The complex of HF2- and H2O: A theoretical study

The complex of HF2- and H2O is studied using B3LYP, MP2, and QCISD methods. Energetics, geometries, and vibrational frequencies of the equilibrium str ucture 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 highe r. 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 degrees and the F-O distance is 2.59 Angstrom. With zero point energy and counterpoise correction, the binding energy is 14.9 kcal/mol and the strong hydrogen bon d of HF2- is weakened by 11.3 kcal/mol (25%). In HF2- the experimental F-F distance is 2.28 Angstrom and the F-H-F bond angle is 180 degrees. The most intense IR vibration is the F-H-F asymmetric stretch at 1331 cm(-1). In HF 2- the calculated F-F distance is 2.30 Angstrom and in the HF2--H2O equilib rium structure the F-H distance for the hydrogen bonded F atom is longer by 0.13 Angstrom but the F-H distance for the free F atom is shorter by 0.10 Angstrom and the F-F distance is only 0.03 Angstrom longer. The F-H-F bond angle is very close to linear at 179.4 degrees. The most intense IR vibrati on 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. (C) 2001 American Institute o f Physics.