Experimental Data on Cl, Br and I Spin Couplings

Experimental spin coupling data for chlorine, bromine and iodine isotopes are summarized in Table 4.2. Spin couplings are reported both as conventional coupling constants, J, and as reduced spin couplings, 

With a few exceptions, the listed spin couplings have been indirectly determined. As discussed in Section 1.5 there are essentially two methods available for determining scalar couplings to quadrupolar halogen nuclei. The first method is based upon the fact that the scalar contribution to the relaxation of a spin 1=1/2 nucleus (I) coupled to a quadrupolar halogen nucleus (S) is different for the two 

When the I nucleus is a proton, contributions other than scalar (usually dipolar) to the experimental relaxation rates, are commonly equal for both and measurement of the difference between 

1/T j and VT2J at one temperature should thus be sufficient to calculate the scalar coupling, Jjg/ provided the relaxation rates (1/T g) and (1/T2g)/ which are usually equal, of the quadrupolar nucleus at that particular temperature are known. Many experimentalists have not been content with measurements at one temperature only but have studied the I nucleus relaxation rates over a range of temperatures. This may be rewarding in the analyses of the experimental data since the dipolar and scalar relaxation contributions have different temperature dependence. 

When the I nucleus for example is C, F or P, spin rotation may give appreciable contributions to the relaxation rate of these nuclei. Studies of both temperature and field dependences of the I relaxation rates are therefore usually mandatory before scalar contributions can be unambiguously identified. 

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Bryce and Autschbach performed the accurate calculation of the isotropic and anisotropic (AT) parts of indirect nuclear spin spin coupling tensors for diatomic alkali metal halides (MX M = Li, Na, K, Rb, Cs X = F, Cl, Br, I) with the relativistic hybrid DFT approach. The calculated coupling tensor components were compared with experimental values obtained from molecular-beam measurements on diatomic molecules in the gas phase. Molecular-beam experiments offer ideal data for testing the success of computational approaches, since the data are essentially free from intermolecular effects. The hyperfine Hamiltonian used in analyzing molecular-beam data contains Hc IkDIi and //C4/a /l terms. The relationships between the parameters C3 and C4, used in molecular-beam experiments, and Rdd, A/, and used in NMR spectroscopy, are summarized in the following equations ... 

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