The Donor-Acceptor Model

Since the electron afSnity of noble gas atoms is zero (see above), chemical bonding can only be estabhshed if the very weak electron donor Ng is combined with a strong electron acceptor X. While the donor ability of Ng is reflected by the ionization potential /j, the acceptor ability of X can be assessed from its electron afiinity. This, in turn, depends on the energy of the lowest unoccupied MO (LUMO) of X. Furthermore, the nature of the LUMO, a or n, symmetric or asymmetric, influences the acceptor ability of X with regard to Ng electrons. 

On the basis of the donor-acceptor model the first calculational search for bonded systems between He and doubly charged cations was performed [7,11, 

---

The authors wish to acknowledge support of this work by the 3M Science Research Laboratory and the Corporate Research Laboratory. Special thanks go to Dr. Allen Sledle for helpful discussions regarding the donor-acceptor model for CO chemisorbed on Pd, and to Dr, Mark Albert for discussions regarding the hydrogen bonding of CO. 

Fig. 1. Schematic representation of the donor-acceptor model for CO adsorption.

While a qualitative understanding of the structures and stabilities of Ng molecules was achieved by the donor-acceptor model, the quantitative analysis of the binding properties was provided by the analysis of the electron density distribution. 

The electrostatic component, the attraction of a bonded hydrogen atom to a region of relatively high electron density of the base (B), is considered the dominant contribntion to most hydrogen bonds. Frontier molecnlar orbitals and the donor-acceptor model can be applied to understand the covalent contribution. 

---