Long-Range Electron Binding to Quadrupolar Molecules

An excess electron can be bound to a molecule in a very diffuse orbital as a result of the long-range contributions of the molecular electrostatic field. Following a systematic search, we report experimental evidence that quadrupole binding occurs for the trans-succinonitrile molecule ($\mathrm{E}\mathrm{A}=20\pm 2\mathrm{m}\mathrm{e}\mathrm{V}$), while the gauche-succinonitrile conformer supports a dipole-bound anion state ($\mathrm{E}\mathrm{A}=108\pm 10\mathrm{m}\mathrm{e}\mathrm{V}$). Theoretical calculations at the DFT/B3LYP level support these interpretations and give electron affinities of 20 and 138 meV, respectively.

Figure 1

Rydberg electron transfer (RET) curves for succinonitrile anion production as a function of the effective Rydberg quantum number $n^{*}$ involved in the electron transfer collisions with either $\mathrm{X}\mathrm{e}(nf)$ states (solid squares) or $\mathrm{R}\mathrm{b}(nd)$ states (open squares). The full line is a fit to these experimental points, using a curve-crossing model [18] with the only parameter being the excess electron binding energy EA.

Figure 2

Anion photoelectron spectrum for succinonitrile anions produced in a supersonic ion beam source. The main peak $a$ corresponds to the transition between the anion and neutral ground states, from which $\mathrm{E}\mathrm{A}=108\pm 10\mathrm{m}\mathrm{e}\mathrm{V}$ is determined, while peaks $b$ and $c$ correspond to transitions towards excited rovibrational levels of the neutral.

Figure 3

Highest occupied molecular orbital (HOMO) for the trans (left) and the gauche (right) conformers of succinonitrile anions, in 3D perspective view, using the gOpenMol visualization software [20]. Both plots correspond to the same isocontour value of 0.003 a.u.

Figure 4

Potential energy surfaces for neutral and anion succinonitrile along the dihedral angle connecting the trans and the gauche conformers. Neutral points (squares) have been directly obtained from DFT/B3LYP calculations while anion points (circles) have been calculated from the neutral electrostatic parameters and a semiempirical model [11] of multipole-bound anions. The order of stability between trans and gauche conformers is reversed in the neutral and the anion but there is still a rather high-energy barrier that prevents interconversion.