The Grotthuss mechanism

Water is of fundamental importance for human life and plays an important role in many biological and chemical systems. Although water is the most abundant compound on earth, it is definitely not a simple liquid. It possesses strongly polar hydrogen bonds which are responsible for a striking set of anomalous physical and chemical properties. For more than a century the combined importance and peculiarity of water inspired scientists to construct conceptual models, which in themselves reproduce the observed behavior of the liquid. The exploration of structural and binding properties of small water complexes provides a key for understanding bulk water in its liquid and solid phase and for understanding solvation phenomena. Modern ab initio quantum chemistry methods and high-resolution spectroscopy methods have been extremely successful in describing such structures. Cluster models for liquid water try to mimic the transition from these clusters to bulk water. The important question is: What cluster properties are required to describe liquid-phase behavior?

Water: From Clusters to the Bulk

We investigated rearrangements of the hydrogen-bond network in water by measuring fluctuations in the OH-stretching frequency of HOD in liquid D2O with femtosecond infrared spectroscopy. Using simulations of an atomistic model of water, we relate these frequency fluctuations to intermolecular dynamics. The model reveals that OH frequency shifts arise from changes in the molecular electric field that acts on the proton. At short times, vibrational dephasing reflects an underdamped oscillation of the hydrogen bond with a period of 170 femtoseconds. At longer times, vibrational correlations decay on a 1.2-picosecond time scale because of collective structural reorganizations.

https://science.sciencemag.org/content/sci/301/5640/1698.full.pdf

Ultrafast hydrogen-bond dynamics in the infrared spectroscopy of water.

Analytic MP2 frequencies without fifth-order storage. Theory and application to bifurcated hydrogen bonds in the water hexamer

We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

Low frequency Δν=0–350 cm−1, Raman intensity data were obtained from liquid water between 3.5 and 89.3 °C using holographic grating double and triple monochromators. The spectra were Bose–Einstein (BE) corrected, I/(1+n), and the total integrated (absolute) contour intensities were treated by an elaboration of the Young–Westerdahl (YW) thermodynamic method, assuming conservation of hydrogen‐bonded (HB) and nonhydrogen‐bonded (NHB=bent and/or stretched, O–H O) nearest‐neighbor O–O pairs. A ΔH°1 value of 2.6±0.1 kcal/mol O–H ⋅⋅⋅ O or 5.2±0.2 kcal/mol H2O (11 kJ/mol O–H ⋅⋅⋅ O, or 22 kJ/mol H2O) resulted for the HB→NHB process. This intermolecular value agrees quantitatively with Raman and infrared ΔH° values from the one‐ and two‐phonon OH‐stretching regions, and from molecular dynamics, depolarized light scattering, neutron scattering, and ultrasonic absorption, thus indicating a common process. A population involving partial covalency of, i.e., charge transfer into, the H ⋅⋅⋅ O units of linear and/or weak...

Temperature dependence of the low‐ and high‐frequency Raman scattering from liquid water

Precise isosbestic points occur in the Raman OH‐stretching spectra from liquid water between 3 and 85 °C if cell alignment is accomplished with Newton’s rings. Isosbestic frequencies measured for the orientations X(Y,X+Z)Y=6β2, X(ZX)Y=3β2, X(Y+Z,X+Z)Y=45α2+13β2, X(Z,X+Z)Y=45α2+7β2, and X(ZZ)Y=45α2+4β2 are 3524, 3522 (note β2 agreement), 3468, 3425, and 3403 cm−1, respectively. Isosbestic points from two different measurements calculated by the relations, X(ZZ)Y‐(4/3)X(ZX)Y and X(Z,X+Z)Y‐(7/6)X(Y,X+Z)Y agree exactly for 45α2, 3370 cm−1. (α and β2 correspond to the mean polarizability and square of the anisotropy.) The pure α2 isosbestic frequency, 3370 cm−1, coincides with the peak of the highest frequency hydrogen‐bonded (HB) Gaussian OH‐stretching component. The pure β2 isosbestic point, 3522–3524 cm−1, coincides with the peak of the nonhydrogen‐bonded (NHB) Gaussian OH‐stretching component, next above in frequency. The α2 and β2 isosbestic points are thus thought to provide an experimental distinction b...

Raman isosbestic points from liquid water

We present the complete intermolecular dynamical spectrum of liquid water, by merging the data sets from femtosecond nonlinear‐optical polarization spectroscopy with the depolarized, Bose–Einstein corrected Raman spectrum to cover the frequency range from 0–1200 cm−1. The impulse response function for liquid water at room temperature is calculated, including all of the intermolecular motions.

The intermolecular dynamics of liquid water

Low-frequency isobaric Raman spectra were obtained from water at 0.5, 1.0, 1.5, and 2.0 kbar and temperatures from −18 to 342 °C. Peaks near ≈60 and ≈175 cm-1 were observed at −18 °C and 2.0 kbar, but only a single, asymmetric 50−60 cm-1 feature, skewed to ≈300 cm-1, remained near ≈170 °C and above. The low-temperature ≈60 and 175 cm-1 peaks arise respectively from transverse acoustic (TA) and longitudinal acoustic (LA) standing waves, whereas the skewed, high-temperature Raman feature is well-fitted by a relative kinetic energy distribution (RKED). The TA phonon and RKED peaks are coincident near 60 cm-1. Spectra taken at intermediate temperatures, e.g., 77 and 124 °C, were fitted by a combination of the appropriate RKED and the (−18 °C, 2.0 kbar) TA + LA phonon spectrum. Temperature rise breaks hydrogen bonds and the solidlike patches which support the TA and LA phonons. This continues until only the gaslike RKED remains at high temperatures. Librational spectra were also obtained isochorically. They in...

Low-Frequency Raman Scattering from Water at High Pressures and High Temperatures†

Raman data have been obtained for vitreous and molten B2O3 from −196 to 1594 °C. A ΔH° of 6.4±0.4 kcal/mole boroxol was obtained from the temperature dependence of ratios of integrated Raman intensities. This heat is thought to correspond to the transformation of BO3 triangles in boroxol rings, to BO3 triangles in a random network. Vitreous B2O3 is composed predominantly of boroxol rings, but the boroxol ring concentration decreases with increasing temperature in the melt, and becomes small at temperatures above 1600 °C.

George E. Walrafen

Papers

Temperature dependence of the low‐ and high‐frequency Raman scattering from liquid water

Raman isosbestic points from liquid water

The intermolecular dynamics of liquid water

Low-Frequency Raman Scattering from Water at High Pressures and High Temperatures†

Raman investigation of vitreous and molten boric oxide