Bonding in hypohalous acids HOX (X=F, Cl, Br, and I) from the topological analysis of the electron localization function

The bonding in hypohalous acids has been investigated from the topological analysis of the electron localization function (ELF) at the Becke3LYP and H artree-Fock levels. The interaction between halogen and oxygen atoms has be en characterized by the presence of bonding, disynaptic attractors V(O,X) X =F, Cl, Br, and I with the mean electron population (N) over bar of 0.32, 0 .61, 0.35e, and 0.35e, respectively. In the case of HOBr, the possibility o f a strong contribution of 3d bromine core electrons to the valence shell h as been observed. On the base of the bonding evolution theory (BET), the O- F bond has been recognized as a covalent, polarized one whereas, the bondin g between O and Cl, Br, and I atoms is of the electron donor-acceptor-type with halogen donating the electron density to valence shell of oxygen. The observed difference between HO+F- and HO-X+ (X=Cl, Br, and I) polarizations is reflected in topology of ELF maps with a large localization domain surr ounding the V(F) and V(F,O) attractors in HOF and a common superbasin encom passing the V(O,H), V(O), and V(X,O) attractors in HOCl, HOBr, and HOI. The very large values of the relative quantum fluctuation (lambda), above 0.8, found for V(O,X) suggest that the covalent electron density is almost enti rely delocalized over other basins. The comparison of the mean electron pop ulation ((N) over bar) of the V(H,X) and V(H,O) basins computed for hydroha lic (HX) and hypohalous acids (HOX) has revealed that the population alters in line with values of Delta E-acid, therefore can be used as a approximat e measure of acidic properties of molecules. The topological analysis of th e ELF function supports the concept of probonded electronegativity and its usefulness as a tool for prediction of the nature of the oxygen-halogen lin kage. (C) 1999 American Institute of Physics. [S0021-9606(99)30130-6].