Hydrolysis of Ferric Ion in Water and Conformational Equilibrium

Martin, Richard L.; Hay, P. Jeffrey; Pratt, Lawrence R.

doi:10.1021/jp980229p

Cited by 439 publications

(482 citation statements)

References 49 publications

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“…68 It is important to recognize that commonly employed entropy evaluations within the rigid rotor, harmonic oscillator and ideal gas approximations normally overestimate the entropic cost for reactions occurring in solution phase, because ideal gas partition functions do not explicitly take into account hindered translation, rotation and vibration of the solute surrounded by solvent molecules. 25,[69][70][71][72][73][74] For example, Huang and coworkers observed that the calculated standard activation entropy values (-T∆S ‡ calc) consistently overestimate the experimental -T∆S ‡ exp values by ~4-5 kcal/mol at 298K. 71,72 Liang and coworkers also observed that -T∆S ‡ exp values are 50-60% of the computed -T∆S ‡ calc, and in some cases activation entropic costs -T∆S ‡ exp are overestimated by ~11 kcal/mol.…”

Section: Methodsmentioning

confidence: 99%

Reduction of CO₂ to Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine

Lim¹,

Holder²,

et al. 2014

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ABSTRACT:We use quantum chemical calculations to elucidate a viable homogeneous mechanism for pyridine-catalyzed reduction of CO2 to methanol. In the first component of the catalytic cycle, pyridine (Py) undergoes a H + transfer (PT) to form pyridinium (PyH + ) followed by an e -transfer (ET) to produce pyridinium radical (PyH 0 ). Examples of systems to effect this ET to populate PyH + 's LUMO (E 0 calc ~ -1.3V vs. SCE) to form the solution phase PyH 0 via highly reducing electrons include the photo-electrochemical p-GaP system (ECBM ~ -1.5V vs. SCE at pH= 5) and the photochemical [Ru(phen)3] 2+ /ascorbate system. We predict that PyH 0 undergoes further PT-ET steps to form the key closed-shell, dearomatized 1,2-dihydropyridine (PyH2) species. Our proposed sequential PT-ET-PT-ET mechanism transforming Py into PyH2 is consistent with the mechanism described in the formation of related dihydropyridines. Because it is driven by its proclivity to regain aromaticity, PyH2 is a potent recyclable organo-hydride donor that mimics the role of NADPH in the formation of C-H bonds in the photosynthetic CO2 reduction process. In particular, in the second component of the catalytic cycle, we predict that the PyH2/Py redox couple is kinetically and thermodynamically competent in catalytically effecting hydride and proton transfers (the latter often mediated by a proton relay chain) to CO2 and its two succeeding intermediates, namely formic acid and formaldehyde, to ultimately form CH3OH. The hydride and proton transfers for the first reduction step, i.e. reduction of CO2, are sequential in nature; by contrast, they are coupled in each of the two subsequent hydride and proton transfers to reduce formic acid and formaldehyde.

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Section: Methodsmentioning

confidence: 99%

Reduction of CO₂ to Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine

Lim¹,

Holder²,

et al. 2014

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“…Therefore, the results of calculations of reaction free energies involving transition-metal ions are highly sensitive to the choice of computational and cluster models. For example, the hydrolysis constants of [Fe(aq)] 3+ calculated from the four independent studies [43][44][45][46] differ by more than 10 pK units. Recent successes in predicting reaction free energies in aqueous solutions have been attributed for the most part to a compensation of errors in the applied cluster/continuum model, density functional, and the basis set.…”

Section: The Hydrated Cumentioning

confidence: 99%

Computational Study of Copper(II) Complexation and Hydrolysis in Aqueous Solutions Using Mixed Cluster/Continuum Models

2009

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We use density functional theory (B3LYP) and the COSMO continuum solvent model to characterize the structure and stability of the hydrated Cu(II) complexes [Cu(MeNH 2 2-x (x ) 1-3) as a function of metal coordination number (4-6) and cluster size (n ) 4-8, 18). The small clusters with n ) 4-8 are found to be the most stable in the nearly square-planar four-coordinate configuration,, which is three-coordinate. In the presence of the two full hydration shells (n ) 18), however, the five-coordinate square-pyramidal geometry is the most favorable for Cu (MeNH 2 ) 2+ (5, 6) and Cu(OH) + (5, 4, 6), and the four-coordinate geometry is the most stable for Cu(OH) 2 (4, 5) and Cu(OH) 3 -(4). (Other possible coordination numbers for these complexes in the aqueous phase are shown in parentheses.) A small energetic difference between these structures (0.23-2.65 kcal/mol) suggests that complexes with different coordination numbers may coexist in solution. Using two full hydration shells around the Cu 2+ ion (18 ligands) gives Gibbs free energies of aqueous reactions that are in excellent agreement with experiment. The mean unsigned error is 0.7 kcal/mol for the three consecutive hydrolysis steps of Cu 2+ and the complexation of Cu 2+ with methylamine. Conversely, calculations for the complexes with only one coordination shell (four equatorial ligands) lead to a mean unsigned error that is >6.0 kcal/mol. Thus, the explicit treatment of the first and the second shells is critical for the accurate prediction of structural and thermodynamic properties of Cu(II) species in aqueous solution.

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“…Structural and thermodynamic issues are connected, of course, and particularly directly by the quasichemical theory (9)(10)(11)(12)(13)(14)(15)(16)(17). A primitive quasichemical approximation has been developed for the hydration free energy of H ϩ (aq) (4, 7).…”

mentioning

confidence: 99%

Ab initio molecular dynamics and quasichemical study of H ⁺ (aq)

Asthagiri

Pratt

Kress

2005

Proc. Natl. Acad. Sci. U.S.A.

100

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The excess proton in water, H ؉ (aq), plays a fundamental role in aqueous solution chemistry. Its solution thermodynamic properties are essential to molecular descriptions of that chemistry and for validation of dynamical calculations. Within the quasichemical theory of solutions those thermodynamic properties are conditional on recognizing underlying solution structures. The quasichemical treatment identifies H 3O ؉ and H2O5 ؉ as natural innershell complexes, corresponding to the cases of n ‫؍‬ 1, 2 water molecule ligands, respectively, of a distinguished H ؉ ion. A quantum-mechanical treatment of the inner-shell complex with both a dielectric continuum and a classical molecular dynamics treatment of the outer-shell contribution identifies the latter case (the Zundel complex) as the more numerous species. Ab initio molecular dynamics simulations, with two different electron density functionals, suggest a preponderance of Zundel-like structures, but a symmetrical ideal Zundel cation is not observed.Eigen cation ͉ Zundel cation T he hydrated proton, H ϩ (aq), plays a fundamental role in aqueous phase chemistry. Here, we address two related issues of the molecular theory of H ϩ (aq) that haven't been resolved: the structural categorization of the local environment of H ϩ (aq) and hydration free energy of this ion under standard conditions.The difficulty of treating H ϩ (aq) on the basis of molecular theory is principally that the interactions are fundamentally chemical, and therefore complicated when viewed on a thermal energy scale. This is also true of other aqueous ions (1, 2), but H ϩ (aq) is an extreme example. In addressing this complexity, computations of thermodynamic properties of these solutions and comparisons with experimental results give the clearest assessment of theory and simulation work (3-8).Structural and thermodynamic issues are connected, of course, and particularly directly by the quasichemical theory (9-17). A primitive quasichemical approximation has been developed for the hydration free energy of H ϩ (aq) (4, 7). The conclusion of that work was that the H-centered Zundel complex ion H 5 O 2 ϩ provided a simple and valid structural description of the local environment of H ϩ (aq); that theory identifies the Eigen complex ion H 9 O 4 ϩ (i.e., hydrated oxonium) as a structural specification of outer-shell hydration of a distinguished H ϩ , and therefore assigns the Eigen cation less significance.A distinguishing feature of the Zundel complex is an OOO distance of 2.4 Å, shorter than OOO distances between water molecules, or within the hydrated oxonium ion. The latter case (the Eigen structure) might be taken as an O-center basis of a quasichemical model of the solution thermodynamics. But a subsequent observation obtained from ab initio molecular dynamics (AIMD) simulation of HO Ϫ (aq) then becomes relevant (1). AIMD results showed that the nearest coordinating O atom to a HO Ϫ oxygen has the 2.45-Å displacement distinctive of the H 3 O 2 Ϫ complex. This separation is analogous to the case o...

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Hydrolysis of Ferric Ion in Water and Conformational Equilibrium

Abstract: 2

Cited by 439 publications

References 49 publications

Reduction of CO₂ to Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine

Reduction of CO₂ to Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine

Computational Study of Copper(II) Complexation and Hydrolysis in Aqueous Solutions Using Mixed Cluster/Continuum Models

Ab initio molecular dynamics and quasichemical study of H ⁺ (aq)

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Hydrolysis of Ferric Ion in Water and Conformational Equilibrium

Abstract: 2

Cited by 439 publications

References 49 publications

Reduction of CO2 to Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine

Reduction of CO2 to Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine

Computational Study of Copper(II) Complexation and Hydrolysis in Aqueous Solutions Using Mixed Cluster/Continuum Models

Ab initio molecular dynamics and quasichemical study of H + (aq)

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Reduction of CO₂ to Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine

Reduction of CO₂ to Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine

Ab initio molecular dynamics and quasichemical study of H ⁺ (aq)