Hydrogen bond cooperativity and electron delocalization in hydrogen fluoride clusters

We investigate the energetic, structural, electronic and thermodynamics properties of hydrogen fluoride cluster, (HF)(n), in the range n=2-8, by ab initio methods and density functional theory (DFT). The ab initio methods chosen were Hartree-Fock (RHF) and second-order Moller-Plesset perturbation theory (MP2). The DFT calculations were based on Becke's hybrid functional and the Lee-Yang-Parr correlation functional (B3LYP). We found that symmetric cyclic clusters are the most stable structure, and that large cooperative effects, particularly from trimer to tetramer are present, in binding energy, and hydrogen bond distance. An analysis of the topology of the electron density reveals a linear correlation between the binding energy per hydrogen bond and the density at the hydrogen bond critical point and the Cioslowski covalent bond order. Based on these correlations, hydrogen bond cooperativity is associated with the electronic delocalization between monomers units. Analysis of the thermodynamics properties shows that the enthalpy changes are determined by the electronic cooperative effects, while the entropic statistical factors are fundamental in the relative stability of these clusters. Finally, for the trimer and tetramer, nonstable linear zigzag chains where found in a detailed analysis of the potential energy surfaces. (C) 2001 American Institute of Physics.