Coupling constants 29Si, calculations

The interaction between the nuclear spin and the electron spin is expected to be proportional to the spin density of the unpaired electron at the nucleus of the central atom. Only s electrons have spin density at the nucleus (in fact this is the position of the highest value of the wave function i/0, whereas p electrons have a node at the nucleus corresponding to zero spin density. Thus the interaction between the nucleus and the electron, and thus the coupling constant, will be proportional to the s character of the orbital. The maximum coupling constants a for an electron in a 2s orbital in 13C and the 3s orbital in 29Si can be calculated to be 111 and 121 mT, respectively. For an electron in an sp3 orbital, the expected values will be 25% of these figures, i.e. 28 and 30 mT, respectively. For an electron in a p orbital, corresponding to sp2 hybridization of the central atom, the expected values of a will be zero in both cases. 

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