Within any molecule or cluster containing one or more positively charged sites, families of Rydberg orbitals exist. Free electrons can attach directly, and anionic reagents with low electron binding energy can transfer an electron into one of these orbitals to form a neutral Rydberg radical. The possibilities that such a radical could form a covalent bond either to another Rydberg radical or to a radical holding its electron in a conventional valence orbital are considered. This Perspective overviews two roles that Rydberg radicals can play, both of which have important chemical consequences. Attachment of an electron into excited Rydberg orbitals is followed by rapid (∼10−6 s) relaxation into the lowest-energy Rydberg orbital to form the ground state radical. Although the excited Rydberg species are stable with respect to fragmentation, the ground-state species is usually quite fragile and undergoes homolytic bond cleavage (e.g., –R2NH dissociates into –R2N + H or into –RNH + R) by overcoming a very small barrier on its potential energy surface, thus generating reactive radicals (H or R). Here, it is shown that as a result of this fragility, any covalent bonds formed by Rydberg radicals are weak and the molecules they form are susceptible to exothermic fragmentations that involve quite small activation barriers. Another role played by Rydberg species arises when the Coulomb potentials provided by the (one or more) positive site(s) in the molecule stabilize low-energy anti-bonding orbitals (e.g., σ∗ orbitals of weak σ bonds or low-lying π∗ orbitals) to the extent that electron attachment into these Coulomb-stabilized orbitals is rendered exothermic. In such cases, the overlap of the Rydberg orbitals on the positive site(s) with the σ∗ or π∗ orbitals allows either a free electron or a weakly bound electron to an anionic reagent that is attracted toward the positive site by its Coulomb force to be guided/transferred into the σ∗ or π∗ orbital instead. After attaching to such an anti-bonding orbital, bond cleavage occurs again, generating reactive radical species. Because of the large radial extent of Rydberg orbitals, this class of bond cleavage events can occur quite distant from the positively charged group. In this Perspective, several examples of both types of phenomena are given for illustrative purposes.
In 1981, Herzberg obtained emission spectra between various electronic states of the ammonium radical NH4 that led him to introduce the terminology of Rydberg molecules, neutral molecules consisting of a closed-shell cationic core to which an electron is bound to an orbital surrounding the entire molecular framework.
Other such molecules include H3O and H3, which have one electron in a Rydberg orbital surrounding the closed-shell H3O+ or H3+ cation, respectively. The electronic energy level spacings observed seemed to approximately fit the expression for the energy levels of a hydrogen atom but with a modified principal quantum number, which is why these species were called Rydberg molecules. In 1982, Gellene, Cleary, and Porter used a neutralized ion beam method to form NH4 and determined that this Rydberg molecule has a life- time with respect to fragmentation of less than 1 μs. It turns out that the excited electronic states of most such Rydberg molecules do not fragment but remain intact at geometries very close to those of their parent cation; only the ground-state Rydberg molecules dissociate (e.g., into NH3 + H for NH4). Although the excited states do not fragment, they do undergo relaxation to lower-energy electronic states at rates in the ∼106 s−1 range. These differences in the behaviors of ground and excited Rydberg states will be important later in this Perspective.
It turns out that closed-shell molecular cations can alternatively attach two electrons to one or more of their Rydberg orbitals to form anions. In 1987, Bowen and Eaton7 carried out photo detachment experiments on the H− anion solvated by one or more NH3 molecules. When studying H−(NH3), they observed a peak in their data showing an electron binding energy more than that of bare H− as expected (because the NH3 molecule differentially stabilizes the anion). However, they also found a peak corresponding to an electron binding energy of 0.5 eV, considerably below the 0.72 eV binding energy of bare H−. Later that same year, Ortiz considered the possibility that a different isomer might be responsible for the 0.5 eV peak and showed using electronic structure calculations that a tetrahedral closed-shell NH4+ cation surrounded by two electrons in a Rydberg-like orbital is predicted to have an electron binding energy of 0.42 eV. This isomer of NH4− was, thus, termed a double-Rydberg anion. The Ortiz group subsequently studied a wide range of double-Rydberg anions and their publication website provides a wealth of information about these studies. Over the past several years, the Miliordos and Ortiz groups have extended the concepts of Rydberg neutrals and double-Rydberg anions to include much larger species in which one or more electrons are bound to the outer surface of a partially solvated cation in what is essentially a Rydberg-like orbital, and they have termed some of the species solvated electron pre- cursors. Interestingly, in those studies, the pattern of ground- and excited-state orbitals has been found to follow that found in the jellium model rather than in the conventional hydrogenic model.
The publication websites of these two research groups as well as my own publication websites offer much detail on these novel radicals and ions.
In this Perspective, I attempt to offer perspective on the roles that Rydberg orbitals can play in chemical bonding and reactivity, but by no means, do I try to review all that is known about Rydberg-based species. This Perspective is organized as follows: First, I introduce and illustrate the primary features (shapes, sizes, and electron binding energies) of Rydberg orbitals; these features are common to small Rydberg molecules and anions as well as to Rydberg species arising from a positively charged group within a larger molecule, such as the solvated electron precursors mentioned earlier or a protonated side chain within an polypeptide or protein. Next, I discuss some of our efforts to examine the possibility that Rydberg molecules could use their Rydberg orbitals to form covalent bonds either to other Rydberg molecules or to a conventional valence orbital of another atom or molecule. In this discussion, I examine the putative bonding involving one, two, and three electrons in cationic, neutral, and anionic molecules, respectively. Finally, I introduce examples from my own research in which Rydberg orbitals on charged groups within polypeptides or on the surface of partially solvated cations can facilitate chemical bond cleavages elsewhere in the molecular framework. These examples relate to the so-called electron capture dissociation processes that have proven very useful in the field of mass spectrometry.
