New Raman method for aqueous solutions: xi-function dispersion evidence for strong F--water H-bonds in aqueous CsF and KF solutions

摘要

The Raman xi-function dispersion method recently elucidated for the strong H-bond breaker, ClO4-, in water [G. E. Walrafen, J. Chem. Phys. 122, 094510 (2005)] is extended to the strongly H-bond forming ion, F-. Measuring the xi function is analogous to measuring Delta G from the thermodynamic activity of the water, a(H2)O, as the stoichiometric mol fraction of the water in the solution decreases due to addition of an electrolyte or nonelectrolyte. xi is the derivative of the OH-stretching part of the Gibbs free energy with respect to the water mol fraction; xi(omega)equivalent to-RT[partial derivative ln(I-omega/I-REF)/partial derivative X-2](T,P). I is the Raman intensity at omega (omega=Raman shift in cm(-1)); I-REF, that at an arbitrary reference omega; and, X-2 is the water mol fraction (X-1=CsF or KF mol fraction). ln(I-omega/I-REF) was found to be linear in X-2 for the complete range of OH-stretching omega's, with correlation coefficients as large as 0.999 96. Linearity of ln(I-omega/I-REF) versus X-2 is an experimental fact for all omega's throughout the spontaneous Raman OH-stretching contour; this fact cannot be negated by relative contributions of ultrafast/fast, homogeneous/inhomogeneous processes which may underlie this linearity. Linearity in ln(I-omega/I-REF) versus 1/T, or in ln(I-omega/I-REF) versus P, was also observed for the Raman H-bond energy Delta E and pair volume Delta V dispersions, respectively. A low-frequency maximum (MAX) and a high-frequency minimum (MIN) were observed in the xi function dispersion curve. Delta xi=xi(MIN)-xi(MAX) values of -7000 +/- 800-cal/mol H2O for CsF, and the experimentally equal Delta xi=-6400 +/- 1000-cal/mol H2O for KF, were obtained. These Delta xi's are opposite in sign but have nearly the same absolute magnitude as the Delta xi value for NaClO4 in water; Delta xi=+8050 +/- 100-cal/mol H2O. A positive Delta xi corresponds to a water-water H-bond breaker; a negative Delta xi to a H-bond former; specifically, a F--water H-bond former, in the instant case. NaClO4 breaks water-water H-bonds and also gives rise to weak, long (3.0-3.3 A), severely bent (approximate to 140 degrees), high-energy, ClO4--water interactions. Fluoride ion scavenges the extremely weak or non-hydrogen-bonded OH groups, thus forming strong, short, linear, low-energy, H-bonds between F- and water. The strength of the F--water H-bond is evident from the fact that the OH-stretching xi-function minimum is centered approximate to 200-300 cm(-1) below that of ice. The diagnostic feature of the Raman spectrum from F- in water is an intense, long, low-frequency OH-stretching tail extending 800 cm(-1) or more below the 3300-cm(-1) peak. A similar intense, long, low-frequency Raman tail is produced by the OH- ion, which is known to H-bond very strongly when protons from water are donated to its oxygen atom.