Dipole-bound anions of highly polar molecules: Ethylene carbonate and vinylene carbonate

Results of experimental and theoretical studies of dipole-bound negative ions of the highly polar molecules ethylene carbonate (EC, $C3H4O3,μ=5.35 D)$ and vinylene carbonate (VC, $C3H2O3,μ=4.55 D)$ are presented. These negative ions are prepared in Rydberg electron transfer (RET) reactions in which rubidium (Rb) atoms, excited to ns or nd Rydberg states, collide with EC or VC molecules to produce $EC−$ or $VC−$ ions. In both cases ions are produced only when the Rb atoms are excited to states described by a relatively narrow range of effective principal quantum numbers, $n*;$ the greatest yields of $EC−$ and $VC−$ are obtained for $nmax*=9.0±0.5$ and $11.6±0.5,$ respectively. Charge transfer from low-lying Rydberg states of Rb is characteristic of a large excess electron binding energy $(Eb)$ of the neutral parent; employing the previously derived empirical relationship $Eb=23/nmax*2.8 eV,$ the electron binding energies are estimated to be $49±8 meV$ for EC and $24±3 meV$ for VC. Electron photodetachment studies of $EC−$ show that the excess electron is bound by $49±5 meV,$ in excellent agreement with the RET results, lending credibility to the empirical relationship between $Eb$ and $nmax*.$ Vertical electron affinities for EC and VC are computed employing aug-cc-pVDZ atom-centered basis sets supplemented with a $(5s5p)$ set of diffuse Gaussian primitives to support the dipole-bound electron; at the CCSD(T) level of theory the computed electron affinities are 40.9 and 20.1 meV for EC and VC, respectively.