Contribution of electron correlation to the stability of dipole-bound anionic states

It has long been assumed that electron correlation effects are relatively unimportant for describing dipole-bound anionic states. It is shown here that this assumption is incorrect: high-level electronic structure calculations on the dipole-bound anion states of CH₃CN, C₃{H}₂$, and ${C}₅H₂ reveal that for these species a large fraction of the electron binding energy derives from the dispersion-type interaction between the loosely bound electron and the neutral molecule. The predicted values of the electron affinities of the dipole bound states of CH₃CN and C₃{H}₂$, 108 and 173 ${cm}^{{{-}}1}$, respectively, are in excellent agreement with the recent experimental results, 93 ${cm}^{{{-}}1}and 171±50{cm}^{{{-}}1}$, respectively. The predicted value for ${C}₅H₂ is 614 cm^-1. {} 1996 The American Physical Society.