Abstract
The complete hamiltonian for a diamagnetic non-rigid molecule with one large-amplitude vibrational coordinate in the presence of a constant magnetic field is derived. Using this hamiltonian a simple approximate theory for the inversional Zeeman effect is elaborated. The theory is used to determine the inversional g-tensor function of 14NH3 and 15NH3 and to estimate the differences in the rotational g-factors for their inversion ground states. In addition, the effective g-factors for inversionally excited states of 14NH3 are estimated. The predictions reveal that the inversional dependence of the g-factors is not only rather strong but also non-monotonic.