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Investigates the double proton transfer in the formic acid dimer using ab initio path integral molecular dynamics (PIMD). Description of the electronic structure by Kohn-Sham formulation of the density functional theory; Details on the PIMD approach of Tuckerman; Results of the thermal and quantum fluctuations around the equilibrium structure.

Examines the empirical valence model for proton transfer in aqueous solutions. Properties of the gas-phase H5O2+ complex; Use of gradient-corrected density functional theory in the study; Simulations of the solvated hydronium.

Abstract: The properties of the hydrated hydroxide ion (aq) stand in sharp contrast to those of most other aqueous ions. Chief among these is its anomalously high mobility, which is shared only by the aqueous hydronium ion (aq). However, while the transport mechanism of (aq) is now well understood, the details of (aq) diffusion at ambient conditions are just beginning to emerge, and the effects of temperature on the transport mechanism remain largely unelucidated. Here, we undertake an ab initio molecular dynamics study of the effect of temperature on the solvation and transport of (aq). In addition to revealing new details of the transport process, our analysis provides an explanation for the experimentally observed temperature dependence of the (aq) reorientation time. The calculations reveal a suppression of proton transfer events that underly the structural diffusion process caused by a pronounced change in the population of dominant (aq) solvation complexes. [Copyright &y& Elsevier]

Intermolecular enol tautomers of Watson-Crick base pairs could emerge spontaneously via interbase double proton transfer. It has been hypothesized that their formation could be facilitated by thermal fluctuations and proton tunneling, and possibly be relevant to DNA damage. Theoretical and computational studies, assuming classical nuclei, have confirmed the dynamic stability of these rare tautomers. However, by accounting for nuclear quantum effects explicitly through Car-Parrinello path integral molecular dynamics calculations, we find the tautomeric enol form to be dynamically metastable, with lifetimes too insignificant to be implicated in DNA damage. [ABSTRACT FROM AUTHOR]

Addresses questions concerning the character and anomalously high mobility of the hydrated excess proton in liquid water using computer simulation techniques. Widely held views of the proton's structure and transport; Conclusion that the proton's quantum-mechanical character plays a more important role than previously thought; Implications of results in areas related to aqueous acid-base chemistry, enzymatic proton transfers, proton transport in biological channels and atmospheric proton transfers on ice surfaces.

Looks at the relative influence of thermal and quantum fluctuations on the proton transfer properties of the charged water complexes H5O2+ and H3O2-. Behavior of the shared protons in each complex; Cause of the differing behavior of the proton in each instance.

Concerted proton-electron transfer (CPET) is documented for the homogeneous reduction of O2 to HO2 in water by the one-electron-reduced heteropolytungstate anion, α-PW12O404- (11e). At 0.01-0.3 M H+, O2 reduction occurs via outer-sphere electron transfer followed by proton transfer (ETPT, with rate constant kET). Between 0.30 and 1.9 M H+, rates increase linearly with [H+] due to a parallel CPET pathway in which H+ is now a reactant: (l/2)kobs = kET + kCPET[H+] (kET = 1.2 M-1 s-1; kCPET = 0.8 M-2 s-1). Control experiments rule out preassociation between H+ and 11e. Analysis of plausible rate expressions shows that the first-order dependence on [H+] is uniquely consistent with multisite CPET, and a deuterium kinetic isotope effect of 1.7 is observed. Reductions of O2 by α-SiW12O405- confirm theoretical predictions that CPET decreases in significance as ET becomes less endergonic. Marcus analysis, including the temperature dependence of ΔG°, gives reorganization energies, λET = 41.5 kcal mol-1 and λCPET = 52.4 kcal mol-1. At 1.5 M H+, ∼75% of the (11e,O2) encounter pairs form within 6 Å of H+ ions. This value (6 ± 1 Å) is the "reaction distance" for proton diffusion and probably close to that for CPET. Even so, the 70-200 Ps lifetimes of the (11e,O2) pairs provide additional time for H+ to diffuse closer to O2. CPET is first-order in [H+] because ke for "cage escape" from (11e,O2) pairs is much larger than kCPET, such that the rate expression for CPET becomes -(1/2)d[11e]/dt= (kd/ke)/kCPET[11e][O2][H+], where kd is the rate constant for (11e,O2) pair formation. Overall, the findings suggest that the emergence of CPET, with hydronium ion as the proton donor, may prove a general feature of sufficiently endergonic reductions of dioxygen by otherwise "outer-sphere" complexes (or electrode reactions) at sufficiently low pH values in water. [ABSTRACT FROM AUTHOR]