Ab initio study of the structure, cooperativity, and vibrational properties of the H2O: (HF)2 hydrogen bonded complex

Ab initio self-consistent field (SCF) and MP2 calculations have been performed in order to investigate the structure, energetics, and spectroscopic properties of H2O: (HF)2. Two conformations of the 1:2 complex are energetically possible. On the one hand is a cyclic complex belonging to the C1 point group and which corresponds to the absolute minimum, and on the other hand is a bifurcated complex with C2v symmetry. In this latter complex the two hydrogen bonds are equivalent whereas in the former there are two strong H-bonds and a weaker one. The energy difference between these two conformers is ∼3 kcal mol−1. From the spectroscopical point of view the bifurcated complex is characterized by νHF frequency shifts smaller than that calculated for the dimer, in the case of the cyclic complex the two different νHF red shifts closely related to the H2O–HF and (HF)2 calculated shifts, respectively, and correspond to those experimentally observed. The cooperative effects on interaction energies and spectroscopic properties have been estimated and analyzed on the basis of the projected induced dipole moment. It is shown that the cyclic complex is favored with respect to both H2O–HF and (HF)2 precursors while the formation of the bifurcated one remains more problematic.