HYDROGEN-BONDING IN POTASSIUM FLUORIDE DIHYDRATE - A CRYSTALLOGRAPHIC, SPECTROSCOPIC, AND THEORETICAL-STUDY

Crystal structure data of orthorhombic KF.2H(2)O are reported that wer e obtained from neutron diffraction refinements at 295 K (Pmc2(1); Z = 2; a = 4.082(2) Angstrom, b = 5.182(2) Angstrom, c = 8.825(5) Angstro m; R = 0.020) and from X-ray diffraction refinements at 298 and 120 K (R = 0.018 and 0.009, respectively). Each of the two independent water molecules is coordinated by two potassium ions and forms two O-H...F hydrogen bonds (neutron values: O-H = 0.966-0.971 Angstrom, H...F = 1. 752-1.796 Angstrom, and O...F = 2.719-2.753 Angstrom). Vibrational spe ctra of partially deuterated samples yielded three rather similar unco upled <(nu)over bar>(OD) frequencies (2462, 2478, and 2491 cm(-1) at 7 5 K) in agreement with the three crystallographically different, but g eometrically similar O-H...F hydrogen bonds. Energy band calculations have been performed using the LAPW method. Total and partial densities of states show that the water-potassium interactions are almost exclu sively electrostatic, while the hydrogen bond interactions with the fl uorine anions have additional covalent contributions or at least polar ization effects of about 20%.