Sammanfattning

The extended dynamic model of liquid water which considers five different hydrogen-bonded states (including three dominant states) and two major positions of the water molecule in equilibrium, is presented based mostly on the analysis of near-infrared overtone spectra for the O-D stretching vibration of pure liquid D2O, obtained and evaluated within the temperature range of 278 - 358 K (5 - 85 degreesC). In agreement with the earlier data of Luck and Ditter (1969), and Angell and Rodgers (1984), it was found that the integrated intensity (but not the shape) of the specified (composite) overtone band, situated within the spectral range of 4250 - 5750 cm(-1), is almost independent of temperature, and displays a quasi-isosbestic point at ca. 5 100 cm(-1). Moreover, the density-corrected difference spectra taken with intervals of 10, 20 and 40 K clearly indicate that the temperature effect is determined by the intensity redistribution between two major components (sub-bands) of the total of five forming the composite overtone contour, These two sub-bands according to their center-of-mass positions, and on the basis of published structural (Bosio et al., 1983), static spectroscopic (Berglund et al., 1978, Walrafen et al., 1986, 1996), and MD simulation (Scortino et al., 1990-1992, Sato and Hirata, 1999) data, can be attributed to the "normal" and temperature-distorted ("T-bifurcated", but not "totally broken") hydrogen-bonded states of O-D oscillators, respectively. In addition, the global Gaussian-contour analysis of obtained composite spectra clearly revealed the existence of O-D oscillator states corresponding to the fraction of presumably pressure-responsive ("P-bifurcated") hydrogen bonds promoted by formation of additional "rigid" five-coordinated water configurations (furnished also by forming of quasi-linear O-O-O trimers within the hexagonal Ih ice-like lattice). The integrated intensity due to this fraction of hydrogen-bonded states does not display any change with temperature, over the whole range of existence of liquid water, as indicated earlier by Luck, Angell, and their co-workers. The whole variety of structural, static spectroscopic, and MD simulation data, together with the results of dielectric relaxation temperature dependence studies (Ronne et al., 1997, 1999, Buchner et al., 1999) can be readily explained in the framework of dynamic "tetrahedral displacement" model first formulated by Agmon (1996). This model implies the temperature-dependent "flip-flop" equilibrium between two major positions for a given water molecule (corresponding to the tetrahedral four-neighbor and the interstitial five-neighbor configurations), the exchange between which is accomplished by the breakage and formation of a net "normal" and a net "T-bifurcated" (and vice versa) hydrogen bond, respectively. The extended van't Hoff analysis indicate that the transition between two states of water at ca. 313 K (40 degreesC) takes place with almost zero equilibrium free energy, but significant counterbalanced enthalpy and entropy contributions (3.3 kcal mol(-1) and 10.5 cal mol(-1) K-1, respectively). In contrast, the effect of high pressure, according to the published data, can be viewed as leading to the "tetrahedral displacements" thanks to the highly-cooperative process of formation of "P-bifurcated" hydrogen bonds only, without altering the quasi-equilibrium between "normal" and "T-bifurcated" fractions.

Publicerad i

Journal of Molecular Liquids
2002, Volym: 96-97, sidor: 45-63
Utgivare: Elsevier Science BV

Konferens

EMLG Meeting on Physical Chemistry of Liquids - Molecules, Macromolecules, Biomolecule

UKÄ forskningsämne

Fysikalisk kemi
Oorganisk kemi


Publikationens identifierare

DOI: https://doi.org/10.1016/S0167-7322(01)00326-9

Permanent länk till denna sida (URI)

https://res.slu.se/id/publ/95560