Electronic Structure of Dipole-Bound Anions

Dipole-bound anionic states of HCN, (HF) 2, CH 3 CN, C 3 H 2, C 4 H 2, C 5 H 2, and stretched CH 3 F are studied using extended one-electron basis sets at the coupled cluster level of theory with single, double, and noniterative triple excitations (CCSD(T)). Orbital relaxation and electron correlation corrections to the Koopmans' theorem prediction of electron binding energy are analyzed, and a physical interpretation of low-order corrections is proposed. It is demonstrated that the second-order dispersion interaction between the loosely bound electron and the electrons of the neutral host should be included into physical models of dipole-bound anions. Higher-order electron correlation corrections are also found to be important, and a slow convergence of the Møller−Plesset series for electron binding energies is documented. Modifications of the potential energy surfaces of the above polar molecules upon electron attachment are studied at the second-order Møller−Plesset level, and Franck−Condon factors for the anion/neutral pairs are calculated. It is predicted that photoelectron spectra of the dipole-bound anions of C 4 H 2 and C 5 H 2 should display vibrational structure.