The Sigma Ring and Other Distinctive Features of Surface Potentials of Group 1 Systems

For a sigma hole induced on a terminal atom, such as a halogen atom (X) in a molecule R-X, it is commonly expected that the maximum potential in that sigma hole will arise along the extension of the R-X bond, opposite the overlap region. We find, however, that for the majority of group 1 halides (MX) that expectation fails. Conventional sigma holes appear on Na in its halides and hydrides, but not so for M = K, Rb, or Cs. Instead, a sigma ring─a latitudinal ring maximum─is observed in the surface potentials of those molecules. That anomaly is examined, and its entailments for structure and bonding in acid-base complexes involving metal halides are found to be substantial. Li generally shows the same behavior as Na, but there are curious exceptions. Certain even more consequential features of the surface potentials themselves─beyond the presence or absence of a sigma hole or ring─including the fact that the less electronegative halides lead to the strongest surface potentials on group 1 atomic centers in MX molecules, are demonstrated and rationalized. Our observations allow us to make and evaluate predictions about metal halides and hydrides as bonding partners in noncovalent interactions.