Isotropic values hyperfine analysis

Gaussian computes isotropic hyperfine coupling constants as part of the population analysis, given in the section labeled "Fermi contact analysis the values are in atomic-units. It is necessary to convert these values to other units in order to compare with experiment we will be converting from atomic units to MHz, using the following expressions ri6ltYg ... 

The impurity interacts with the band structure of the host crystal, modifying it, and often introducing new levels. An analysis of the band structure provides information about the electronic states of the system. Charge densities, and spin densities in the case of spin-polarized calculations, provide additional insight into the electronic structure of the defect, bonding mechansims, the degree of localization, etc. Spin densities also provide a direct link with quantities measured in EPR or pSR, which probe the interaction between electronic wavefunctions and nuclear spins. First-principles spin-density-functional calculations have recently been shown to yield reliable values for isotropic and anisotropic hyperfine parameters for hydrogen or muonium in Si (Van de Walle, 1990) results will be discussed in Section IV.2. 

Thus, as a consequence of the selection of orientation subsets by choice of field value, the ENDOR spectrum changes as a function of the external field position or g value at which it is measured. A series of ENDOR spectra collected at fields across the EPR envelope samples different sets of molecular orientations. An example of this is shown in Fig. 6B. It can be seen that at the extreme g values (g, and g ) the ENDOR spectrum is the least complex, whereas at intervening g values the ENDOR spectrum shows multiple frequencies. The analysis of such spectra taken at several magnetic field positions gives the full tensor of the hyperfine interaction A, from which the isotropic and anisotropic components can be deduced. Procedures for this analysis have been described in detail elsewhere. 

The exchange coupling can in certain cases be determined by an analysis of the hyperfine pattern due to nitrogen observed for instance in nitroxide biradicals in liquid solution. A procedure to extract the value of J was developed many years ago [43]. This procedure does not depend on the microwave frequency and measurements were usually made at X-band. Three cases may appear, (1) / [Pg.195]

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