Microsolvation of small anions by aromatic molecules: An exploratory study

This work was motivated by the experimental finding that the O2−/benzene interaction energy is unexpectedly large. To further explore the interactions of small anions with aromatic molecules, anion photoelectron spectroscopy was utilized to measure interaction strengths of the seed anions, O2− and NO−, complexed with several aromatic molecules, including benzene, naphthalene, pyridine, and pyrimidine. As in the case of O2−(benzene), the anion(aromatic)1 binding energies for the other complexes studied were also higher than one might have anticipated. In addition, the interaction energy of O2− complexed with a given aromatic molecule was, in every case studied, higher (by a factor of ∼1.5) than that of NO− complexed with the same aromatic. While the dependence of interaction strengths on solvent dipole moments and/or polarizabilities implied a substantial electrostatic component to the binding in these complexes, differences in the binding of O2− and NO− with these aromatic molecules showed that there is a distinct covalent aspect to the interaction as well. A significant portion of this interaction was attributed to the fact that O2− and NO− are both open-shell anions that are interacting with closed-shell aromatic molecules. In the accompanying paper [J. Chem. Phys. 116, 9672 (2002)] calculations on O2−(benzene)1 by Jalbout and Adamowicz shed additional light on the nature of small anion–aromatic molecule interactions. Last, results are also presented comparing the interaction energies of several multisolvent, anion–molecule complexes.