An electron localization function study of the lone pair

Electron localization function (ELF) theory is used to characterize lone pa irs in a variety of situations. Using the lone pair basin attractor locatio ns, 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 at oms and bond basin centers. Studies of the first and second row hydrides sh ow, with some exceptions, that lone Fairs are basically tetrahedrally dispo sed with distances dependent essentially only on the atomic number (Z) of t he heavy atom, decreasing with increasing Z. The examination of three-to-si x membered heterorings involving nitrogen and phosphorus show that only in the most constrained systems are large effects noticeable. Two relatively w eak hydrogen bonding systems (HOH . . . OH2 and FH . . . NH3) show that onl y 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 i s shown that both the acceptor lone pair and the donor proton lie off the l ine connecting the two oxygen atoms. Finally, examination of several model gauche and anti isomers containing ether oxygens shows that in the more sta ble gauche form a small but noticeable transfer of charge occurs from the i nteracting lone Fair to the adjoining carbon bonding basin, a result consis tent with the generally accepted double-bond/no-bond sigma* orbital interac tion model of the anomeric effect in organic stereochemistry.