Hyperfine coupling constants HFCC

A great deal of information on the electronic structure and geometry of radicals in solution can be extracted from their ESR spectra, as it is well established that the values of hyperfine coupling constants (hfcc), arising from the spin density of the s-orbitals, markedly increase with increasing of the SOMO s-character. The pyramidalization of the radicals is manifested in higher values of their hfccs (o-radicals), whereas smaller values of the hfccs are indicative of the more planar radicals (tt-radicals). 

Several thiadiazole-fused organic radicals have been characterized by ESR spectroscopy, and significant spin delocalization can be seen from the hyperfine coupling constants (hfcc s) N (Table 5). 

The contact interaction is also referred to as the Fermi contact term. In a given atomic orbital basis -0, the isotropic hyperfine coupling constant (hfcc) for a particular nucleus N,, is given by the expression,... 

Hyperfine coupling constants (hfcc) are a property on which much computational (as well as experimental) effort has been spent. Early success with semiempirical methods, such as INDO (16), in reproducing hfccs from ESR was accompanied by inconsistency of results. It was discovered that fortuitous cancellation of errors was behind the apparent agreement (17). As technical capabilities improved, it became clear that accurate computation of hfccs is a subtle problem, requiring both a basis set with flexibility near the magnetic nuclei and a substantial amount of electron correlation. At the present time, calculation of hfccs is essentially a resolved question The effort needed in ab initio calculations to achieve accurate results is known (18), and recent work with DFT theory seems to indicate that hfccs calculated by DFT methods are in quite satisfactory agreement with experiment (19). Even molecules that have been considered "pathological" can be correctly described with a systematic theoretical approach (20). 

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