Empirical Magnetic Parameters

The concept of a spin Hamiltonian is thus central to this discussion, in which it plays a twofold role From an experimental viewpoint, effective spin Hamiltonians are used to convey a description of the experimental spectral behaviors in terms of numerical values of the magnetic parameters thus, the structural and dynamical information on the system under examination is summarized and encoded into these empirical parametCTS. From the viewpoint of computational spectroscopy, the spin Hamiltonian is first of all decomposed into a set of individual operators corresponding to specific physical effects. Once suitable theoreticaFcomputational descriptions are established for these operators a viable link is obtained between computed and observed spectral parameters. In the case of NMR spectroscopy, a general formulation of the spin Hamiltonian is the following ... 

Nevertheless, the calculation of is an important issue. In experiment, it is considered as an empirical parameter fitted to experimental data so that the corresponding Heisenberg Hamiltonian describes the experimentally observed magnetic behavior (100,101). Although it would be more desirable from a quantum chemical point of view to directly calculate experimentally accessible properties, e.g., the magnetic susceptibility (102), the quantum chemical calculation of Ky provides a means to compare experimental and calculated results—though in a somewhat indirect fashion. 

Many spectroscope and magnetic studies have been concerned with empirical correlations between these parameters and features of structural and chemical interest in the molecules. It should be noticed, however, that these symmetry-based parameters are global (like /HDvv which is discussed earlier), referring to the field of all ligands as a whole. [The same is true of recent more comprehensive symmetry-defined parameters proposed by Donini et al. (17).] Being based on the minimum assumptions of ligand field theory, and hence, for some, preferred as more basic, these parameters lack possibilities for immediate chemical relevance and appeal. 

Magnetic nuclei associated with orbitals in the 7r-electron system can also exhibit HFS. The interactions that describe the HFSC of such nuclei involve electron exchange between adjacent atoms as well as between the unpaired electron and the core electrons of the particular atom. These expressions are complicated and are derived on theoretical grounds in References 10 and 11, and empirical parameters for many heteroatoms are tabulated in References 1, 6, and 11. 

In principle one can use this wave function to calculate the magnetic and electric hyperfine parameters. In practice, it is interesting to follow the arguments of Gerry, Merer, Sassenberg and Steimle [61 ] as they attempt to find a semi-empirical description of the bonding in CuO which also gives a reasonable quantitative interpretation of the molecular constants. It is, evidently, not easy to find a satisfactory compromise between the physically visual semi-empirical model, and the frill blown ab initio calculations. 

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