration of the Cr + ion is usually expressed as tP under atomic notation, and the quartet and doublet state correspond to the high spin state and low spin state, respectively. Since the ground. state Mz is a quartet state, the transitions to the quartet states are the spin-allowed transitions which have large intensities and broad bands in the absorption spectra, on the other hand, the transitions to the doublet states are the spin-restricted transitions which have more than one order smaller intensities and narrow line peaks which are not obvious in the experimental spectra in Fig. 3. 

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