Comparison of lithium and hydrogen bonds in (X•••Li•••X)- and (X•••H•••X)- (X=F, Cl and Br) complexes:: Topological analysis of electron localization function

The topological analysis of the electron localization function (ELF) is used to elucidate the nature of the hydrogen and lithium bonds in the (X...Y...X)(X F, Cl, Br; Y=H, Li) complexes. The lithium bond that exhibits an electrostatic nature is characterized by a constant mean electron population of the lithium core basin C(Li) equal to 2.03e. The values of the ELF within Li...X regions are smaller than 0.5 corresponding to a totally delocalized electron density. For the X...H hydrogen bonds a covalent nature is found with an essential electron pairing, as implied by values of ELF in the H...X regions larger than 0.5. The mean electron population of the V(H) basin depends on electronegativity of a halogen atom, and it ranges from 0.17e for (F...H...F)(-) to 2.11e for (Br...H...Br)(-). The polarization of halogen anions X- by lithium or hydrogen cations results in an anisotropy of the nonbonding electron density, which is reflected by two valence attractors V-1(X-i=1,X-2) and V-2(X-i=1,X-2) found inside and outside a complex, respectively. The analysis of the H...Br hydrogen bond reveals essential differences as compared with the H...X (X F and Cl) hydrogen bonds. The topology of nonbonding electrons of Br yields only one valence, monosynaptic basin V(Br-i=1,Br-2). Furthermore, similarity of the H...Br hydrogen bonds to a bond in an isolated HBr molecule is noted. The increased Pauli repulsion between the V(H) and V-1(X-i=1,X-2) basins in the (X...H...X)(-) complexes leads to a larger electron delocalization of V-1(X-i=1,X-2) as compared with the (X...Li...X)(-) systems. (C) 2002 Wiley Periodicals, Inc.