Spin-pairing effect

From the odd-even effects observed in the ionization potential and electron affinity of small clusters (see Sect. 2.4) we can also expect odd-even oscillations in the reactivity. For instance, the ionization potential I(Np) of a small alkali cluster with an even number of atoms (Np) is larger than I(Np -i- 1) and I(Np — 1). At the same time the electron affinity A(Np) is smaller than A(Np -I- 1) and A(Np — 1). Consequently I(Np) —A(Np) will be larger than I(Np-I- 1) — A(Np -I- 1) or I(Np — 1) — A(Np — 1). In summary, the spin pairing effect induces odd-even oscillations in the reactivity. 

It is relevant to note at this point that, because the metal ions are isoelcctronic, the spectra of low-spin Fe complexes might be expected to be similar to those of low-spin Co ". However, Fe" requires a much stronger crystal field to effect spin-pairing and the ligands which provide such a field also give rise to low-energy charge-transfer bands which almost always obscure the d-d bands. Nevertheless, the spectrum of the pale-yellow [Fe(CN)f,] shows a shoulder at... 

The stabilisation of the [HS—LS] mixed-spin pair results from a synergistic effect between intramolecular and cooperative intermolecular interactions (see below). 

Absorption bands arising from adjacent protons are split into multiplet peaks by a mutual interaction of the spins. The effect is due to small variations in the effective field experienced by a proton when neighbouring nuclei can occupy two or more energy levels or spin states. It is transmitted through the intervening bonds by a tendency for electron and nuclear spins to be paired. 

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