Energy of spin state

VP-16 phenoxyl radicals [detection, 234, 632-637 reduction reactions in aqueous solution, 234, 638-640 reduction reactions in cell and nuclear homogenates, 234, 641-642] Zeeman energy of spin states, 246, 538, 593 zero-field splitting, 246,550-551. 

Not only the electron, but also the proton, or hydrogen nucleus, and other atomic nuclei, have their own spin. Spin is a quantum property, and therefore must be described by a wavefiinction and by quantum numbers, and the energies of spin states must be quantized. The usual procedure of writing down a classical hamiltonian and then transforming into the QM hamiltonian operator cannot be followed since there is no classical spin energy. The theory proceeds from a number of postulates that will now be discussed. 

Fig. 4. Lowest energy set of spin states of a [2Fe-2S] + center as a function of the ratio BU, as calculated from Eq. (4) in the case of U = lOJ (full lines) and U = 2J (dashed lines). In both cases, the energy of the S = 1/2 state when B = 0 was taken to be zero.

So far, we have considered the phenomenon of spin-state transition employing both the properties of the macroscopic complexes as well as those of the isolated molecules. However, with respect to the nature of the energy variation at the transition, these two situations should be clearly distinguished. Thus, if... 

In a discussion of the mechanism of spin-state interconversion on the basis of the simplified potential energy curves presented above, considerable attention... 

The dynamics of spin-state transitions may be rationalized in various ways. Thus, if the geometry of eomplexes does not depart too far from octahedral, onedimensional cross sections through Gibbs free energy surfaces G = G R) along... 

Table 5-8. Relative energies (eV) of spin states of 11 V4S1 (SCI I )4 ]...

One of the first uses of ESR spectra to measure the rate of a chemical reaction was by Ward and Weissman in the early 1950s.1 They made use of a form of the Heisenberg uncertainty principle (eqn 5.1) to relate the lifetime of a spin state to the uncertainty in the energy of the state. 

We (as well as many others (4-8), for reviews see (9,10)) have been interested for some time (11,12) in the effect of spin-state changes on reactivity. In particular, we have used computational methods to explore these effects in transition metal chemistry ((13-15), for reviews see (10,16)). The key factor affecting reactivity is the relative energy at which the zeroth-order potential energy surfaces corresponding to the individual spin-states cross (Scheme 2). This factor will determine, among other things, whether spin-forbidden reactivity is competitive with... 

All the structure and energy changes discussed so far refer to the S = 0 state. Additional calculations were, however, also performed for higher spin states. The relative energy of these states with regard to Fe-O-O bending is illustrated in Fig. 3.7. 

The iron-sulfur cluster of the iron-protein in its [Fc4S4] + redox state exists in a mixture of spin states of similar energy... 

---