Spin state sextet

The amount of spin contamination is given by the expectation value of die operator, (S ). The theoretical value for a pure spin state is S S + 1), i.e. 0 for a singlet (Sz = 0), 0.75 for a doublet (S = 1/2), 2.00 for a triplet (S = 1) etc. A UHF singlet wave function will contain some amounts of triplet, quintet etc. states, increasing the (S ) value from its theoretical value of zero for a pure spin state. Similarly, a UHF doublet wave function will contain some amounts of quartet, sextet etc. states. Usually the contribution from the next higher spin state from the desired is... 

Let us now turn to the description of the reaction pathways. Figure 13-5 schematically depicts the shapes of the corresponding potential energy curves for the sextet and quartet spin-states and Table 13-11 contains the thermochemical information obtained at different levels of theory. 

Figure 3.10 Partial energy level diagram for the Fe3+ or Mn2+ ions with 3tfi configurations in high-spin states in an octahedral crystal field. Only sextet and quartet spectroscopic terms and crystal field states are shown. Note that the same energy level diagram applies to the cations in tetrahedral crystal fields (with g subscripts omitted from the state symbols for the acentric coordination site).

Model Heme Complexes. Presented in Table I are the calculated relative energy differences in kcal/mol for the sextet, quartet, and doublet states of the model ferric heme complexes included in the present study. Also included in Table I are the calculated quadrupole splittings (AEq) for the relevant spin state, along with the experimentally observed values of AEq and the measured effective magnetic moments. 

The observed effective magnetic moments (43-46) for the 5- and 6-coordinated complexes found to have sextet ground states are all in the range of 5.9-6 i typical of high-spin complexes. The calculated AEqS for the high-spin state of these complexes are also in good agreement with the experimental values known for three of them (44-46, 50). 

For the dicyano and azide-pyridine complexes, our calculated results indicate that in each case a S = 1/2 spin state is the lowest energy state with the quartet and sextet states much higher in energy. The observed effective magnetic moments (Table I) of both... 

Turning now to comparisons of CCP, MMB, and CAT, the relative energies of the germane doublet, quartet, and sextet spin states have been calculated using the same INDO-RHF-SCF method as for the model complexes and the results are presented in Tables V and VI. 

The calculated relative spin state energies for CAT (Table V) at the crystal geometry shows that the the sextet state is the most... 

The calculated spin state ordering in CAT with a sextet ground state and a low-lying quartet state is also consistent with and helps explain the observed values ( ) between 5.9 and 4.45 yu the temperature range of 4.2-293°K. The calculated AEq of 0.72 mm/sec for the sextet spin state of CAT (Table V) is in good agreement with the experimental value of 0.84 mm/sec from MOssbauer spectra (12). 

Because there has been no clear study of the MFE on any chemical reaction involving a higher spin state than 3/2, we recently tried to get clear MFEs on the electron transfer reaction from the excited trip-sextet ( Ti) state of chloro-(3-methylimidazol)-(me5o-tetraphenylporphyrinato)chromium(III) (Cr P) to viologens (V ) in acetonitrile at 293 K [16]. The scheme of this reaction is shown in Fig. 13-7. 

The electronic stmcture, spectroscopy, and reduction potential have been thoroughly studied for all common classes of iron-sulphur clusters [52,89,182-191]. In particular, Noodleman and coworkers have performed detailed quantum chemical calculations on iron-sulphur clusters in various spin states [192-198]. It is now settled that rubredoxin contains an iron ion in the high-spin state (quintet for Fe , sextet for Fe ), whereas in the [2Fe-2S] clusters, the two iron ions are both in the high-spin state, but antiferromagntically coupled to form a singlet or doublet state for the oxidised (Ill+ni) and reduced (mixed-valence n+ni) forms, respectively [112,162]. In variance to the Cua site, the unpaired spin is trapped at one of the iron ions in the mixed-valence state. 

Similarly, a UHF doublet wave function will contain some amounts of quartet, sextet etc. states. Usually the contribution from the next higher spin state from the desired is... 

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