Structure and Bonding in Double Rydberg Anions and Related Species

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J. V. (Vince) Ortiz of Auburn University is supported by an award from the Chemical Theory, Models and Computational program in the Chemistry division to develop and apply computational approaches based on quantum mechanics to study an unusual type of molecular ion, known as double Rydberg anions (DRAS). At an early stage in their education, students of chemistry learn the importance of pairs of negatively charged electrons in understanding chemical bonding. In two familiar types of electron pairs, those involved in chemical bonding between atoms and those that are confined in a non-bonding fashion to a single atom, the electron pairs are called "valence electrons" because they can be found relatively close to the atomic nucleus. In quantum mechanics, these electrons are described by interference between atomic-orbital waves. Recent experiments and calculations have shown that positively charged molecular ions are capable of binding two electrons with diffuse orbital waves in species known as double Rydberg anions (DRAs). A new kind of electron pair in which the electrons may be found further from the atomic nucleus occurs in DRAs, for example, NH4-, a positively charged ammonium cation with two diffuse weakly bound electrons. Ortiz and his research group develop and use sophisticated theoretical and computational methods to predict the existence of new DRAs and related species and to understand the interactions between electrons which permit DRAs to exist. Educational activities that are coordinated with this research are directed toward students and scientists with a variety of cultural experiences and academic backgrounds. New, fundamental concepts in chemical bonding will be integrated immediately into educational and outreach activities and will enable students and scientists to predict and understand a greater variety of chemical phenomena. To gain a better understanding of the electron correlation between two diffuse electrons that is essential for the formation of double Rydberg anions, transition amplitudes that connect states of these anions to those of uncharged radicals or closed-shell cations are analyzed. These amplitudes, known as Dyson orbitals and Dyson geminals, are generated in electron propagator and two-electron propagator calculations of electron binding energies. The effects of hydrogen bonds and of chemical substitutions on the structure and stability of the double Rydberg anions are explored and the photoelectron spectra of larger double Rydberg anions are being assigned. Correlation states in these photoelectron spectra which correspond to excited states of uncharged, Rydberg radicals also are also being assigned and predicted. Potential generalizations of the double Rydberg concept to anions with atoms of the fourth through sixth periods are based on closed-shell configurations of united-atom limits and isoelectronic relationships. Experimentalists who synthesize, isolate and characterize molecular anions in the gas phase are or will be provided reliable predictions on the stability and spectral properties of species with unprecedented patterns of electronic structure. Deeper insight into chemical bonding in double Rydberg anions will follow from an analysis of Dyson orbitals and geminals that connect these ions to Rydberg radicals and their cationic cores. Novel theoretical and computational techniques that are adapted to the search for such species will be published for the benefit of those who develop ab initio methodology. Ortiz continues his many efforts to recruit, educate and promote female and minority scientists. Among these activities are the training of graduate students, seminars and short courses, lectures to student and professional groups, popularizations for secondary-school students, scientific conferences, exchange programs, and organizational efforts for minority students and professionals. Students and young professionals are exposed to highly novel patterns of chemical bonding that alter the paradigms encountered in basic chemistry courses.