Hydrogen bond cooperativity and electron delocalization in hydrogen fluoride clusters

We investigate the energetic, structural, electronic and thermodynamics pro perties of hydrogen fluoride cluster, (HF)(n), in the range n=2-8, by ab in itio methods and density functional theory (DFT). The ab initio methods cho sen were Hartree-Fock (RHF) and second-order Moller-Plesset perturbation th eory (MP2). The DFT calculations were based on Becke's hybrid functional an d 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 energ y, and hydrogen bond distance. An analysis of the topology of the electron density reveals a linear correlation between the binding energy per hydroge n bond and the density at the hydrogen bond critical point and the Cioslows ki covalent bond order. Based on these correlations, hydrogen bond cooperat ivity is associated with the electronic delocalization between monomers uni ts. Analysis of the thermodynamics properties shows that the enthalpy chang es are determined by the electronic cooperative effects, while the entropic statistical factors are fundamental in the relative stability of these clu sters. Finally, for the trimer and tetramer, nonstable linear zigzag chains where found in a detailed analysis of the potential energy surfaces. (C) 2 001 American Institute of Physics.