Fermi contact mechanism

The nuclear spin coupling constants in liquids (and gases) are mediated by the electrons. The main contribution is usually the so-called Fermi contact mechanism between the nuclear spins and the electron spins. For such interactions the only relevant orbitals are the s orbitals, which are the only ones having non-vanishing density at the corresponding nucleus. 

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. 

In methyllithium, INDO calculations assuming only the Fermi contact mechanism predict U(C-7Li) 25-6 Hz. (23)... 

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,... 

Such expressions are based on the dominance of the Fermi contact mechanism. (38) Negative values of /(C-C) are not possible on the basis of equation (7). 

For the amine derivatives in Table VII it is probable that the Fermi contact mechanism dominates in almost all cases. 

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). 

A second determining factor in the Fermi contact mechanism is the requirement that the wavefimction of the bonding orbital has a significant density at each nucleus, in order for the nuclear and the electron magnets to interact. One consequence of this is that K correlates with nuclear volume and therefore rises sharply for heavier nuclei. Thus the constants in the XH series with X = Si, Ge, Sn and Pb are... 

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. ... 

With one exception (see below) V(C,C) spin coupling constants in alkanes are all positive. According to a theoretical treatment by the INDO-FPT method the Fermi contact mechanism for spin coupling dominates, whereas orbital and dipolar... 

The behaviour of the coupling constants involving nitrogen is fre-quently discussed by analogy with the case of the C-H coupling constants. Thus, to the extent that the Fermi contact mechanism is considered as governing J yi correlations with bond hybridization, simi-... 

Moreover, as in the case of J C-H, attention is also drawn to the possible effect of variations in the effective nuclear charge. Hence an attempt has been made to establish certain relationships between J N-X and parameters such as substituent electronegativities or Hammett constants. However in reality, mechanisms other than the Fermi contact mechanism may contribute, namely interactions with the orbital motion of the electrons or with the spin of the extra-nuclear electrons. The Behaviour of is therefore expected to be complex and... 

One-bond Couplings. As expected from the Fermi contact mechanism, is negative for most molecules, and for many, eq. 5, which relates s character at the bonded atoms to... 

Since the Fermi contact mechanism leads to a purely isotropic contribution to the coupling constant, large anisotropies are usually taken to indicate that noncontact mechanisms are making significant contributions to the isotropic average as well. 

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