Inversion of the N atom in ammonia

How is this possible when the vibrational energy is less than one third of the energy required for the molecule to pass over the top of the barrier When discussing the properties of one-electron atoms (Chapter 1) we noted that the total energy of the electron is given by the sum of the kinetic and potential energies and that the latter is given by 

If the total energy E is negative, then, according to classical mechanics, the maximum distance between electron and nucleus (rmax) is given by 

Nevertheless examination of the Is wavefunction of a one-electron atom showed that the probability of finding the electron outside a sphere with radius equal to rmax was greater than 20% (Problem 1.12). The phenomenon of the electron being in places where the potential energy is greater than the total energy is referred to as tunneling. 

The inversion barrier of phosphine has not been determined experimentally. The best quantum chemical calculations yield a barrier of 141 kJ mol , i.e. more than five times higher than in ammonia [2]. Since the barrier to internal rotation in Si2H is smaller than in ethane, it may seem surprising that the inversion barrier of PH3 is higher than in NH3. The reason may be that the pyramidal structure of NH3 is significantly destabilized by repulsion between the H atoms. 

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