Direct evidence for the ground state, quadrupole-bound anion of succinonitrile

The details of how an electron attaches to an isolated neutral molecule are important to understand situations like the charging of a molecule in contact with an electrode or how electrons move through a molecular wire. These issues motivate the study of negative ions formed due to electron capture by long-range, electrostatic forces and/or dispersion effects. One intriguing type of these weakly-bound anions is the quadrupole-bound anion, where the excess electron is bound by charge-quadrupole attraction.

Evidence for quadrupole-bound anions is scarce, but a 2004 study indirectly concluded that the trans-succinonitrile anion is a quadrupole-bound anion. A new article revisits succinonitrile anions to tackle unfinished business from the previous study and present more direct evidence of the quadrupole-bound anion state.

The earlier work combined results from anion photoelectron spectroscopy, which revealed the dipole-bound anion state of the gauche-succinonitrile conformer, and Rydberg electron transfer, which revealed its quadrupole-bound anion state. While the two types should co-exist in the molecular beam, each of these techniques revealed just one type of a multipole-bound anion but was unable to discern the other type. Therefore, the conclusion that trans-succinonitrile anion is a quadrupole-bound anion was based on several separate pieces of evidence.

The current research used a unique combination of Rydberg electron transfer to prepare the ground diffuse-electron anion states and negative ion photoelectron spectroscopy to measure the electron binding energies. The resulting anion photoelectron spectra exhibited spectral signatures for both quadrupole-bound and dipole-bound anions in the same spectrum.

The findings provide direct spectroscopic confirmation of previous indirect conclusions. The work also introduces the combination of Rydberg electron transfer and anion photoelectron spectroscopy as a powerful technique for studying diffuse electron states.

Source: “The ground state, quadrupole-bound anion of succinonitrile revisited,” by Gaoxiang Liu, Sandra M. Ciborowski, Jacob D. Graham, Allyson M. Buytendyk, and Kit H. Bowen, The Journal of Chemical Physics (2019). The article can be accessed at http://doi.org/10.1063/1.5114617 .