Hyperfine coupling constants Subject

As we will see in Chapter 4, g-matrices are often difficult to interpret reliably. The interpretation of isotropic g-values is even less useful and subject to misinterpretation. Thus isotropic ESR spectra should be used to characterize a radical by means of the hyperfine coupling pattern, to study its dynamical properties through line width effects, or to measure its concentration by integration of the spectrum and comparison with an appropriate standard but considerable caution should be exercised in interpreting the g-value or nuclear hyperfine coupling constants. 

Linear hydrocarbon radicals have been the subject of intensive laboratory spectroscopic and radio-astronomical research since the early 1980s. In recent years, a considerable number of rotational spectroscopic studies of medium to longer hydrocarbon chains such as C5H, CeH, CgH, and ChH have been carried out using a pulsed molecular beam FTMW spectrometer. The high resolution offered by such a spectrometer allowed the detection of the hyperfine sphtting of rotational transitions. These measurements improved fine and hyperfine coupling constants and provided rest frequencies with accuracies better than 0.30 km s in equivalent radial velocity up to 50 GHz. Indeed, some of the small C H radicals with n < 9 have subsequently been detected in space, in molecular cloud cores, and in certain circumstellar shells. These hydrocarbon chains are among the most abundant reactive space molecules known. 

Several theoretical studies have been dealing with the hyperfine coupling constants of F centers in MgO. One of the earliest studies on this subject was reported by Sharma and Stoneham [136], who performed HF calculations on simple models of the surface defect and obtained values in reasonable agreement with the Tench model [122,123]. The problem has been reconsidered more recently at various levels of theory [125,39,55,137]. In general, excellent agreement is obtained for the bulk where a firm assignment is possible. On the... 

Theoretical considerations can provide values for the hyperfine coupling constants and spin intensities to be used in different situations and the reader is referred to authorative texts on the subject for further guidance ... 

Consequently, although the conclusion that the spin density is located essentially in the 7r-framework of the ring is in accordance with the established experimental data, it is necessary to reserve final judgement upon the subject until values of the anisotropic susceptibilities are known experimentally. Similarly, the conclusion that the extent of covalent interaction in U(Me4Cot)2 is somewhat greater than in Np(Me4Cot)2 is not unequivocably established since it depends upon the approximate identity of the hyperfine coupling constants in the two species (as deduced from the calculated Fermi contact terms), which contain respectively two and three unpaired electrons. 

Semiempirical calculations have been carried out by an unparameterized SCF-MO method with integral approximations [5], various versions of the CNDO [37 to 42] and INDO [6, 38, 43 to 45] methods, the MNDO [46, 47] and MINDO [48] methods, the extended Hiickel method [3, 4, 49, 50] (presumably also [51 ]), a Pariser-Parr-Pople-type open-shell method [49] (presumably also [51]), and a simple MO approach [52]. Besides some other molecular properties, the charge distribution (atomic charges and/or overlap populations) [5, 38,40,41,43,49 to 51] and the spin density distribution (and thus, the hyperfine coupling constants, compare above and p. 241) [3 to 6, 46, 48] have been the subjects of many of these studies. 

Contents Introduction. - ENDOR-Instrumentation. - Analysis of ENDOR Spectra. - Advances ENDOR Techniques. - Interpretation of Hyperfine and Quadrupole Data. - Discussion of the Literature. - Concluding Remarks. - Appendix A Abbreviations Used in this Paper. - Appendix B Second Order ENDOR Frequencies. - Appendix C Relations Between Nuclear Quadrupole Coupling Constants in Different Expressions of Hq (Sect.5.2). - References. - Subject Index. 

The choice of factors for converting the unpaired a.o. spin densities so obtained to hyperfine interactions is a subject of some contention. Empirical conversion factors have been suggested for use with INDO - derived spin densities (28). Alternatively, one may employ atomic coupling constants derived ab initio from a particular set of atomic wave functions. This is the approach which we favour, and, if only for the sake of consistency, we have used the atomic constants derived fromFroese s (29) wave function (Table H). 

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