Proton Transfer In Ice Research Articles

Fast and Slow Proton Transfer in Ice: The Role of the Quasi-Liquid Layer and Hydrogen-Bond Network

The proton mobility in ice is studied through molecular dynamics simulations carried out with a newly developed ab initio-based reactive force field, aMS-EVB3/ice. The analysis of both structural and dynamical properties of protonated ice as a function of temperature indicates that the mobility of excess protons at the surface is largely suppressed, with protons becoming essentially immobile at temperatures below 200 K. In contrast, fast proton transfer/transport can exist in bulk ice Ih at low temperature through connected regions of the proton-disordered hydrogen-bond network. Based on the simulation results, it is shown that the mechanisms associated with proton transfer/transport in both bulk and interfacial regions of ice are largely dependent on the local hydrogen-bond structure surrounding the charge defect. A molecular-level picture of the mechanisms responsible for proton transfer/transport in ice is then developed and used to interpret the available experimental data.

Periodic local MP2 method for the study of electronic correlation in crystals: Theory and preliminary applications

A computational technique for solving the MP2 equations for periodic systems using a local-correlation approach and implemented in the CRYSCOR code is presented. The Hartree-Fock solution provided by the CRYSTAL program is used as a reference. The motivations for the implementation of the new code are discussed, and the techniques adopted are briefly recalled. With respect to the original formulation (Pisani et al., J Chem Phys 2005, 122, 094113), many new features have been introduced in CRYSCOR to improve its efficiency and robustness. In particular, an adaptation of the density fitting scheme to translationally periodic systems is described, based on Fourier transformation techniques. Three examples of application are provided, concerning the CO(2) crystal, proton transfer in ice XI, and the adsorption of methane on MgO (001). The results obtained with the periodic LMP2 method for these systems appear more reliable than the ones obtained using density functional theory.

Temperature Dependence of Excited State Proton Transfer in Ice

We have studied the excited-state proton-transfer rate of four photoacids in ice as a function of temperature. For all four photoacids, we have found a non Arrhenius behavior of the proton-transfer rate constant, k(PT). d(ln k(PT))/d(1/T) decreases as the temperature decreases. The average slope of ln(k(PT))versus 1/T depends on the photoacid strength (pK*). The stronger the photoacid is, the smaller the slope. For the strongest photoacid 2-naphthol-6,8-disulfonate (2N68DS) the largest slope is 35 kJ/mol at about 270 K, and the smallest measured slope is about 8 kJ/mol at about 215 K. We propose that the temperature dependence of k(PT) in ice at the temperature range 270 > T > 200 K can be explained as arising from contributions of two proton-transfer mechanisms over the barrier and tunneling under the barrier. At very low temperatures T < 200 K, the slope of ln(k(PT)) versus 1/T increases again. At about 170 K, the proton-transfer rate is much slower than the radiative rate, and the deprotonated form of the photoacid cannot be detected in the steady-state emission spectrum. At lower temperatures, T < 200 K, the rate further decreases because of a limitation on the reaction caused by the restrictions on the H2O hydrogen reorientations.

Mechanism of proton transfer in ice. II. Hydration, modes, and transport

The mechanism of the excess-proton transfer in ice is investigated by analyzing the potential energy surface, the normal modes, and the interaction between the excess proton and defects. It is found that the solvation from water molecules in long-distance shells is essential for the smooth transport of the proton. The solvation shells up to, for example, about the 18th shell are needed to attain a convergence of the excess-proton solvation energies. The potential energy surface of the excess-proton transfer calculated with including these distant hydration shells is very smooth even for a long distance proton transport. Normal modes are calculated along the reaction paths of the proton transfer. An analysis is done to find how the character of these normal modes changes along the proton transfer. The structure and energetics of hydronium ion and L-defect complex are also examined to explain the temperature dependence of the proton transport.

Erratum: “Mechanism of fast proton transfer in ice: Potential energy surface and reaction coordinate analyses” [J. Chem. Phys. 113, 9090 (2000)

First Page

Mechanism of fast proton transfer in ice: Potential energy surface and reaction coordinate analyses

The mechanism of proton transfer in ice is investigated theoretically by examining the potential energy surfaces and determining the reaction coordinates. It is found to be quite different from that in liquid water. As shown by many authors, proton transfer in liquid water is promoted by the structure fluctuation, creating three-coordinated water molecules in the hydrogen bond network rearrangement, and the excess proton makes transitions among these three-coordinated water molecules as forming a so-called Zundel structure, (H5O2)+. This kind of large structural rearrangement cannot take place in ice. Nevertheless, the proton transfer in ice can be very fast. It is found that the strong constraint on the molecular geometry in ice is the source of the facile proton transfer. This constraint reduces the stabilization of the excess proton state in two ways: (1) as O–O cannot shrink freely, it cannot form a stable Zundel structure in which two water molecules share the excess proton locating at the center of the shortened O–O bond, and (2) as the existence of the repulsive force, an Eigen structure cannot be much stabilized. This repulsive force also contributes to partially shorten the O–O distance and thus facilitating a proton transfer. As the result, the excess proton is not trapped in a deep energy minimum but makes the transfers on small energy barriers. The molecular geometry relaxation along the proton transfer is analyzed; it is found that O–O stretchings/shrinkages at the excess proton moiety are mutually coupled to assist the sequential proton transfers in a concerted fashion. The energetics and geometrical changes along these reaction coordinates are analyzed. The potential energies are found to be fairly flat for different locations of the excess proton. The nature of the excess proton solvation from the surrounding water molecules are analyzed; it is shown that the solvation by even distant shells yields a significant contribution to the potential energy surface of the proton transfer.

Proton transfer in ice

Born—Oppenheimer energy surfaces for proton transfer in H 2O cubic ice are calculated using gradient-corrected density-functional theory. Recombination of a pair of H 3O + and OH − ionic defects is found to occur without barrier for both first-neighbor and second-neighbor pairs. The energy barriers for proton transfer in the configurations (H 2OHH 2O)+ and (OHHOH) − are calculated to be only 0.069 and 0.058 eV, respectively. Relaxation of the atomic coordinates is found to be critical in determining the magnitude of the energy barriers. The relaxed geometries are not consistent with a solitonic description of the proton motion.

The dielectric relaxation spectra of water, ice, and aqueous solutions, and their interpretation. III. Proton organization and proton transfer in ice

For pt.II see ibid., vol.23, no.5, Oct. 1988, p.817-23. After a short presentation of the various ice phases and of the oxygen sublattices for ice I, the proton array of ice I is considered, simple approximate derivation of the zero-point entropy for random distribution given, and evidence examined from some proton ordering along the hexagonal axis of ice I/sub h/. Next, the experimental facts are summarized on orientation polarization and conduction, and N. Bjerrum's ideas (1951) are given about defect-pair and ion-pair formation. The double-well model for polarization and conduction in ice, is used to describe the behavior of the individual defects and ions, after the pairs have formed, drifted apart, and settled down. The author argues that this model seems in conflict with the observed dielectric relaxation time when the proper activation energy is used. Therefore, another model is presented, linking the formation and separation of defects to the dielectric relaxation spectrum and to infrared spectroscopy.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Continuum model for proton transfer in ice containing symmetrical hydrogen bonds

Continuum model for proton transfer in ice containing symmetrical hydrogen bonds

Kinetics of proton transfer in ice via the pH-jump method: Evaluation of the proton diffusion rate in polycrystalline doped ice

The value of the proton diffusion coefficient D H + in ice was extracted from the diffusion-controlled rate k D of the proton recombination reaction RO − + H 3O + → k D ROH + H 2O in polycrystalline doped ice. At −10°C, D H + was estimated to lie between 3.5×10 −6 and 1.3×10 −5 cm 2 s −1, well below the corresponding value of (4.1 ± 0.1)×10 −5 cm 2 s −1 found in supercooled water.

Is the serine protease “charge relay system” a charge relay system?

Summary The catalytic mechanism of the serine proteases as currently conceived specifies that the charge relay system Asp is ionized at pH's where the enzymes are active. This choice is questioned on the basis that external anions are found associated with the charge relay system of both chymotrypsin and subtilisin at pH's below the charge relay system pK, and no evidence has been found for the presence of an associated cation at pH's above the charge relay system pK. An alternative catalytic mechanism is proposed wherein Nδl of the His imidazole is hydrogen bonded to a neutral Asp, and His-Nɛ2 functions to relay a proton from Ser-Oγ to the substrate in a manner analogous to proton transfer in ice.

Transfer of Protons through “Pure” Ice Ih Single Crystals. I. Polarization Spectra of Ice Ih

After a short introduction of the subject “proton transfer in ice” and some background information on previous polarization measurements, the growing and preparation of our single crystals and the instrumentation for a–c measurements from 105–8 × 10−3 Hz, 0 to − 180°C, are briefly described. By rigorous control of error limits and an improved computerized evaluation procedure it was possible to resolve the dielectric spectrum of these “pure” ice single crystals into several significant components: the intrinsic Debye spectrum, two weak spectra preceding it at a high frequency, and “space-charge-polarization” spectra. Important characteristics of these spectra and inherent difficulties in quoting their “accurate” parameters are presented. The interpretation of these data requires new molecular models; they are developed in Part II; the analysis of polarization data is resumed in Part III in conjunction with new transconductance measurements.

The infrared spectra of crystalline H 2O, D 2O and HDO

The infrared spectra of normal and deuterated ice prepared by freezing the liquid or by condensing the vapor were studied at temperatures down to −190°C. No important spectral differences between the various specimens were found. The infrared spectra of H 2O, HDO and D 2O at −190°C were compared and the spectrum of crystalline HOD identified. Starting from the HOD spectrum the spectra of H 2O and D 2O could be clarified. The band at 2230 cm −1 in ice is identified as the second overtone of a librational mode. The stretching region of ice consists of 2ν 2 at about 3150 cm −1, ν 3 at about 3240 cm −1 and ν 1 at about 3350 cm −1; D 2O is analogous. The cross-over between the frequency of ν 1 and ν 3 in passing from gas to solid is caused by a change in the stretch-stretch interaction constant. Its magnitude in ice, f 12 = 0.3 cm dyn A ̊ , can be roughly explained on electrostatic grounds. The spectrum of the overtone region shows that the barrier to proton transfer in ice is greater than 27 kcal mole .