Spin Dimer Model

The parameters of these spectra are calculated on a six-spin dimer model , and are as follows for the polymers in o-dichlorobenzene at 110°. 

In (d) is shown the spectrum in this region calculated using the six-spin dimer model and essentially the same chemical shift parameters as obtained for the atactic 5-polymers, which are very close to the chemical shifts obtained from the isotactic and syndiotactic polymer spectra. It must be emphasized that there are thus no arbitrary chemical shift adjustments in these calculations. The vicinal couplings of a and jS protons are likewise close to those observed for the stereoregular polymers. The parameters used are summarized in Table 2 below. 

A partial explanation for this surprising difference is provided by a combination of an analysis of the role of magnetic site coordination polyhedron in determining the electronic structure and application of the Spin Dimer Model.The octahedron about Os in Li50s06 is subject to a much stronger psuedo-tetragonal compression, 4 Os - O bonds at 1.908 A... 

Table 2.4 Relative values for the exchange pathways Ji -J5 (Figure 2.13) calculated on the Spin Dimer Model for occupation of only and equal occupation of dxy, dxz and dy for Li50s06. The values for Li4MgRe06 follow the same pattern...

Figure 45 Effective two-dimensional Heisenberg model for localized spins in the dimer model for X-(BETS)2GaX2Y4 j.

Fig. 24. Spin-coupling model used for pair-of-Mn fOlaMn" dimers (134,141).

Fig. 7 Modeling a high-resolution dimeric structure, (a) Estimate of the mean distance on the example of spin label K-221R1. The primary DEER trace (P(t)A (0)), the form factor and the distance distribution obtained by Tikhonov regularization (the L curve is shown in the inset) with the software Deer Analysis are presented, (b) Fits of primary experimental DEER data black lines) by simulated data red lines) corresponding to the final structure of the NhaA dimer and a distribution of spin label conformations modeled by a rotamer library, (c) C2 symmetry axis of the dimer created with the EPR constraints, (d) Comparison between the EPR stmcture and the electron density projection to the membrane plane obtained by cryo-EM on 2D crystals, (e) Comparison between the EPR structure and the dimer modeled on the refined cryo-EM data (PDB 3EI1). Adapted from [77]...

The MCSCF method has been used to calculate the intermolecular spin-spin coupling constants in formamide-formamide and formamide-formamidine dimers, modelling the hydrogen bond in peptides and adenine-tymine (or adenine-uracil) pair in a nucleic acid, respectively. The obtained hydrogen-bond-transmitted coupling constants are visualized in Fig-... 

Analysis of the effect of O disorder on the Mo - O - Mo superexchange interaction, using spin dimer analysis, shows a significant perturbation to the average structure value. This observation fits well with the Saunders-Chalker model for the appearance of the spin glass state for systems with... 

Reflecting these differences of the crystal structure of the a and P phase, the temperature dependence of susceptibilities, % T vs. T plot, and the magnetization isotherms, shown in Figures 12.9 and 12.10, are quite diffoent. The results for the Ot-phase are easily analyzed by a dimer model with Jlkg = —11 K, in which the spin Hamiltonian is described by H = -27X 2, However, it is not clear at this moment... 

A Hiickel MO treatment of a simple dimer model is introduced to rationalize the observed spin density asymmetry of primary donor radical cations in terms of differences of molecular orbital energies of the isolated monomeric moieties. Such a model can provide a semiquantitative method to trace spin density asymmetries back to structural and/or environmental asymmetries. 

Fig. 3. Results of HMO calculations on a simple dimer model (ethylene-dimer), (a) dimer configuration and the chosen Huckel parametrization. (b) and (c) spin densities 2 in the dimer halves L and M, and ratios of the sums of spin densites Pl/p for two different strengths of coupling = 0.01 (b) and =...

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