The story of the ammonia molecule: Ten years of investigation of molecular inversion

The application of the Hougen-Bunker-Johns (HBJ) Hamiltonian to quasiplanar molecules is discussed with special emphasis on ammonia. The double-minimum potential functions of ammonia obtained in the approximation of the rigid invertor, “harmonic” nonrigid invertor, and “anharmonic” nonrigid invertor are compared from the point of view of their isotopic invariancy. The HBJ Hamiltonian can also be applied with advantage to the quantitative analysis of the transition frequencies obtained by high and very high resolution spectroscopy, as well as to the interpretation of the anomalous inversional dependence of the dipole moment function, quadrupole coupling or of the g-tensor function.The newly determined “true” barrier height for ammonia inversion by fitting the experimental data on vibration-inversion-rotation transitions is 1834 cm^?1.