Trimers of Hydrogen Fluoride and Water

After this excursion into a world of pairwise additive potentials we summarize the available data on (HF)3 and (H20)3. We start with the results of ab initio calculations. The most stable configurations of the hydrogen-bonded trimers (HF)j and (H20)3 are cyclic structures (Fig. 13). Because the enormous numerical efforts which are inevitable in large scale computations on trimers no satisfactory equilibrium geometries are available at present. Most calculations were performed assuming frozen monomer geometries or applying some other constraints. 

In the case of the water trimer the situation is much more complex. An extensive study has been performed with frozen monomer geometries and under the assumption of pairwise additive potentials 20). This structure (Fig. 13) is characterized by an almost planar arrangement of the tree oxygen and the three hydrogen atoms 

The approximate equilibrium geometry of the open-chain structure of (HF is shown in Fig. 14. The energy of interaction relative to three HF molecules is —9.4 kcal/mol. The open-chain trimer structure is thus less stable than the cyclic trimer by approximately 2.2 kcal/mol. For the former structure we calculate an energy per hydrogen bond of —4.7 kcal/mol whereas for the latter we find only a value of —3.9 kcal/mol. Thus, the ring strain in the cyclic structure reduces the energy of interaction. The cyclic conformer, however, contains one more bond which ultimately leads to the higher stability. 

Hydrogen fluoride trimer has also been used as an appropriate model system for the discussion of three-body forces 64 65). The data reported on (HF)3 indicate a stabilizing effect caused by the three-body interactions. The energy per hydrogen 

Anton Beyer, Alfred Karpfen and Peter Schuster 

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