The complex of HF2− and H2O: A theoretical study

Abstract

The complex of HF2− and H2O is studied using B3LYP, MP2, and QCISD methods. Energetics, geometries, and vibrational frequencies of the equilibrium structure and two transition states are calculated using 6-311++G(d,p), 6-311++G(2d,2p), and 6-311++G(2df,2pd) basis sets. For the equilibrium structure there is a hydrogen bond between one of the F atoms of HF2− and one of the H atoms of H2O. The two transition states are only about 0.5 kcal/mol higher. The HF2−–H2O equilibrium structure is planar and, at the B3LYP/6-311++G(2df,2pd) level, the F–H–O bond angle is nearly linear at 174.4° and the F–O distance is 2.59 Å. With zero point energy and counterpoise correction, the binding energy is 14.9 kcal/mol and the strong hydrogen bond of HF2− is weakened by 11.3 kcal/mol (25%). In HF2− the experimental F–F distance is 2.28 Å and the F–H–F bond angle is 180°. The most intense IR vibration is the F–H–F asymmetric stretch at 1331 cm−1. In HF2− the calculated F–F distance is 2.30 Å and in the HF2−–H2O equilibrium structure the F–H distance for the hydrogen bonded F atom is longer by 0.13 Å but the F–H distance for the free F atom is shorter by 0.10 Å and the F–F distance is only 0.03 Å longer. The F–H–F bond angle is very close to linear at 179.4°. The most intense IR vibration remains the F–H–F asymmetric stretch, blueshifted by 648 cm−1. The F–H–O asymmetric stretch is also an intense IR vibration, redshifted by 729 cm−1 from the O–H local mode stretch for H2O.