The pyramidalization and energies of inversion of group 15 :MR (M = N, P, As, Sb, Bi) and group 14 •MR molecules can also be predicted and rationalized using a second-order Jahn-Teller distortion treatment. The “parent” planar molecule possessing D symmetry has frontier orbitals of a” (HOMO) and a’ (LUMO) symmetries. The pyramidalizing vibration mode has symmetry a”. The triple product yields the totally symmetric representation a’, indicating that the molecule will indeed pyramidalize into C symmetry.
The energies of inversion can also be predicted and compared. Due to lower energy overlap between the 3p and 1s orbitals in PH (versus between 2p and 1s in NH), the HOMO-LUMO energy gap in PH will be smaller than that of NH. This allows for a stronger interaction between the HOMO and LUMO in second-order Jahn-Teller fashion. The distortion stabilizes the HOMO and destabilizes the LUMO, resulting in a larger barrier to inversion in PH.