Barrier to inversion In ammonia

All the homoleptic, hypervalent compounds of the Group 15 elements described in this chapter are fluxional. The activation energies are not accurately known, but have been estimated to about 20 kJ mol in PF5 and PCI5 13 kJ mol in AsFs and about 8 kJ mol in SbCls and Sb(CH3)5 the activation energies appear to decease as the group is descended. The magnirnde of these barriers to pseudorotation is comparable to the magnitude of the rotational barrier in ethane, 12 kJ mol, or the barrier to inversion in ammonia, 21 kJ mol . ... 

The repulsion between the hydrogen atoms could be still further reduced if the molecule became planar. The nitrogen atom would then have sp hybridization (Fig. 3.27c) with three electrons in orbitals which are (js + Ip) and two in the remaining 2p AO. The effective valence state would then be sp. To get here from the tetrahedral configuration would require further promotion of one-fourth of an electron from the 2s to the 2p AO apparently the hydrogen atoms are sufficiently content with their situation when nitrogen is tetrahedral for this further promotion not to be worthwhile. There is therefore a barrier to inversion in ammonia and in its alkyl derivatives of about 6 kcal/mole. This is the energy required to convert the pyramidal molecule into the planar intermediate. 

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