Parameter Reductions in Force Fields

Within the SH formalism the MPs (gx, gy, gz, D, E, /tip) are thought of as physical constants associated with each particular system. The electronic-magnetic theory beyond the SH formalism reveals that there are only the electronic-structure parameters (like B, C, ) associated with the electron configuration and the CF parameters [like F2(L) and F4(I)] for each ligand. A more realistic approach brings the orbital reduction factors k (which must be anisotropic) and in a particular case of the degenerate electronic states also the force-field and vibronic coupling parameters (like Kee, Xe, Xee, and eventually Ktt, Xt, Xtt, or even more parameters). 

In Sections 17.10-17.12 you developed and used some cobalt(III)-amine force field parameters that are different from those of MOMEC. Refine the cobalt(III) structures of Table 17.18.3 with some of these parameter sets, recompute the reduction potentials and see what the difference is. This gives you a feeling of how accurate the data need to be to make reasonable predictions in terms of the reduction potentials. 

The latter development has taken two paths. In one, parameters are obtained by a least-squares fit to energies and first and second derivatives at a number of points on the ab initio potential energy surf ace.In this method, F,y terms in Kq are selected for their presumed relevance. In the second method, that of the spectroscopically determined force field (SDFF), a direct transformation is made from the ab initio structures and second derivatives into the MM function. All F,y are initially retained, with subsequent reduction being determined by preset conditions of frequency agreement. Ab initio energies... 

In a study of phenacylphenylsulfone photolysis, CIDNP data were taken as evidence that the primary radical pairs cannot recombine to regenerate the starting material because the micelle forces a certain orientation of the radicals [63], From low-field 13C CIDNP and SNP measurements on cleavage of benzylic ketones in sodium dodecyl sulfate micelles, it was inferred [64] that the exchange interaction in these systems is several orders of magnitude smaller ( 10lorads 1 at a reduction distance of 6 A cf. the values in Section IV.B) and the distance dependence is much weaker (a x 0.5 A" cf. the discussion of Eq. 10) than generally assumed for radical pairs. By numerical solutions of the stochastic Liouville equation for a model of the micelle where one of the radicals is kept fixed at the center of the micelle while the other radical is allowed to diffuse, the results of MARY experiments, 13C CIDNP experiments at variable fields, and SNP experiments could be reproduced with the same set of parameters [65],... 

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