The Fermi contact coupling mechanism

Linderberg, Y. Ohm, Propagators in Quantum Chemistry , 2nd edition, John Wiley Sons, Ltd, 2004. 

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The Fermi Contact Coupling Mechanism Gauge-Invariant Atomic Orbitals (GIAOs) (4<1))... 

There are no simple rules, but usually the most important contribution to Jab comes from the Fermi contact term (7jg), the next most important is paramagnetic spin-orbit term J, other terms, including the mixed contributions 7 , are of little importance. Let us consider the Fermi contact coupling mechanism between two protons through a single bond (the coupling constant Jab denoted qs Jhh)- The proton and the electron close to it prefer to have opposite spins. Fig. 12.17. Then the other electron of the bond (being closer to the other nucleus) shows the other nucleus the spin of first nucleus, so the second nucleus prefers to have the opposite... 

The principal M—F coupling mechanism in inorganic fluorides also is believed to be via the Fermi contact interaction. Here, even semiquantita-tive treatment is much more difficult. M—F coupling is possible through... 

The greater the s-character of the hybrid orbital involved in the bond, the greater the coupling via the Fermi contact mechanism. However, it has been... 

Figure 4-5 Diagram of the Fermi contact mechanism for the indirect coupling of two spins.

Methods of measuring nuclear coupling by tin have been reviewed by Wrackmeyer.62 Coupling to the lighter elements appears to be mainly by the Fermi contact mechanism, which increases with increasing -character of the bonds. Thus the values of. /(SnC) and 2/(Sn,H) for methyltin chlorides are shown in Table 2-10. 

The naturally occurring isotopes of boron,, 0B (19-58%) and UB (80-42%) have spin quantum numbers of 3 and 3/2, respectively. Experimental V(C-UB) values and those calculated using an INDO-MO method, assuming only the Fermi contact mechanism, are given in Table II. (24-27) It is apparent that the calculations account for the major patterns of substituent effects on /(C-B) and that, if the orbital and dipolar mechanisms are important for these coupling constants,... 

Values of /(C-N) across the triple bond in nitriles and isonitriles are negative. (106, 109) In both acetonitrile and methylisocyanide the Fermi contact contribution to /(N-C) is predicted to be positive and, for the methylisocyanide, quite large. The orbital and spin-dipolar mechanisms are predicted to make a negative contribution thus it appears that the latter two mechanisms are important for coupling across triple bonds. 

Fluorine-carbon coupling constants, measured for 2-, 3-, and 4-fluoropyridine and pyridinium ions, (155) have been compared with values calculated assuming only the Fermi contact mechanism. Many of the experimental trends are paralleled by those for V(C-H) values observed in pyridine and fluorobenzene. The INDO calculations qualitatively reproduce the experimental trends. 

The so-called heavy-atom chemical shift of light nuclei in nuclear magnetic resonance (NMR) had been identified as a spin-orbit effect early on by Nomura etal. (1969). The theory had been formulated by Pyykktt (1983) and Pyper (1983), and was previously treated in the framework of semi-empirical MO studies (PyykktJ et al. 1987). The basis for the interpretation of these spin-orbit effects in analogy to the Fermi contact mechanism of spin-spin coupling has been discussed by Kaupp et al. (1998b). 

Although the Fermi contact mechanism dominates most couplings, there are smaller contributions where a nuclear dipole physically distorts an orbital, not necessarily of s type [18]. There are many useful compilations of/and Kvalues, especially for HH couplings (see [9], ch 4, 7-21 and [12, JA, 14 and 15]). 

Compounds containing A1—C Bonds.— The A1- C coupling constants in dimeric AlMcs and its derivatives show remarkable similarities to B- H coupling constants in diborane derivatives. This arises from the similarity in the electronic structures of the two series, and can be accounted for by the Fermi-contact mechanism for spin-spin coupling. ... 

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