Proton-transfer dynamics in the (HCO3-)(2) dimer of KHCO3 from Car-Parrinello and path-integrals molecular dynamics calculations
ABSTRACT The proton motion in the (HCO(3)(-))(2) dimer of KHCO(3) at 298 K has been studied with Car-Parrinello molecular dynamics (CPMD) and path-integrals molecular dynamics (PIMD) simulations. According to earlier neutron diffraction studies at 298 K hydrogen is disordered and occupies two positions with an occupancy ratio of 0.804/0.196. A simulation with only one unit cell is not sufficient to reproduce the disorder of the protons found in the experiments. The CPMD results with four cells, 0.783/0.217, are in close agreement with experiment. The motion of the two protons along the O...O bridge is highly correlated inside one dimer, but strongly uncoupled between different dimers. The present results support a mechanism for the disorder which involves proton transfer from donor to acceptor and not orientational disordering of the entire dimer. The question of simultaneous or successive proton transfer in the two hydrogen bonds in the dimer remains unanswered. During the simulation situations with almost simultaneous proton transfer with a time gap of around 1 fs were observed, as well as successive processes where first one proton is transferred and then the second one with a time gap of around 20 fs. The calculated vibrational spectrum is in good agreement with the experimental IR spectrum, but a slightly different assignment of the bands is indicated by the present simulations.
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ABSTRACT: Car-Parrinello molecular dynamics and acoustic method have been applied to determine the hydration of HCO3- and CO32- ions in aqueous solution. From CPMD simulations the hydration numbers are 5.3 and 8.5 for the HCO3- and CO32- ions, respectively. From speed of sound and density measurements the hydration numbers for HCO3- and CO32- are 5.5 and 10.2, respectively. Our attempt to answer the old question concerning the status of HCO3 in solution quite well support the earlier spectroscopic results that no (HCO3)(2) is present in the solution but the existence of the minor amount of dimers cannot be definitely excluded.Chemical Physics Letters 04/2011; 507:89. DOI:10.1016/j.cplett.2011.03.065 · 1.99 Impact Factor
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ABSTRACT: The reversible decomposition of formic acid (HCOOH ⇌ CO(2) + H(2)) has been attracting attention for its potential utility in hydrogen storage and production. It is therefore of interest to explore the influence of solvents on the decomposition reaction. To this end, Born-Oppenheimer (BO) molecular dynamics (MD) calculations have been performed to explore the mechanism involved in hydrogen (H(2)) evolution from formic acid decomposition in an ionic liquid solvent. Specifically, for a solvent consisting of 1,3-dimethylimidazolium cations and formate anions, evolution of hydrogen (H(2)) and carbon dioxide (CO(2)) was observed within a few picoseconds when BO-MD trajectories were carried out at an elevated temperature of 3000 K. The observed dehydrogenation involved a reaction between a formic acid solute and a nearby solvent formate anion. The observed mechanism contrasts with the unimolecular mechanism proposed in the gas phase. Specifically, in the ionic liquid, the reaction is initiated from a C-H bond dissociation of a formate anion to produce a short-lived hydride anion, which subsequently captures the acidic proton of a nearby formic acid molecule. The present BO-MD computations suggest that the high reducing ability of formic acid in the ionic liquid is due in part to its acid-dissociated form: the formate anion, which is encouraged to dissociate into a hydride anion and CO(2) by the strong electrostatic field of the ionic liquid solvent.The Journal of Physical Chemistry B 07/2011; 115(48):14136-40. DOI:10.1021/jp204007w · 3.38 Impact Factor