Electron affinities of polar molecules

All molecules having dipole moments greater than 1.625 D (0.639 ea0) have positive electron affinities in the Born–Oppenheimer approximation (i.e., when the nuclei are stationary). However, when nuclear motion is treated exactly, the above sufficient condition for binding an extra electron is modified. We have determined the magnitudes of Born–Oppenheimer electron affinities which are required in order to insure that the negative ions of polar molecules (μ≳1.625 D) are still stable when the nuclei are free to move.