Explicit Hydration of the Beryllium Trifluoride Anion with One to Three Water Molecules: BeF₃^-(H₂O)_n=1-3

This work investigates the microhydration of the beryllium trifluoride anion (BeF₃^-) with up to three water molecules (BeF₃^-(H₂O)_n where n = 1-3). Full geometry optimizations and harmonic vibrational frequencies were computed on various BeF₃^-(H₂O)_n stationary points using density functional theory methods (B3LYP-D3BJ, B3LYP, ωB97XD, and M06-2X) as well as the CCSD(T) and MP2 ab initio methods with a triple-ζ correlation-consistent basis set augmented with diffuse functions on all non-hydrogen atoms (haTZ). One BeF₃^-(H₂O)₁, five BeF₃^-(H₂O)₂ and 11 BeF₃^-(H₂O)₃ minima were identified, most of which have not been reported in the literature to date. Two of the new BeF₃^-(H₂O)₃ configurations have lower electronic energies than the previously reported trihydrate structure by nearly 1 kcal mol^-1, but the previous structure has the lowest energy when harmonic zero-point vibrational energies are included. For the mono- and dihydrates, the water molecule(s) prefer to bind directly to the beryllium trifluoride anion via double ionic hydrogen bonds (DIHBs) to form planar structures with C_2v symmetry. With the introduction of a third water molecule, the hydration pattern associated with the lowest-energy structure (with C₃ symmetry) changes from solely DIHBs to a hydrogen-bonded network that also includes water-water interactions. The CCSD(T)/haTZ electronic dissociation energies for the BeF₃^-(H₂O)_n global minima are 16.0, 30.1, and 43.6 kcal mol^-1 for n = 1, 2, and 3, respectively, and these values decrease by about 5% when a counterpoise procedure is applied. The CCSD(T)/haTZ harmonic OH stretching frequencies of H₂O decrease appreciably when donating hydrogen bonds to BeF₃^- and/or H₂O molecules. The magnitude of these shifts is strongly dependent on the hydration motif. For the symmetric structures exhibiting only DIHB contacts, the largest shifts at this level of theory are -168 cm^-1 for n = 1 (C_2v), -138 cm^-1 for n = 2 (C_2v), and -122 cm^-1 for n = 3 (D_3h). In contrast, the C₃ trihydrate global minimum also has hydrogen bonding between the water molecules, and the largest shift exceeds 200 cm^-1. The magnitude can even exceed 300 cm^-1 in structures with an H atom that does not participate in hydrogen bonding.