An electron localization function study of the lone pair

Electron localization function (ELF) theory is used to characterize lone pairs in a variety of situations. Using the lone pair basin attractor locations, lone pair distances from their parent nucleus may be defined as well as the angular disposition of the lone pair with regard to its neighboring atoms and bond basin centers. Studies of the first and second row hydrides show, with some exceptions, that lone Fairs are basically tetrahedrally disposed with distances dependent essentially only on the atomic number (Z) of the heavy atom, decreasing with increasing Z. The examination of three-to-six membered heterorings involving nitrogen and phosphorus show that only in the most constrained systems are large effects noticeable. Two relatively weak hydrogen bonding systems (HOH . . . OH2 and FH . . . NH3) show that only a very small transfer of charge occurs upon dimer formation and involves mainly the proton donor and the acceptor lone pair; in the water dimer it is shown that both the acceptor lone pair and the donor proton lie off the line connecting the two oxygen atoms. Finally, examination of several model gauche and anti isomers containing ether oxygens shows that in the more stable gauche form a small but noticeable transfer of charge occurs from the interacting lone Fair to the adjoining carbon bonding basin, a result consistent with the generally accepted double-bond/no-bond sigma* orbital interaction model of the anomeric effect in organic stereochemistry.