An entire band of perturbation allowed transitions to the ν4 state of 14NH3 has been assigned in the Fourier transform spectra recorded using a 192-m path length. More than 900 of the forbidden transitions provide necessary information on the spacing between the energy levels with different quantum numbers k, inaccessible from allowed transitions. These data were combined with all other relevant data (MW, submillimetrewave, FIR, IR) published in the literature to derive precise values of inversion–rotation energy levels. This extensive data set completely describes the ground state energy levels of 14NH3 up to quantum number J = 16 for all possible values of the quantum number k.

A review is given of the forbidden ( more precisely: perturbation allowed) transistions between molecular vibrational-rotational states including transistions which are induced by the electric dipole and quadrupole moments and the magnetic dipole moment. The basic theory of these transistions is outlined starting with the overall symmetry selection rules, followed by the discussion of the spin statistics isomers, approximate selection rules for the usual vibrational-rotational transistions, and forbidden transistions induced by the electric quadrupole and magnetic dipole moments. Forbidden transistions due to the vibrationally and rotationally induced electric dipole moments are the discussed in detail for symmetric top and spherical top molecules with the emphasis on the physical nature of the various phenomena leading to these transistions. A summary is also given of the most important experimental work on the forbidden transistions in diatomic molecules and polar as well as nonpolar polyatomics.

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